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Swift Intermediate Language (SIL)

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Abstract

SIL is an SSA-form IR with high-level semantic information designed to implement the Swift programming language. SIL accommodates the following use cases:

  • A set of guaranteed high-level optimizations that provide a predictable baseline for runtime and diagnostic behavior.
  • Diagnostic dataflow analysis passes that enforce Swift language requirements, such as definitive initialization of variables and constructors, code reachability, switch coverage.
  • High-level optimization passes, including retain/release optimization, dynamic method devirtualization, closure inlining, promoting heap allocations to stack allocations, promoting stack allocations to SSA registers, scalar replacement of aggregates (splitting aggregate allocations into multiple smaller allocations), and generic function instantiation.
  • A stable distribution format that can be used to distribute "fragile" inlineable or generic code with Swift library modules, to be optimized into client binaries.

In contrast to LLVM IR, SIL is a generally target-independent format representation that can be used for code distribution, but it can also express target-specific concepts as well as LLVM can.

For more information on developing the implementation of SIL and SIL passes, see:

SIL in the Swift Compiler

At a high level, the Swift compiler follows a strict pipeline architecture:

  • The Parse module constructs an AST from Swift source code.
  • The Sema module type-checks the AST and annotates it with type information.
  • The SILGen module generates raw SIL from an AST.
  • A series of Guaranteed Optimization Passes and Diagnostic Passes are run over the raw SIL both to perform optimizations and to emit language-specific diagnostics. These are always run, even at -Onone, and produce canonical SIL.
  • General SIL Optimization Passes optionally run over the canonical SIL to improve performance of the resulting executable. These are enabled and controlled by the optimization level and are not run at -Onone.
  • IRGen lowers canonical SIL to LLVM IR.
  • The LLVM backend (optionally) applies LLVM optimizations, runs the LLVM code generator and emits binary code.

The stages pertaining to SIL processing in particular are as follows:

SILGen

SILGen produces raw SIL by walking a type-checked Swift AST. The form of SIL emitted by SILGen has the following properties:

  • Variables are represented by loading and storing mutable memory locations instead of being in strict SSA form. This is similar to the initial alloca-heavy LLVM IR emitted by frontends such as Clang. However, Swift represents variables as reference-counted "boxes" in the most general case, which can be retained, released, and captured into closures.
  • Dataflow requirements, such as definitive assignment, function returns, switch coverage (TBD), etc. have not yet been enforced.
  • transparent function optimization has not yet been honored.

These properties are addressed by subsequent guaranteed optimization and diagnostic passes which are always run against the raw SIL.

Guaranteed Optimization and Diagnostic Passes

After SILGen, a deterministic sequence of optimization passes is run over the raw SIL. We do not want the diagnostics produced by the compiler to change as the compiler evolves, so these passes are intended to be simple and predictable.

  • Mandatory inlining inlines calls to "transparent" functions.
  • Memory promotion is implemented as two optimization phases, the first of which performs capture analysis to promote alloc_box instructions to alloc_stack, and the second of which promotes non-address-exposed alloc_stack instructions to SSA registers.
  • Constant propagation folds constant expressions and propagates the constant values. If an arithmetic overflow occurs during the constant expression computation, a diagnostic is issued.
  • Return analysis verifies that each function returns a value on every code path and doesn't "fall off the end" of its definition, which is an error. It also issues an error when a noreturn function returns.
  • Critical edge splitting splits all critical edges from terminators that don't support arbitrary basic block arguments (all non cond_branch terminators).

If all diagnostic passes succeed, the final result is the canonical SIL for the program.

TODO:

  • Generic specialization
  • Basic ARC optimization for acceptable performance at -Onone.

General Optimization Passes

SIL captures language-specific type information, making it possible to perform high-level optimizations that are difficult to perform on LLVM IR.

  • Generic Specialization analyzes specialized calls to generic functions and generates new specialized version of the functions. Then it rewrites all specialized usages of the generic to a direct call of the appropriate specialized function.
  • Witness and VTable Devirtualization for a given type looks up the associated method from a class's vtable or a type witness table and replaces the indirect virtual call with a call to the mapped function.
  • Performance Inlining
  • Reference Counting Optimizations
  • Memory Promotion/Optimizations
  • High-level domain specific optimizations The Swift compiler implements high-level optimizations on basic Swift containers such as Array or String. Domain specific optimizations require a defined interface between the standard library and the optimizer. More details can be found here: HighLevelSILOptimizations

Syntax

SIL is reliant on Swift's type system and declarations, so SIL syntax is an extension of Swift's. A .sil file is a Swift source file with added SIL definitions. The Swift source is parsed only for its declarations; Swift func bodies (except for nested declarations) and top-level code are ignored by the SIL parser. In a .sil file, there are no implicit imports; the swift and/or Builtin standard modules must be imported explicitly if used.

Here is an example of a .sil file:

sil_stage canonical

import Swift

// Define types used by the SIL function.

struct Point {
  var x : Double
  var y : Double
}

class Button {
  func onClick()
  func onMouseDown()
  func onMouseUp()
}

// Declare a Swift function. The body is ignored by SIL.
func taxicabNorm(_ a:Point) -> Double {
  return a.x + a.y
}

// Define a SIL function.
// The name @_T5norms11taxicabNormfT1aV5norms5Point_Sd is the mangled name
// of the taxicabNorm Swift function.
sil @_T5norms11taxicabNormfT1aV5norms5Point_Sd : $(Point) -> Double {
bb0(%0 : $Point):
  // func Swift.+(Double, Double) -> Double
  %1 = function_ref @_Tsoi1pfTSdSd_Sd
  %2 = struct_extract %0 : $Point, #Point.x
  %3 = struct_extract %0 : $Point, #Point.y
  %4 = apply %1(%2, %3) : $(Double, Double) -> Double
  return %4 : Double
}

// Define a SIL vtable. This matches dynamically-dispatched method
// identifiers to their implementations for a known static class type.
sil_vtable Button {
  #Button.onClick: @_TC5norms6Button7onClickfS0_FT_T_
  #Button.onMouseDown: @_TC5norms6Button11onMouseDownfS0_FT_T_
  #Button.onMouseUp: @_TC5norms6Button9onMouseUpfS0_FT_T_
}

SIL Stage

decl ::= sil-stage-decl
sil-stage-decl ::= 'sil_stage' sil-stage

sil-stage ::= 'raw'
sil-stage ::= 'canonical'

There are different invariants on SIL depending on what stage of processing has been applied to it.

  • Raw SIL is the form produced by SILGen that has not been run through guaranteed optimizations or diagnostic passes. Raw SIL may not have a fully-constructed SSA graph. It may contain dataflow errors. Some instructions may be represented in non-canonical forms, such as assign and destroy_addr for non-address-only values. Raw SIL should not be used for native code generation or distribution.
  • Canonical SIL is SIL as it exists after guaranteed optimizations and diagnostics. Dataflow errors must be eliminated, and certain instructions must be canonicalized to simpler forms. Performance optimization and native code generation are derived from this form, and a module can be distributed containing SIL in this (or later) forms.

SIL files declare the processing stage of the included SIL with one of the declarations sil_stage raw or sil_stage canonical at top level. Only one such declaration may appear in a file.

SIL Types

sil-type ::= '$' '*'? generic-parameter-list? type

SIL types are introduced with the $ sigil. SIL's type system is closely related to Swift's, and so the type after the $ is parsed largely according to Swift's type grammar.

Type Lowering

A formal type is the type of a value in Swift, such as an expression result. Swift's formal type system intentionally abstracts over a large number of representational issues like ownership transfer conventions and directness of arguments. However, SIL aims to represent most such implementation details, and so these differences deserve to be reflected in the SIL type system. Type lowering is the process of turning a formal type into its lowered type.

It is important to be aware that the lowered type of a declaration need not be the lowered type of the formal type of that declaration. For example, the lowered type of a declaration reference:

  • will usually be thin,
  • may have a non-Swift calling convention,
  • may use bridged types in its interface, and
  • may use ownership conventions that differ from Swift's default conventions.

Abstraction Difference

Generic functions working with values of unconstrained type must generally work with them indirectly, e.g. by allocating sufficient memory for them and then passing around pointers to that memory. Consider a generic function like this:

func generateArray<T>(n : Int, generator : () -> T) -> [T]

The function generator will be expected to store its result indirectly into an address passed in an implicit parameter. There's really just no reasonable alternative when working with a value of arbitrary type:

  • We don't want to generate a different copy of generateArray for every type T.
  • We don't want to give every type in the language a common representation.
  • We don't want to dynamically construct a call to generator depending on the type T.

But we also don't want the existence of the generic system to force inefficiencies on non-generic code. For example, we'd like a function of type () -> Int to be able to return its result directly; and yet, () -> Int is a valid substitution of () -> T, and a caller of generateArray<Int> should be able to pass an arbitrary () -> Int in as the generator.

Therefore, the representation of a formal type in a generic context may differ from the representation of a substitution of that formal type. We call such differences abstraction differences.

SIL's type system is designed to make abstraction differences always result in differences between SIL types. The goal is that a properly- abstracted value should be correctly usable at any level of substitution.

In order to achieve this, the formal type of a generic entity should always be lowered using the abstraction pattern of its unsubstituted formal type. For example, consider the following generic type:

struct Generator<T> {
  var fn : () -> T
}
var intGen : Generator<Int>

intGen.fn has the substituted formal type () -> Int, which would normally lower to the type @callee_owned () -> Int, i.e. returning its result directly. But if that type is properly lowered with the pattern of its unsubstituted type () -> T, it becomes @callee_owned () -> @out Int.

When a type is lowered using the abstraction pattern of an unrestricted type, it is lowered as if the pattern were replaced with a type sharing the same structure but replacing all materializable types with fresh type variables.

For example, if g has type Generator<(Int, Int) -> Float>, g.fn is lowered using the pattern () -> T, which eventually causes (Int, Int) -> Float to be lowered using the pattern T, which is the same as lowering it with the pattern U -> V; the result is that g.fn has the following lowered type:

@callee_owned () -> @owned @callee_owned (@in (Int, Int)) -> @out Float.

As another example, suppose that h has type Generator<(Int, inout Int) -> Float>. Neither (Int, inout Int) nor inout Int are potential results of substitution because they aren't materializable, so h.fn has the following lowered type:

@callee_owned () -> @owned @callee_owned (@in Int, @inout Int) -> @out Float

This system has the property that abstraction patterns are preserved through repeated substitutions. That is, you can consider a lowered type to encode an abstraction pattern; lowering T by R is equivalent to lowering T by (S lowered by R).

SILGen has procedures for converting values between abstraction patterns.

At present, only function and tuple types are changed by abstraction differences.

Legal SIL Types

The type of a value in SIL is either:

  • an object type $T, where T is a legal loadable type, or
  • an address type $*T, where T is a legal SIL type (loadable or address-only).

A type T is a legal SIL type if:

  • it is a function type which satisfies the constraints (below) on function types in SIL,
  • it is a metatype type which describes its representation,
  • it is a tuple type whose element types are legal SIL types,
  • it is Optional<U>, where U is a legal SIL type,
  • it is a legal Swift type that is not a function, tuple, optional, metatype, or l-value type, or
  • it is a @box containing a legal SIL type.

Note that types in other recursive positions in the type grammar are still formal types. For example, the instance type of a metatype or the type arguments of a generic type are still formal Swift types, not lowered SIL types.

Address Types

The address of T $*T is a pointer to memory containing a value of any reference or value type $T. This can be an internal pointer into a data structure. Addresses of loadable types can be loaded and stored to access values of those types.

Addresses of address-only types (see below) can only be used with instructions that manipulate their operands indirectly by address, such as copy_addr or destroy_addr, or as arguments to functions. It is illegal to have a value of type $T if T is address-only.

Addresses are not reference-counted pointers like class values are. They cannot be retained or released.

Address types are not first-class: they cannot appear in recursive positions in type expressions. For example, the type $**T is not a legal type.

The address of an address cannot be directly taken. $**T is not a representable type. Values of address type thus cannot be allocated, loaded, or stored (though addresses can of course be loaded from and stored to).

Addresses can be passed as arguments to functions if the corresponding parameter is indirect. They cannot be returned.

Box Types

Captured local variables and the payloads of indirect value types are stored on the heap. The type @box T is a reference-counted type that references a box containing a mutable value of type T. Boxes always use Swift-native reference counting, so they can be queried for uniqueness and cast to the Builtin.NativeObject type.

Metatype Types

A concrete or existential metatype in SIL must describe its representation. This can be:

  • @thin, meaning that it requires no storage and thus necessarily represents an exact type (only allowed for concrete metatypes);
  • @thick, meaning that it stores a reference to a type or (if a concrete class) a subclass of that type; or
  • @objc, meaning that it stores a reference to a class type (or a subclass thereof) using an Objective-C class object representation rather than the native Swift type-object representation.

Function Types

Function types in SIL are different from function types in Swift in a number of ways:

  • A SIL function type may be generic. For example, accessing a generic function with function_ref will give a value of generic function type.

  • A SIL function type may be declared @noescape. This is required for any function type passed to a parameter not declared with @escaping declaration modifier. @noescape function types may be either @convention(thin) or @callee_guaranteed. They have an unowned context--the context's lifetime must be independently guaranteed.

  • A SIL function type declares its conventional treatment of its context value:

    • If it is @convention(thin), the function requires no context value. Such types may also be declared @noescape, which trivially has no effect passing the context value.
    • If it is @callee_guaranteed, the context value is treated as a direct parameter. This implies @convention(thick). If the function type is also @noescape, then the context value is unowned, otherwise it is guaranteed.
    • If it is @callee_owned, the context value is treated as an owned direct parameter. This implies @convention(thick) and is mutually exclusive with @noescape.
    • If it is @convention(block), the context value is treated as an unowned direct parameter.
    • Other function type conventions are described in Properties of Types and Calling Convention.

  • A SIL function type declares the conventions for its parameters. The parameters are written as an unlabeled tuple; the elements of that tuple must be legal SIL types, optionally decorated with one of the following convention attributes.

    The value of an indirect parameter has type *T; the value of a direct parameter has type T.

    • An @in parameter is indirect. The address must be of an initialized object; the function is responsible for destroying the value held there.
    • An @inout parameter is indirect. The address must be of an initialized object. The memory must remain initialized for the duration of the call until the function returns. The function may mutate the pointee, and furthermore may weakly assume that there are no aliasing reads from or writes to the argument, though must preserve a valid value at the argument so that well-ordered aliasing violations do not compromise memory safety. This allows for optimizations such as local load and store propagation, introduction or elimination of temporary copies, and promotion of the @inout parameter to an @owned direct parameter and result pair, but does not admit "take" optimization out of the parameter or other optimization that would leave memory in an uninitialized state.
    • An @inout_aliasable parameter is indirect. The address must be of an initialized object. The memory must remain initialized for the duration of the call until the function returns. The function may mutate the pointee, and must assume that other aliases may mutate it as well. These aliases however can be assumed to be well-typed and well-ordered; ill-typed accesses and data races to the parameter are still undefined.
    • An @owned parameter is an owned direct parameter.
    • A @guaranteed parameter is a guaranteed direct parameter.
    • An @in_guaranteed parameter is indirect. The address must be of an initialized object; both the caller and callee promise not to mutate the pointee, allowing the callee to read it.
    • An @in_constant parameter is indirect. The address must be of an initialized object; the function will treat the value held there as read-only.
    • A @pack_owned parameter is indirect. The parameter must be of pack type and is always an address. Whether the pack elements are direct values or addresses of values is encoded in the pack type. In either case, both the pack elements and their referenced storage (if they are addresses) must be initialized prior to the call. The callee is responsible for destroying the values and is permitted to modify both the pack elements and their referenced storage. The caller is not permitted to access either the pack or the referenced storage during the call.
    • A @pack_guaranteed parameter is indirect. The parameter must be of pack type and is always an address. Whether the pack elements are direct values or addresses of values is encoded in the pack type. In either case, both the pack elements and their referenced storage (if they are addresses) must be initialized prior to the call. Neither the callee nor the caller is permitted to modify or destroy the pack elements or their referenced storage during the call.
    • A @pack_inout parameter is indirect. The parameter must be of pack type and is always an address. The pack elements must also always be addresses. The element addresses are set in the pack prior to the call, and the same addresses must be in the pack following the call, but the callee is permitted to modify the pack on a temporary basis if it wishes. The referenced storage of each element address must be initialized prior to the call, and it must still be initialized after the call, but the callee may modify the value stored there and potentially even leave it temporarily uninitialized. The caller is not permitted to access either the pack elements or their referenced storage during the call.
    • Otherwise, the parameter is an unowned direct parameter.

  • A SIL function type declares the conventions for its results. The results are written as an unlabeled tuple; the elements of that tuple must be legal SIL types, optionally decorated with one of the following convention attributes. Indirect and direct results may be interleaved.

    Indirect results correspond to implicit arguments of type *T in function entry blocks and in the arguments to apply and try_apply instructions. These arguments appear in the order in which they appear in the result list, always before any parameters.

    Direct results correspond to direct return values of type T. A SIL function type has a return type derived from its direct results in the following way: when there is a single direct result, the return type is the type of that result; otherwise, it is the tuple type of the types of all the direct results, in the order they appear in the results list. The return type is the type of the operand of return instructions, the type of apply instructions, and the type of the normal result of try_apply instructions.

    • An @out result is indirect.

      If the result type is not a pack type, then the address must be of an uninitialized object, and the callee is required to leave an initialized value there unless it terminates with a throw or has a non-Swift calling convention.

      If the result type is a pack type, then the pack must contain addresses. The addresses must be set in the pack prior to the call, and these same addresses must be in the pack after the call, but the callee may modify the pack elements on a temporary basis if it wishes. The addresses must be of uninitialized objects, and the callee is require to initialize them unless it terminates with a throw or has a non-Swift calling convention.

    • An @owned result is an owned direct result.

    • An @autoreleased result is an autoreleased direct result. If there is an autoreleased result, it must be the only direct result.

    • Otherwise, the parameter is an unowned direct result.

A direct parameter, yield, or result of trivial type must always be unowned.

A parameter or yield of pack type must always use one of the three pack conventions. A result of pack type must always be @out.

An owned direct parameter or result is transferred to the recipient, which becomes responsible for destroying the value. This means that the value is passed at +1.

An unowned direct parameter or result is instantaneously valid at the point of transfer. The recipient does not need to worry about race conditions immediately destroying the value, but should copy it (e.g. by strong_retaining an object pointer) if the value will be needed sooner rather than later.

A guaranteed direct parameter is like an unowned direct parameter value, except that it is guaranteed by the caller to remain valid throughout the execution of the call. This means that any strong_retain, strong_release pairs in the callee on the argument can be eliminated.

An autoreleased direct result must have a type with a retainable pointer representation. Autoreleased results are nominally transferred at +0, but the runtime takes steps to ensure that a +1 can be safely transferred, and those steps require precise code-layout control. Accordingly, the SIL pattern for an autoreleased convention looks exactly like the SIL pattern for an owned convention, and the extra runtime instrumentation is inserted on both sides when the SIL is lowered into LLVM IR. An autoreleased apply of a function that is defined with an autoreleased result has the effect of a +1 transfer of the result. An autoreleased apply of a function that is not defined with an autoreleased result has the effect of performing a strong retain in the caller. A non-autoreleased apply of a function that is defined with an autoreleased result has the effect of performing an autorelease in the callee.

  • SIL function types may provide an optional error result, written by placing @error on a result. An error result is always implicitly @owned. Only functions with a native calling convention may have an error result.

    A function with an error result cannot be called with apply. It must be called with try_apply. There is one exception to this rule: a function with an error result can be called with apply [nothrow] if the compiler can prove that the function does not actually throw.

    return produces a normal result of the function. To return an error result, use throw.

    Type lowering lowers the throws annotation on formal function types into more concrete error propagation:

    • For native Swift functions, throws is turned into an error result.
    • For non-native Swift functions, throws is turned in an explicit error-handling mechanism based on the imported API. The importer only imports non-native methods and types as throws when it is possible to do this automatically.

  • SIL function types may provide a pattern signature and substitutions to express that values of the type use a particular generic abstraction pattern. Both must be provided together. If a pattern signature is present, the component types (parameters, yields, and results) must be expressed in terms of the generic parameters of that signature. The pattern substitutions should be expressed in terms of the generic parameters of the overall generic signature, if any, or else the enclosing generic context, if any.

    A pattern signature follows the @substituted attribute, which must be the final attribute preceding the function type. Pattern substitutions follow the function type, preceded by the for keyword. For example:

    @substituted <T: Collection> (@in T) -> @out T.Element for Array<Int>

    The low-level representation of a value of this type may not match the representation of a value of the substituted-through version of it:

    (@in Array<Int>) -> @out Int

    Substitution differences at the outermost level of a function value may be adjusted using the convert_function instruction. Note that this only works at the outermost level and not in nested positions. For example, a function which takes a parameter of the first type above cannot be converted by convert_function to a function which takes a parameter of the second type; such a conversion must be done with a thunk.

    Type substitution on a function type with a pattern signature and substitutions only substitutes into the substitutions; the component types are preserved with their exact original structure.

  • In the implementation, a SIL function type may also carry substitutions for its generic signature. This is a convenience for working with applied generic types and is not generally a formal part of the SIL language; in particular, values should not have such types. Such a type behaves like a non-generic type, as if the substitutions were actually applied to the underlying function type.

Async Functions

SIL function types may be @async. @async functions run inside async tasks, and can have explicit suspend points where they suspend execution. @async functions can only be called from other @async functions, but otherwise can be invoked with the normal apply and try_apply instructions (or begin_apply if they are coroutines).

In Swift, the withUnsafeContinuation primitive is used to implement primitive suspend points. In SIL, @async functions represent this abstraction using the get_async_continuation[_addr] and await_async_continuation instructions. get_async_continuation[_addr] accesses a continuation value that can be used to resume the coroutine after it suspends. The resulting continuation value can then be passed into a completion handler, registered with an event loop, or scheduled by some other mechanism. Operations on the continuation can resume the async function's execution by passing a value back to the async function, or passing in an error that propagates as an error in the async function's context. The await_async_continuation instruction suspends execution of the coroutine until the continuation is invoked to resume it. A use of withUnsafeContinuation in Swift:

func waitForCallback() async -> Int {
  return await withUnsafeContinuation { cc in
    registerCallback { cc.resume($0) }
  }
}

might lower to the following SIL:

sil @waitForCallback : $@convention(thin) @async () -> Int {
entry:
  %cc = get_async_continuation $Int
  %closure = function_ref @waitForCallback_closure
    : $@convention(thin) (UnsafeContinuation<Int>) -> ()
  apply %closure(%cc)
  await_async_continuation %cc, resume resume_cc

resume_cc(%result : $Int):
  return %result
}

The closure may then be inlined into the waitForCallback function:

sil @waitForCallback : $@convention(thin) @async () -> Int {
entry:
  %cc = get_async_continuation $Int
  %registerCallback = function_ref @registerCallback
    : $@convention(thin) (@convention(thick) () -> ()) -> ()
  %callback_fn = function_ref @waitForCallback_callback
  %callback = partial_apply %callback_fn(%cc)
  apply %registerCallback(%callback)
  await_async_continuation %cc, resume resume_cc

resume_cc(%result : $Int):
  return %result
}

Every continuation value must be used exactly once to resume its associated async coroutine once. It is undefined behavior to attempt to resume the same continuation more than once. On the flip side, failing to resume a continuation will leave the async task stuck in the suspended state, leaking any memory or other resources it owns.

Coroutine Types

A coroutine is a function which can suspend itself and return control to its caller without terminating the function. That is, it does not need to obey a strict stack discipline. SIL coroutines have control flow that is tightly integrated with their callers, and they pass information back and forth between caller and callee in a structured way through yield points. Generalized accessors and generators in Swift fit this description: a read or modify accessor coroutine projects a single value, yields ownership of that one value temporarily to the caller, and then takes ownership back when resumed, allowing the coroutine to clean up resources or otherwise react to mutations done by the caller. Generators similarly yield a stream of values one at a time to their caller, temporarily yielding ownership of each value in turn to the caller. The tight coupling of the caller's control flow with these coroutines allows the caller to borrow values produced by the coroutine, where a normal function return would need to transfer ownership of its return value, since a normal function's context ceases to exist and be able to maintain ownership of the value after it returns.

To support these concepts, SIL supports two kinds of coroutine: @yield_many and @yield_once. Either of these attributes may be written before a function type to indicate that it is a coroutine type. @yield_many and @yield_once coroutines are allowed to also be @async. (Note that @async functions are not themselves modeled explicitly as coroutines in SIL, although the implementation may use a coroutine lowering strategy.)

A coroutine type may declare any number of yielded values, which is to say, values which are provided to the caller at a yield point. Yielded values are written in the result list of a function type, prefixed by the @yields attribute. A yielded value may have a convention attribute, taken from the set of parameter attributes and interpreted as if the yield site were calling back to the calling function.

Currently, a coroutine may not have normal results.

Coroutine functions may be used in many of the same ways as normal function values. However, they cannot be called with the standard apply or try_apply instructions. A non-throwing yield-once coroutine can be called with the begin_apply instruction. There is no support yet for calling a throwing yield-once coroutine or for calling a yield-many coroutine of any kind.

Coroutines may contain the special yield and unwind instructions.

A @yield_many coroutine may yield as many times as it desires. A @yield_once coroutine may yield exactly once before returning, although it may also throw before reaching that point.

Variadic Generics

Swift's variadic generics feature introduces the concepts of pack parameters, pack arguments, and pack expansions. When these features are used in formal types embedded in SIL, they follow the same rules as they do in Swift. However, in its own type system and operations, SIL largely uses a different (if closely related) language model.

Pack types

In (current) Swift, packs only exist as parameters, either type parameters or value parameters. These parameters can then only be used in pack expansions, which can only appear in certain naturally-variadic positions, such as the elements list of a tuple type or the arguments list of a call expression. Formally, substitution flattens these pack expansions into the surrounding structure.

This language model poses similar problems for direct implementation at runtime as many of Swift's other generics features. Normal compilation paths (without unportable assembly-level heroics) require functions to take a fixed list of parameters. Calling a generic function with packs of different lengths cannot result in different parameters being mapped to different registers (or positions in the stack arguments area). SIL must therefore organize pack expansions in function parameters and results into concrete variadic packs that can be passed with a fixed ABI.

SIL must also directly model temporary packs, not just pack parameters. After all, a pack parameter must be bound to something concretely provided by the caller.

Packs are therefore something closer to a first-class type in the SIL type system. A pack type is written with the syntax Pack { ... }. By default, a pack type is indirect, meaning that its elements are addresses. SIL does not currently support direct packs, but the description in this document tries to leave room for them.

Pack types are always address-only and are always passed around by address. Values of pack type cannot be moved or copied except by explicitly iterating over the pack elements. They are allocated and deallocated with special instructions.

Pack types are only allowed in two positions: - at the top level of a type, e.g. %0 : $Pack{Int, Float} - as a parameter or result type of a function type, e.g.

%fn : $@convention(thin) (@pack_in Pack{Int, Float}) -> ()

Note in particular that they are not allowed in tuple types. Pack expansions in tuple types are still flattened into the surrounding tuple structure like they are in Swift (unless the tuple is exploded, as tuples normally are in function parameters or results). There are specific instructions for manipulating tuples with variadic elements.

This explicit pack syntax can also be used to delimit type argument packs in positions that expect formal types, such as the substitution list of an apply instruction. This is necessary because SIL functions can be parameterized over multiple packs, and unlike in Swift, the type arguments to such functions are explicit on calls. If Swift ever gains syntax to delimit packs in type argument lists, the SIL syntax will switch to use it. Other features of SIL pack types, such as direct-ness, are not allowed in these positions; a formal pack type is purely a list of types and type expansions.

Pack expansions

Pack expansions (repeat) are allowed in a reduced set of situations in lowered types: - the elements list of a tuple type - the elements list of a pack type Note in particular that pack expansions cannot appear in the parameters list or results list of a lowered function type. The function type must traffic in packs explicitly.

If substitution into a lowered tuple type that is not a single unlabeled element that is not a pack expansion would produce a tuple with a single unlabeled element that is not a pack expansion, it actually produces that element type. Certain instructions must be rewritten to accommodate this when cloned under substitution.

Opened pack element archetypes

SIL must be able to work directly with the element types of a pack with statically unknown elements. For example, it might need to move each element of a pack into a tuple. To do this, it must be able to give a temporary name to the element type. This type is called an opened pack element archetype. It is spelled like this:

::
@pack_element("<uuid>") T

There must be an open_pack_element in the current SIL function with the given UUID. The name T must resolve to a pack parameter within the generic signature of this instruction, and that pack parameter must have the same shape class in the signature as the opened shape class pack parameter. The pack parameter is then translated to the corresponding element archetype.

Opened pack element archetypes can appear in both formal and lowered types. As with opened existential archetypes, the open_pack_element which introduces an opened pack element archetype must dominate all uses of the archetype.

The current SIL parser does not support references to opened element archetypes prior to the open_pack_element instruction. This can occur when basic blocks are not in dominance order. Fixing this will likely require changes to the syntax, at least for forward references.

Properties of Types

SIL classifies types into additional subgroups based on ABI stability and generic constraints:

  • Loadable types are types with a fully exposed concrete representation:

    • Reference types
    • Builtin value types
    • Fragile struct types in which all element types are loadable
    • Tuple types in which all element types are loadable
    • Class protocol types
    • Archetypes constrained by a class protocol

    Values of loadable types are loaded and stored by loading and storing individual components of their representation. As a consequence:

    • values of loadable types can be loaded into SIL SSA values and stored from SSA values into memory without running any user-written code, although compiler-generated reference counting operations can happen.
    • values of loadable types can be take-initialized (moved between memory locations) with a bitwise copy.

    A loadable aggregate type is a tuple or struct type that is loadable.

    A trivial type is a loadable type with trivial value semantics. Values of trivial type can be loaded and stored without any retain or release operations and do not need to be destroyed.

  • Runtime-sized types are restricted value types for which the compiler does not know the size of the type statically:

    • Resilient value types
    • Fragile struct or tuple types that contain resilient types as elements at any depth
    • Archetypes not constrained by a class protocol

  • Address-only types are restricted value types which cannot be loaded or otherwise worked with as SSA values:

    • Runtime-sized types
    • Non-class protocol types
    • @weak types
    • Types that can't satisfy the requirements for being loadable because they care about the exact location of their value in memory and need to run some user-written code when they are copied or moved. Most commonly, types "care" about the addresses of values because addresses of values are registered in some global data structure, or because values may contain pointers into themselves. For example:
      • Addresses of values of Swift @weak types are registered in a global table. That table needs to be adjusted when a @weak value is copied or moved to a new address.
      • A non-COW collection type with a heap allocation (like std::vector in C++) needs to allocate memory and copy the collection elements when the collection is copied.
      • A non-COW string type that implements a small string optimization (like many implementations of std::string in C++) can contain a pointer into the value itself. That pointer needs to be recomputed when the string is copied or moved.

    Values of address-only type ("address-only values") must reside in memory and can only be referenced in SIL by address. Addresses of address-only values cannot be loaded from or stored to. SIL provides special instructions for indirectly manipulating address-only values, such as copy_addr and destroy_addr.

Some additional meaningful categories of type:

  • A heap object reference type is a type whose representation consists of a single strong-reference-counted pointer. This includes all class types, the Builtin.NativeObject and AnyObject types, and archetypes that conform to one or more class protocols.
  • A reference type is more general in that its low-level representation may include additional global pointers alongside a strong-reference-counted pointer. This includes all heap object reference types and adds thick function types and protocol/protocol composition types that conform to one or more class protocols. All reference types can be retain-ed and release-d. Reference types also have ownership semantics for their referenced heap object; see Reference Counting below.
  • A type with retainable pointer representation is guaranteed to be compatible (in the C sense) with the Objective-C id type. The value at runtime may be nil. This includes classes, class metatypes, block functions, and class-bounded existentials with only Objective-C-compatible protocol constraints, as well as one level of Optional or ImplicitlyUnwrappedOptional applied to any of the above. Types with retainable pointer representation can be returned via the @autoreleased return convention.

SILGen does not always map Swift function types one-to-one to SIL function types. Function types are transformed in order to encode additional attributes:

  • The convention of the function, indicated by the

    @convention(convention)

    attribute. This is similar to the language-level @convention attribute, though SIL extends the set of supported conventions with additional distinctions not exposed at the language level:

    • @convention(thin) indicates a "thin" function reference, which uses the Swift calling convention with no special "self" or "context" parameters.
    • @convention(thick) indicates a "thick" function reference, which uses the Swift calling convention and carries a reference-counted context object used to represent captures or other state required by the function. This attribute is implied by @callee_owned or @callee_guaranteed.
    • @convention(block) indicates an Objective-C compatible block reference. The function value is represented as a reference to the block object, which is an id-compatible Objective-C object that embeds its invocation function within the object. The invocation function uses the C calling convention.
    • @convention(c) indicates a C function reference. The function value carries no context and uses the C calling convention.
    • @convention(objc_method) indicates an Objective-C method implementation. The function uses the C calling convention, with the SIL-level self parameter (by SIL convention mapped to the final formal parameter) mapped to the self and _cmd arguments of the implementation.
    • @convention(method) indicates a Swift instance method implementation. The function uses the Swift calling convention, using the special self parameter.
    • @convention(witness_method) indicates a Swift protocol method implementation. The function's polymorphic convention is emitted in such a way as to guarantee that it is polymorphic across all possible implementors of the protocol.

Layout Compatible Types

(This section applies only to Swift 1.0 and will hopefully be obviated in future releases.)

SIL tries to be ignorant of the details of type layout, and low-level bit-banging operations such as pointer casts are generally undefined. However, as a concession to implementation convenience, some types are allowed to be considered layout compatible. Type T is layout compatible with type U iff:

  • an address of type $*U can be cast by address_to_pointer/pointer_to_address to $*T and a valid value of type T can be loaded out (or indirectly used, if T is address- only),
  • if T is a nontrivial type, then retain_value/release_value of the loaded T value is equivalent to retain_value/release_value of the original U value.

This is not always a commutative relationship; T can be layout-compatible with U whereas U is not layout-compatible with T. If the layout compatible relationship does extend both ways, T and U are commutatively layout compatible. It is however always transitive; if T is layout-compatible with U and U is layout-compatible with V, then T is layout-compatible with V. All types are layout-compatible with themselves.

The following types are considered layout-compatible:

  • Builtin.RawPointer is commutatively layout compatible with all heap object reference types, and Optional of heap object reference types. (Note that RawPointer is a trivial type, so does not have ownership semantics.)
  • Builtin.RawPointer is commutatively layout compatible with Builtin.Word.
  • Structs containing a single stored property are commutatively layout compatible with the type of that property.
  • A heap object reference is commutatively layout compatible with any type that can correctly reference the heap object. For instance, given a class B and a derived class D inheriting from B, a value of type B referencing an instance of type D is layout compatible with both B and D, as well as Builtin.NativeObject and AnyObject. It is not layout compatible with an unrelated class type E.
  • For payloaded enums, the payload type of the first payloaded case is layout-compatible with the enum (not commutatively).

Values and Operands

sil-identifier ::= [A-Za-z_0-9]+
sil-value-name ::= '%' sil-identifier
sil-value ::= sil-value-name
sil-value ::= 'undef'
sil-operand ::= sil-value ':' sil-type

SIL values are introduced with the % sigil and named by an alphanumeric identifier, which references the instruction or basic block argument that produces the value. SIL values may also refer to the keyword 'undef', which is a value of undefined contents.

Unlike LLVM IR, SIL instructions that take value operands only accept value operands. References to literal constants, functions, global variables, or other entities require specialized instructions such as integer_literal, function_ref, global_addr, etc.

Functions

decl ::= sil-function
sil-function ::= 'sil' sil-linkage? sil-function-attribute+
                   sil-function-name ':' sil-type
                   '{' effects* sil-basic-block* '}'
sil-function-name ::= '@' [A-Za-z_0-9]+

SIL functions are defined with the sil keyword. SIL function names are introduced with the @ sigil and named by an alphanumeric identifier. This name will become the LLVM IR name for the function, and is usually the mangled name of the originating Swift declaration. The sil syntax declares the function's name and SIL type, and defines the body of the function inside braces. The declared type must be a function type, which may be generic.

If a function body does not contain atleast one sil-basic-block, the function is an external declaration.

Function Attributes

sil-function-attribute ::= '[canonical]'

The function is in canonical SIL even if the module is still in raw SIL.

sil-function-attribute ::= '[ossa]'

The function is in OSSA (ownership SSA) form.

sil-function-attribute ::= '[transparent]'

Transparent functions are always inlined and don't keep their source information when inlined.

sil-function-attribute ::= '[' sil-function-thunk ']'
sil-function-thunk ::= 'thunk'
sil-function-thunk ::= 'signature_optimized_thunk'
sil-function-thunk ::= 'reabstraction_thunk'

The function is a compiler generated thunk.

sil-function-attribute ::= '[dynamically_replacable]'

The function can be replaced at runtime with a different implementation. Optimizations must not assume anything about such a function, even if the SIL of the function body is available.

sil-function-attribute ::= '[dynamic_replacement_for' identifier ']'
sil-function-attribute ::= '[objc_replacement_for' identifier ']'

Specifies for which function this function is a replacement.

sil-function-attribute ::= '[exact_self_class]'

The function is a designated initializers, where it is known that the static type being allocated is the type of the class that defines the designated initializer.

sil-function-attribute ::= '[without_actually_escaping]'

The function is a thunk for closures which are not actually escaping.

sil-function-attribute ::= '[' sil-function-purpose ']'
sil-function-purpose ::= 'global_init'

The implied semantics are:

  • side-effects can occur any time before the first invocation.
  • all calls to the same global_init function have the same side-effects.
  • any operation that may observe the initializer's side-effects must be preceded by a call to the initializer.

This is currently true if the function is an addressor that was lazily generated from a global variable access. Note that the initialization function itself does not need this attribute. It is private and only called within the addressor.

sil-function-purpose ::= 'lazy_getter'

The function is a getter of a lazy property for which the backing storage is an Optional of the property's type. The getter contains a top-level switch_enum (or switch_enum_addr), which tests if the lazy property is already computed. In the None-case, the property is computed and stored to the backing storage of the property.

After the first call of a lazy property getter, it is guaranteed that the property is computed and consecutive calls always execute the Some-case of the top-level switch_enum.

sil-function-attribute ::= '[weak_imported]'

Cross-module references to this function should always use weak linking.

sil-function-attribute ::= '[stack_protection]'

Stack protectors are inserted into this function to detect stack related buffer overflows.

sil-function-attribute ::= '[available' sil-version-tuple ']'
sil-version-tuple ::= [0-9]+ ('.' [0-9]+)*

The minimal OS-version where the function is available.

sil-function-attribute ::= '[' sil-function-inlining ']'
sil-function-inlining ::= 'noinline'

The function is never inlined.

sil-function-inlining ::= 'always_inline'

The function is always inlined.

sil-function-attribute ::= '[' sil-function-optimization ']'
sil-function-inlining ::= 'Onone'
sil-function-inlining ::= 'Ospeed'
sil-function-inlining ::= 'Osize'

The function is optimized according to this attribute, overriding the setting from the command line.

sil-function-attribute ::= '[' sil-function-effects ']'
sil-function-effects ::= 'readonly'
sil-function-effects ::= 'readnone'
sil-function-effects ::= 'readwrite'
sil-function-effects ::= 'releasenone'

The specified memory effects of the function.

sil-function-attribute ::= '[_semantics "' [A-Za-z._0-9]+ '"]'

The specified high-level semantics of the function. The optimizer can use this information to perform high-level optimizations before such functions are inlined. For example, Array operations are annotated with semantic attributes to let the optimizer perform redundant bounds check elimination and similar optimizations.

sil-function-attribute ::= '[_specialize "' [A-Za-z._0-9]+ '"]'

Specifies for which types specialized code should be generated.

sil-function-attribute ::= '[clang "' identifier '"]'

The clang node owner.

sil-function-attribute ::= '[' performance-constraint ']'
performance-constraint :: 'no_locks'
performance-constraint :: 'no_allocation'

Specifies the performance constraints for the function, which defines which type of runtime functions are allowed to be called from the function.

Argument Effects

The function effects, especially for function arguments. For details see the documentation in SwiftCompilerSources/Sources/SIL/Effects.swift.

effects ::= '[' argument-name ':' argument-effect (',' argument-effect)*]'
effects ::= '[' 'global' ':' global-effect (',' global-effect)*]'
argument-name ::= '%' [0-9]+

argument-effect ::= 'noescape' defined-effect? projection-path?
argument-effect ::= 'escape' defined-effect? projection-path? '=>' arg-or-return  // exclusive escape
argument-effect ::= 'escape' defined-effect? projection-path? '->' arg-or-return  // not-exclusive escape
argument-effect ::= side-effect

global-effect ::= 'traps'
global-effect ::= 'allocates'
global-effect ::= side-effect

side-effect ::= 'read' projection-path?
side-effect ::= 'write' projection-path?
side-effect ::= 'copy' projection-path?
side-effect ::= 'read' projection-path?

arg-or-return ::= argument-name ('.' projection-path)?
arg-or-return ::= '%r' ('.' projection-path)?
defined-effect ::= '!'    // the effect is defined in the source code and not
                          // derived by the optimizer

projection-path ::= path-component ('.' path-component)*
path-component ::= 's' [0-9]+        // struct field
path-component ::= 'c' [0-9]+        // class field
path-component ::= 'ct'              // class tail element
path-component ::= 'e' [0-9]+        // enum case
path-component ::= [0-9]+            // tuple element
path-component ::= 'v**'             // any value fields
path-component ::= 'c*'              // any class field
path-component ::= '**'              // anything

Basic Blocks

sil-basic-block ::= sil-label sil-instruction-def* sil-terminator
sil-label ::= sil-identifier ('(' sil-argument (',' sil-argument)* ')')? ':'
sil-value-ownership-kind ::= @owned
sil-value-ownership-kind ::= @guaranteed
sil-value-ownership-kind ::= @unowned
sil-argument ::= sil-value-name ':' sil-value-ownership-kind? sil-type

sil-instruction-result ::= sil-value-name
sil-instruction-result ::= '(' (sil-value-name (',' sil-value-name)*)? ')'
sil-instruction-source-info ::= (',' sil-scope-ref)? (',' sil-loc)?
sil-instruction-def ::=
  (sil-instruction-result '=')? sil-instruction sil-instruction-source-info

A function body consists of one or more basic blocks that correspond to the nodes of the function's control flow graph. Each basic block contains one or more instructions and ends with a terminator instruction. The function's entry point is always the first basic block in its body.

In SIL, basic blocks take arguments, which are used as an alternative to LLVM's phi nodes. Basic block arguments are bound by the branch from the predecessor block:

sil @iif : $(Builtin.Int1, Builtin.Int64, Builtin.Int64) -> Builtin.Int64 {
bb0(%cond : $Builtin.Int1, %ifTrue : $Builtin.Int64, %ifFalse : $Builtin.Int64):
  cond_br %cond : $Builtin.Int1, then, else
then:
  br finish(%ifTrue : $Builtin.Int64)
else:
  br finish(%ifFalse : $Builtin.Int64)
finish(%result : $Builtin.Int64):
  return %result : $Builtin.Int64
}

Arguments to the entry point basic block, which has no predecessor, are bound by the function's caller:

sil @foo : $@convention(thin) (Int) -> Int {
bb0(%x : $Int):
  return %x : $Int
}

sil @bar : $@convention(thin) (Int, Int) -> () {
bb0(%x : $Int, %y : $Int):
  %foo = function_ref @foo
  %1 = apply %foo(%x) : $(Int) -> Int
  %2 = apply %foo(%y) : $(Int) -> Int
  %3 = tuple ()
  return %3 : $()
}

When a function is in Ownership SSA, arguments additionally have an explicit annotated convention that describe the ownership semantics of the argument value:

sil [ossa] @baz : $@convention(thin) (Int, @owned String, @guaranteed String, @unowned String) -> () {
bb0(%x : $Int, %y : @owned $String, %z : @guaranteed $String, %w : @unowned $String):
  ...
}

Note that the first argument (%x) has an implicit ownership kind of @none since all trivial values have @none ownership.

Debug Information

sil-scope-ref ::= 'scope' [0-9]+
sil-scope ::= 'sil_scope' [0-9]+ '{'
                 sil-loc
                 'parent' scope-parent
                 ('inlined_at' sil-scope-ref)?
              '}'
scope-parent ::= sil-function-name ':' sil-type
scope-parent ::= sil-scope-ref
sil-loc ::= 'loc' string-literal ':' [0-9]+ ':' [0-9]+

Each instruction may have a debug location and a SIL scope reference at the end. Debug locations consist of a filename, a line number, and a column number. If the debug location is omitted, it defaults to the location in the SIL source file. SIL scopes describe the position inside the lexical scope structure that the Swift expression a SIL instruction was generated from had originally. SIL scopes also hold inlining information.

Declaration References

sil-decl-ref ::= '#' sil-identifier ('.' sil-identifier)* sil-decl-subref?
sil-decl-subref ::= '!' sil-decl-subref-part ('.' sil-decl-lang)? ('.' sil-decl-autodiff)?
sil-decl-subref ::= '!' sil-decl-lang
sil-decl-subref ::= '!' sil-decl-autodiff
sil-decl-subref-part ::= 'getter'
sil-decl-subref-part ::= 'setter'
sil-decl-subref-part ::= 'allocator'
sil-decl-subref-part ::= 'initializer'
sil-decl-subref-part ::= 'enumelt'
sil-decl-subref-part ::= 'destroyer'
sil-decl-subref-part ::= 'deallocator'
sil-decl-subref-part ::= 'globalaccessor'
sil-decl-subref-part ::= 'ivardestroyer'
sil-decl-subref-part ::= 'ivarinitializer'
sil-decl-subref-part ::= 'defaultarg' '.' [0-9]+
sil-decl-lang ::= 'foreign'
sil-decl-autodiff ::= sil-decl-autodiff-kind '.' sil-decl-autodiff-indices
sil-decl-autodiff-kind ::= 'jvp'
sil-decl-autodiff-kind ::= 'vjp'
sil-decl-autodiff-indices ::= [SU]+

Some SIL instructions need to reference Swift declarations directly. These references are introduced with the # sigil followed by the fully qualified name of the Swift declaration. Some Swift declarations are decomposed into multiple entities at the SIL level. These are distinguished by following the qualified name with ! and one or more .-separated component entity discriminators:

  • getter: the getter function for a var declaration
  • setter: the setter function for a var declaration
  • allocator: a struct or enum constructor, or a class's allocating constructor
  • initializer: a class's initializing constructor
  • enumelt: a member of a enum type.
  • destroyer: a class's destroying destructor
  • deallocator: a class's deallocating destructor
  • globalaccessor: the addressor function for a global variable
  • ivardestroyer: a class's ivar destroyer
  • ivarinitializer: a class's ivar initializer
  • defaultarg.n: the default argument-generating function for the n-th argument of a Swift func
  • foreign: a specific entry point for C/Objective-C interoperability

Linkage

sil-linkage ::= 'public'
sil-linkage ::= 'hidden'
sil-linkage ::= 'shared'
sil-linkage ::= 'private'
sil-linkage ::= 'public_external'
sil-linkage ::= 'hidden_external'
sil-linkage ::= 'non_abi'

A linkage specifier controls the situations in which two objects in different SIL modules are linked, i.e. treated as the same object.

A linkage is external if it ends with the suffix external. An object must be a definition if its linkage is not external.

All functions, global variables, and witness tables have linkage. The default linkage of a definition is public. The default linkage of a declaration is public_external. (These may eventually change to hidden and hidden_external, respectively.)

On a global variable, an external linkage is what indicates that the variable is not a definition. A variable lacking an explicit linkage specifier is presumed a definition (and thus gets the default linkage for definitions, public.)

Definition of the linked relation

Two objects are linked if they have the same name and are mutually visible:

  • An object with public or public_external linkage is always visible.
  • An object with hidden, hidden_external, or shared linkage is visible only to objects in the same Swift module.
  • An object with private linkage is visible only to objects in the same SIL module.

Note that the linked relationship is an equivalence relation: it is reflexive, symmetric, and transitive.

Requirements on linked objects

If two objects are linked, they must have the same type.

If two objects are linked, they must have the same linkage, except:

  • A public object may be linked to a public_external object.
  • A hidden object may be linked to a hidden_external object.

If two objects are linked, at most one may be a definition, unless:

  • both objects have shared linkage or
  • at least one of the objects has an external linkage.

If two objects are linked, and both are definitions, then the definitions must be semantically equivalent. This equivalence may exist only on the level of user-visible semantics of well-defined code; it should not be taken to guarantee that the linked definitions are exactly operationally equivalent. For example, one definition of a function might copy a value out of an address parameter, while another may have had an analysis applied to prove that said value is not needed.

If an object has any uses, then it must be linked to a definition with non-external linkage.

Public non-ABI linkage

The non_abi linkage is a special linkage used for definitions which only exist in serialized SIL, and do not define visible symbols in the object file.

A definition with non_abi linkage behaves like it has shared linkage, except that it must be serialized in the SIL module even if not referenced from anywhere else in the module. For example, this means it is considered a root for dead function elimination.

When a non_abi definition is deserialized, it will have shared_external linkage.

There is no non_abi_external linkage. Instead, when referencing a non_abi declaration that is defined in a different translation unit from the same Swift module, you must use hidden_external linkage.

Summary

  • public definitions are unique and visible everywhere in the program. In LLVM IR, they will be emitted with external linkage and default visibility.
  • hidden definitions are unique and visible only within the current Swift module. In LLVM IR, they will be emitted with external linkage and hidden visibility.
  • private definitions are unique and visible only within the current SIL module. In LLVM IR, they will be emitted with private linkage.
  • shared definitions are visible only within the current Swift module. They can be linked only with other shared definitions, which must be equivalent; therefore, they only need to be emitted if actually used. In LLVM IR, they will be emitted with linkonce_odr linkage and hidden visibility.
  • public_external and hidden_external objects always have visible definitions somewhere else. If this object nonetheless has a definition, it's only for the benefit of optimization or analysis. In LLVM IR, declarations will have external linkage and definitions (if actually emitted as definitions) will have available_externally linkage.

VTables

decl ::= sil-vtable
sil-vtable ::= 'sil_vtable' identifier '{' sil-vtable-entry* '}'

sil-vtable-entry ::= sil-decl-ref ':' sil-linkage? sil-function-name

SIL represents dynamic dispatch for class methods using the class_method, super_method, objc_method, and objc_super_method instructions.

The potential destinations for class_method and super_method are tracked in sil_vtable declarations for every class type. The declaration contains a mapping from every method of the class (including those inherited from its base class) to the SIL function that implements the method for that class:

class A {
  func foo()
  func bar()
  func bas()
}

sil @A_foo : $@convention(thin) (@owned A) -> ()
sil @A_bar : $@convention(thin) (@owned A) -> ()
sil @A_bas : $@convention(thin) (@owned A) -> ()

sil_vtable A {
  #A.foo: @A_foo
  #A.bar: @A_bar
  #A.bas: @A_bas
}

class B : A {
  func bar()
}

sil @B_bar : $@convention(thin) (@owned B) -> ()

sil_vtable B {
  #A.foo: @A_foo
  #A.bar: @B_bar
  #A.bas: @A_bas
}

class C : B {
  func bas()
}

sil @C_bas : $@convention(thin) (@owned C) -> ()

sil_vtable C {
  #A.foo: @A_foo
  #A.bar: @B_bar
  #A.bas: @C_bas
}

Note that the declaration reference in the vtable is to the least-derived method visible through that class (in the example above, B's vtable references A.bar and not B.bar, and C's vtable references A.bas and not C.bas). The Swift AST maintains override relationships between declarations that can be used to look up overridden methods in the SIL vtable for a derived class (such as C.bas in C's vtable).

In case the SIL function is a thunk, the function name is preceded with the linkage of the original implementing function.

Witness Tables

decl ::= sil-witness-table
sil-witness-table ::= 'sil_witness_table' sil-linkage?
                      normal-protocol-conformance '{' sil-witness-entry* '}'

SIL encodes the information needed for dynamic dispatch of generic types into witness tables. This information is used to produce runtime dispatch tables when generating binary code. It can also be used by SIL optimizations to specialize generic functions. A witness table is emitted for every declared explicit conformance. Generic types share one generic witness table for all of their instances. Derived classes inherit the witness tables of their base class.

protocol-conformance ::= normal-protocol-conformance
protocol-conformance ::= 'inherit' '(' protocol-conformance ')'
protocol-conformance ::= 'specialize' '<' substitution* '>'
                         '(' protocol-conformance ')'
protocol-conformance ::= 'dependent'
normal-protocol-conformance ::= identifier ':' identifier 'module' identifier

Witness tables are keyed by protocol conformance, which is a unique identifier for a concrete type's conformance to a protocol.

  • A normal protocol conformance names a (potentially unbound generic) type, the protocol it conforms to, and the module in which the type or extension declaration that provides the conformance appears. These correspond 1:1 to protocol conformance declarations in the source code.
  • If a derived class conforms to a protocol through inheritance from its base class, this is represented by an inherited protocol conformance, which simply references the protocol conformance for the base class.
  • If an instance of a generic type conforms to a protocol, it does so with a specialized conformance, which provides the generic parameter bindings to the normal conformance, which should be for a generic type.

Witness tables are only directly associated with normal conformances. Inherited and specialized conformances indirectly reference the witness table of the underlying normal conformance.

sil-witness-entry ::= 'base_protocol' identifier ':' protocol-conformance
sil-witness-entry ::= 'method' sil-decl-ref ':' sil-function-name
sil-witness-entry ::= 'associated_type' identifier
sil-witness-entry ::= 'associated_type_protocol'
                      '(' identifier ':' identifier ')' ':' protocol-conformance

Witness tables consist of the following entries:

  • Base protocol entries provide references to the protocol conformances that satisfy the witnessed protocols' inherited protocols.
  • Method entries map a method requirement of the protocol to a SIL function that implements that method for the witness type. One method entry must exist for every required method of the witnessed protocol.
  • Associated type entries map an associated type requirement of the protocol to the type that satisfies that requirement for the witness type. Note that the witness type is a source-level Swift type and not a SIL type. One associated type entry must exist for every required associated type of the witnessed protocol.
  • Associated type protocol entries map a protocol requirement on an associated type to the protocol conformance that satisfies that requirement for the associated type.

Default Witness Tables

decl ::= sil-default-witness-table
sil-default-witness-table ::= 'sil_default_witness_table'
                              identifier minimum-witness-table-size
                              '{' sil-default-witness-entry* '}'
minimum-witness-table-size ::= integer

SIL encodes requirements with resilient default implementations in a default witness table. We say a requirement has a resilient default implementation if the following conditions hold:

  • The requirement has a default implementation
  • The requirement is either the last requirement in the protocol, or all subsequent requirements also have resilient default implementations

The set of requirements with resilient default implementations is stored in protocol metadata.

The minimum witness table size is the size of the witness table, in words, not including any requirements with resilient default implementations.

Any conforming witness table must have a size between the minimum size, and the maximum size, which is equal to the minimum size plus the number of default requirements.

At load time, if the runtime encounters a witness table with fewer than the maximum number of witnesses, the witness table is copied, with default witnesses copied in. This ensures that callers can always expect to find the correct number of requirements in each witness table, and new requirements can be added by the framework author, without breaking client code, as long as the new requirements have resilient default implementations.

Default witness tables are keyed by the protocol itself. Only protocols with public visibility need a default witness table; private and internal protocols are never seen outside the module, therefore there are no resilience issues with adding new requirements.

sil-default-witness-entry ::= 'method' sil-decl-ref ':' sil-function-name

Default witness tables currently contain only one type of entry:

  • Method entries map a method requirement of the protocol to a SIL function that implements that method in a manner suitable for all witness types.

Global Variables

decl ::= sil-global-variable
static-initializer ::= '=' '{' sil-instruction-def* '}'
sil-global-variable ::= 'sil_global' sil-linkage identifier ':' sil-type
                           (static-initializer)?

SIL representation of a global variable.

Global variable access is performed by the alloc_global, global_addr and global_value instructions.

A global can have a static initializer if its initial value can be composed of literals. The static initializer is represented as a list of literal and aggregate instructions where the last instruction is the top-level value of the static initializer:

sil_global hidden @$S4test3varSiv : $Int {
  %0 = integer_literal $Builtin.Int64, 27
  %initval = struct $Int (%0 : $Builtin.Int64)
}

If a global does not have a static initializer, the alloc_global instruction must be performed prior an access to initialize the storage. Once a global's storage has been initialized, global_addr is used to project the value.

If the last instruction in the static initializer is an object instruction the global variable is a statically initialized object. In this case the variable cannot be used as l-value, i.e. the reference to the object cannot be modified. As a consequence the variable cannot be accessed with global_addr but only with global_value.

Differentiability Witnesses

decl ::= sil-differentiability-witness
sil-differentiability-witness ::=
    'sil_differentiability_witness'
    sil-linkage?
    '[' differentiability-kind ']'
    '[' 'parameters' sil-differentiability-witness-function-index-list ']'
    '[' 'results' sil-differentiability-witness-function-index-list ']'
    generic-parameter-clause?
    sil-function-name ':' sil-type
    sil-differentiability-witness-body?

differentiability-kind ::= 'forward' | 'reverse' | 'normal' | 'linear'

sil-differentiability-witness-body ::=
    '{' sil-differentiability-witness-entry?
        sil-differentiability-witness-entry? '}'

sil-differentiability-witness-entry ::=
    sil-differentiability-witness-entry-kind ':'
    sil-entry-name ':' sil-type

sil-differentiability-witness-entry-kind ::= 'jvp' | 'vjp'

SIL encodes function differentiability via differentiability witnesses.

Differentiability witnesses map a "key" (including an "original" SIL function) to derivative SIL functions.

Differentiability witnesses are keyed by the following:

  • An "original" SIL function name.
  • Differentiability parameter indices.
  • Differentiability result indices.
  • A generic parameter clause, representing differentiability generic requirements.

Differentiability witnesses may have a body, specifying derivative functions for the key. Verification checks that derivative functions have the expected type based on the key.

sil_differentiability_witness hidden [normal] [parameters 0] [results 0] <T where T : Differentiable> @id : $@convention(thin) (T) -> T {
  jvp: @id_jvp : $@convention(thin) (T) -> (T, @owned @callee_guaranteed (T.TangentVector) -> T.TangentVector)
  vjp: @id_vjp : $@convention(thin) (T) -> (T, @owned @callee_guaranteed (T.TangentVector) -> T.TangentVector)
}

During SILGen, differentiability witnesses are emitted for the following:

  • @differentiable declaration attributes.
  • @derivative declaration attributes. Registered derivative functions become differentiability witness entries.

The SIL differentiation transform canonicalizes differentiability witnesses, filling in missing entries.

Differentiability witness entries are accessed via the differentiability_witness_function instruction.

Dataflow Errors

Dataflow errors may exist in raw SIL. Swift's semantics defines these conditions as errors, so they must be diagnosed by diagnostic passes and must not exist in canonical SIL.

Definitive Initialization

Swift requires that all local variables be initialized before use. In constructors, all instance variables of a struct, enum, or class type must be initialized before the object is used and before the constructor is returned from.

Unreachable Control Flow

The unreachable terminator is emitted in raw SIL to mark incorrect control flow, such as a non-Void function failing to return a value, or a switch statement failing to cover all possible values of its subject. The guaranteed dead code elimination pass can eliminate truly unreachable basic blocks, or unreachable instructions may be dominated by applications of functions returning uninhabited types. An unreachable instruction that survives guaranteed DCE and is not immediately preceded by a no-return application is a dataflow error.

Ownership SSA

A SILFunction marked with the [ossa] function attribute is considered to be in Ownership SSA form. Ownership SSA is an augmented version of SSA that enforces ownership invariants by imbuing value-operand edges with semantic ownership information. All SIL values are assigned a constant ownership kind that defines the ownership semantics that the value models. All SIL operands that use a SIL value are required to be able to be semantically partitioned in between "non-lifetime ending uses" that just require the value to be live and "lifetime ending uses" that end the lifetime of the value and after which the value can no longer be used. Since by definition operands that are lifetime ending uses end their associated value's lifetime, we must have that, ignoring program ending Dead End Blocks, the lifetime ending use points jointly post-dominate all non-lifetime ending use points and that a value must have exactly one lifetime ending use along all reachable program paths, preventing leaks and use-after-frees. As an example, consider the following SIL example with partitioned defs/uses annotated inline:

sil @stash_and_cast : $@convention(thin) (@owned Klass) -> @owned SuperKlass {
bb0(%kls1 : @owned $Klass): // Definition of %kls1

  // "Normal Use" kls1.
  // Definition of %kls2.
  %kls2 = copy_value %kls1 : $Klass

  // "Consuming Use" of %kls2 to store it into a global. Stores in ossa are
  // consuming since memory is generally assumed to have "owned"
  // semantics. After this instruction executes, we can no longer use %kls2
  // without triggering an ownership violation.
  store %kls2 to [init] %globalMem : $*Klass

  // "Consuming Use" of %kls1.
  // Definition of %kls1Casted.
  %kls1Casted = upcast %kls1 : $Klass to $SuperKlass

  // "Consuming Use" of %kls1Casted
  return %kls1Casted : $SuperKlass
}

Notice how every value in the SIL above has a partitionable set of uses with normal uses always before consuming uses. Any such violations of ownership semantics would trigger a SILVerifier error allowing us to know that we do not have any leaks or use-after-frees in the above code.

Ownership Kind

The semantics in the previous example is of just one form of ownership semantics supported: "owned" semantics. In SIL, we map these "ownership semantics" into a form that a compiler can reason about by mapping semantics onto a lattice with the following elements: None, Owned, Guaranteed, Unowned, Any. We call this the lattice of "Ownership Kinds" and each individual value an "Ownership Kind". This lattice is defined as a 3-level lattice with:

1. None being Top.
2. Any being Bottom.
3. All non-Any, non-None OwnershipKinds being defined as a mid-level elements of the lattice

We can graphically represent the lattice via a diagram like the following:

              +------+
    +-------- | None | ---------+
    |         +------+          |
    |            |              |
    v            v              v         ^
+-------+  +-----+------+  +---------+    |
| Owned |  | Guaranteed |  | Unowned |    +--- Value Ownership Kinds and
+-------+  +-----+------+  +---------+         Ownership Constraints
    |            |              |
    |            v              |         +--- Only Ownership Constraints
    |         +-----+           |         |
    +-------->| Any |<----------+         v
              +-----+

One moves down the lattice by performing a "meet" operation:

None meet OtherOwnershipKind -> OtherOwnershipKind
Unowned meet Owned -> Any
Owned meet Guaranteed -> Any

and one moves up the lattice by performing a "join" operation, e.x.:

Any join OtherOwnershipKind -> OtherOwnershipKind
Owned join Any -> Owned
Owned join Guaranteed -> None

This lattice is applied to SIL by requiring well formed SIL to:

  1. Define a map of each SIL value to a constant OwnershipKind that classify the semantics that the SIL value obeys. This ownership kind may be static (i.e.: the same for all instances of an instruction) or dynamic (e.x.: forwarding instructions set their ownership upon construction). We call this subset of OwnershipKind to be the set of Value Ownership Kind: None, Unowned, Guaranteed, Owned (note conspicuously missing Any). This is because in our model Any represents an unknown ownership semantics and since our model is strict, we do not allow for values to have unknown ownership.
  2. Define a map from each operand of a SILInstruction, i, to a constant Ownership Kind, Boolean pair called the operand's Ownership Constraint. The Ownership Kind element of the Ownership Constraint determines semantically which ownership kind's the operand's value can take on. The Boolean value is used to know if an operand will end the lifetime of the incoming value when checking dataflow rules. The dataflow rules that each Value Ownership Kind obeys is documented for each Value Ownership Kind in its detailed description below.

Then we take these two maps and require that valid SIL has the property that given an operand, op(i) of an instruction i and a value v that op(i) can only use v if the join of OwnershipConstraint(operand(i)) with ValueOwnershipKind(v) is equal to the ValueOwnershipKind of v. In symbols, we must have that:

join : (OwnershipConstraint, ValueOwnershipKind) -> ValueOwnershipKind
OwnershipConstraint(operand(i)) join ValueOwnershipKind(v) = ValueOwnershipKind(v)

In words, a value can be passed to an operand if applying the operand's ownership constraint to the value's ownership does not change the value's ownership. Operationally this has a few interesting effects on SIL:

  1. We have defined away invalid value-operand (aka def-use) pairing since the SILVerifier validates the aforementioned relationship on all SIL values, uses at all points of the pipeline until ossa is lowered.
  2. Many SIL instructions do not care about the ownership kind that their value will take. They can just define all of their operand's as having an ownership constraint of Any.

Now lets go into more depth upon Value Ownership Kind and Ownership Constraint.

Value Ownership Kind

As mentioned above, each SIL value is statically mapped to an Ownership Kind called the value's "ValueOwnershipKind" that classify the semantics of the value. Below, we map each ValueOwnershipKind to a short summary of the semantics implied upon the parent value:

  • None. This is used to represent values that do not require memory management and are outside of Ownership SSA invariants. Examples: trivial values (e.x.: Int, Float), non-payloaded cases of non-trivial enums (e.x.: Optional<T>.none), all address types.
  • Owned. A value that exists independently of any other value and is consumed exactly once along all paths through a function by either a destroy_value (actually destroying the value) or by a consuming instruction that rebinds the value in some manner (e.x.: apply, casts, store).
  • Guaranteed. A value with a scoped lifetime whose liveness is dependent on the lifetime of some other "base" owned or guaranteed value. Consumed by instructions like end_borrow. The "base" value is statically guaranteed to be live at all of the value's paired end_borrow instructions.
  • Unowned. A value that is only guaranteed to be instantaneously valid and must be copied before the value is used in an @owned or @guaranteed context. This is needed both to model argument values with the ObjC unsafe unowned argument convention and also to model the ownership resulting from bitcasting a trivial type to a non-trivial type. This value should never be consumed.

We describe each of these semantics in below in more detail.

Owned

Owned ownership models "move only" values. We require that each such value is consumed exactly once along all program paths. The IR verifier will flag values that are not consumed along a path as a leak and any double consumes as use-after-frees. We model move operations via forwarding uses such as casts and transforming terminators (e.x.: switch_enum, checked_cast_br) that transform the input value, consuming it in the process, and producing a new transformed owned value as a result.

Putting this all together, one can view each owned SIL value as being effectively a "move only value" except when explicitly copied by a copy_value. This of course implies that ARC operations can be assumed to only semantically effect the specific value that they are applied to /and/ that each ARC constraint is able to be verified independently for each owned SILValue derived from the ARC object. As an example, consider the following Swift/SIL:

// testcase.swift.
func doSomething(x : Klass) -> OtherKlass? {
  return x as? OtherKlass
}

// testcase.sil. A possible SILGen lowering
sil [ossa] @doSomething : $@convention(thin) (@guaranteed Klass) -> () {
bb0(%0 : @guaranteed Klass):
  // Definition of '%1'
  %1 = copy_value %0 : $Klass

  // Consume '%1'. This means '%1' can no longer be used after this point. We
  // rebind '%1' in the destination blocks (bbYes, bbNo).
  checked_cast_br %1 : $Klass to $OtherKlass, bbYes, bbNo

bbYes(%2 : @owned $OtherKlass): // On success, the checked_cast_br forwards
                                // '%1' into '%2' after casting to OtherKlass.

  // Forward '%2' into '%3'. '%2' can not be used past this point in the
  // function.
  %3 = enum $Optional<OtherKlass>, case #Optional.some!enumelt, %2 : $OtherKlass

  // Forward '%3' into the branch. '%3' can not be used past this point.
  br bbEpilog(%3 : $Optional<OtherKlass>)

bbNo(%3 : @owned $Klass): // On failure, since we consumed '%1' already, we
                          // return the original '%1' as a new value '%3'
                          // so we can use it below.
  // Actually destroy the underlying copy (``%1``) created by the copy_value
  // in bb0.
  destroy_value %3 : $Klass

  // We want to return nil here. So we create a new non-payloaded enum and
  // pass it off to bbEpilog.
  %4 = enum $Optional<OtherKlass>, case #Optional.none!enumelt
  br bbEpilog(%4 : $Optional<OtherKlass>)

bbEpilog(%5 : @owned $Optional<OtherKlass>):
  // Consumes '%5' to return to caller.
  return %5 : $Optional<OtherKlass>
}

Notice how our individual copy (%1) threads its way through the IR using forwarding uses of @owned ownership. These forwarding uses partition the lifetime of the result of the copy_value into a set of disjoint individual owned lifetimes (%2, %3, %5).

Guaranteed

Guaranteed ownership models values that have a scoped dependent lifetime on a "base value" with owned or guaranteed ownership. Due to this lifetime dependence, the base value is required to be statically live over the entire scope where the guaranteed value is valid.

These explicit scopes are introduced into SIL by begin scope instructions (e.x.: begin_borrow, load_borrow) that are paired with sets of jointly post-dominating scope ending instructions (e.x.: end_borrow):

sil [ossa] @guaranteed_values : $@convention(thin) (@owned Klass) -> () {
bb0(%0 : @owned $Klass):
  %1 = begin_borrow %0 : $Klass
  cond_br ..., bb1, bb2

bb1:
  ...
  end_borrow %1 : $Klass
  destroy_value %0 : $Klass
  br bb3

bb2:
  ...
  end_borrow %1 : $Klass
  destroy_value %0 : $Klass
  br bb3

bb3:
  ...
}

Notice how the end_borrow allow for a SIL generator to communicate to optimizations that they can never shrink the lifetime of %0 by moving destroy_value above %1.

Values with guaranteed ownership follow a dataflow rule that states that non-consuming forwarding uses of the guaranteed value are also guaranteed and are recursively validated as being in the original values scope. This was a choice we made to reduce idempotent scopes in the IR:

sil [ossa] @get_first_elt : $@convention(thin) (@guaranteed (String, String)) -> @owned String {
bb0(%0 : @guaranteed $(String, String)):
  // %1 is validated as if it was apart of %0 and does not need its own begin_borrow/end_borrow.
  %1 = tuple_extract %0 : $(String, String)
  // So this copy_value is treated as a use of %0.
  %2 = copy_value %1 : $String
  return %2 : $String
}

None

Values with None ownership are inert values that exist outside of the guarantees of Ownership SSA. Some examples of such values are:

  • Trivially typed values such as: Int, Float, Double
  • Non-payloaded non-trivial enums.
  • Address types.

Since values with none ownership exist outside of ownership SSA, they can be used like normal SSA without violating ownership SSA invariants. This does not mean that code does not potentially violate other SIL rules (consider memory lifetime invariants):

sil @none_values : $@convention(thin) (Int, @in Klass) -> Int {
bb0(%0 : $Int, %1 : $*Klass):

  // %0, %1 are normal SSA values that can be used anywhere in the function
  // without breaking Ownership SSA invariants. It could violate other
  // invariants if for instance, we load from %1 after we destroy the object
  // there.
  destroy_addr %1 : $*Klass

  // If uncommented, this would violate memory lifetime invariants due to
  // the ``destroy_addr %1`` above. But this would not violate the rules of
  // Ownership SSA since addresses exist outside of the guarantees of
  // Ownership SSA.
  //
  // %2 = load [take] %1 : $*Klass

  // I can return this object without worrying about needing to copy since
  // none objects can be arbitrarily returned.
  return %0 : $Int
}

Unowned

This is a form of ownership that is used to model two different use cases:

  • Arguments of functions with ObjC convention. This convention requires the callee to copy the value before using it (preferably before any other code runs). We do not model this flow sensitive property in SIL today, but we do not allow for unowned values to be passed as owned or guaranteed values without copying it first.
  • Values that are a conversion from a trivial value with None ownership to a non-trivial value. As an example of this consider an unsafe bit cast of a trivial pointer to a class. In that case, since we have no reason to assume that the object will remain alive, we need to make a copy of the value.

Ownership Constraint

NOTE: We assume that one has read the section above on Ownership Kind.

As mentioned above, every operand operand(i) of a SIL instruction i has statically mapped to it:

  1. An ownership kind that acts as an "Ownership Constraint" upon what "Ownership Kind" a value can take.
  2. A boolean value that defines whether or not the execution of the operand's instruction will cause the operand's value to be invalidated. This is often times referred to as an operand acting as a "lifetime ending use".

Forwarding Uses

NOTE: In the following, we assumed that one read the section above, Ownership Kind, Value Ownership Kind and Ownership Constraint.

A subset of SIL instructions define the value ownership kind of their results in terms of the value ownership kind of their operands. Such an instruction is called a "forwarding instruction" and any use with such a user instruction a "forwarding use". This inference generally occurs upon instruction construction and as a result:

  • When manipulating forwarding instructions programmatically, one must manually update their forwarded ownership since most of the time the ownership is stored in the instruction itself. Don't worry though because the SIL verifier will catch this error for you if you forget to do so!

  • Textual SIL does not represent the ownership of forwarding instructions explicitly. Instead, the instruction's ownership is inferred normally from the parsed operand. In some cases the forwarding ownership kind is different from the ownership kind of its operand. In such cases, textual SIL represents the forwarding ownership kind explicitly. Eg:

    %cast = unchecked_ref_cast %val : $Klass to $Optional<Klass>, forwarding: @unowned

    Since the SILVerifier runs on Textual SIL after parsing, you can feel confident that ownership constraints were inferred correctly.

Forwarding has slightly different ownership semantics depending on the value ownership kind of the operand on construction and the result's type. We go through each below:

  • Given an @owned operand, the forwarding instruction is assumed to end the lifetime of the operand and produce an @owned value if non-trivially typed and @none if trivially typed. Example: This is used to represent the semantics of casts:

    sil @unsafelyCastToSubClass : $@convention(thin) (@owned Klass) -> @owned SubKlass {
bb0(%0 : @owned $Klass): // %0 is defined here.

  // %0 is consumed here and can no longer be used after this point.
  // %1 is defined here and after this point must be used to access the object
  // passed in via %0.
  %1 = unchecked_ref_cast %0 : $Klass to $SubKlass

  // Then %1's lifetime ends here and we return the casted argument to our
  // caller as an @owned result.
  return %1 : $SubKlass
}

  • Given a @guaranteed operand, the forwarding instruction is assumed to produce @guaranteed non-trivially typed values and @none trivially typed values. Given the non-trivial case, the instruction is assumed to begin a new implicit borrow scope for the incoming value. Since the borrow scope is implicit, we validate the uses of the result as if they were uses of the operand (recursively). This of course means that one should never see end_borrows on any guaranteed forwarded results, the end_borrow is always on the instruction that "introduces" the borrowed value. An example of a guaranteed forwarding instruction is struct_extract:

    // In this function, I have a pair of Klasses and I want to grab some state
// and then call the hand off function for someone else to continue
// processing the pair.
sil @accessLHSStateAndHandOff : $@convention(thin) (@owned KlassPair) -> @owned State {
bb0(%0 : @owned $KlassPair): // %0 is defined here.

  // Begin the borrow scope for %0. We want to access %1's subfield in a
  // read only way that doesn't involve destructuring and extra copies. So
  // we construct a guaranteed scope here so we can safely use a
  // struct_extract.
  %1 = begin_borrow %0 : $KlassPair

  // Now we perform our struct_extract operation. This operation
  // structurally grabs a value out of a struct without safety relying on
  // the guaranteed ownership of its operand to know that %1 is live at all
  // use points of %2, its result.
  %2 = struct_extract %1 : $KlassPair, #KlassPair.lhs

  // Then grab the state from our left hand side klass and copy it so we
  // can pass off our klass pair to handOff for continued processing.
  %3 = ref_element_addr %2 : $Klass, #Klass.state
  %4 = load [copy] %3 : $*State

  // Now that we have finished accessing %1, we end the borrow scope for %1.
  end_borrow %1 : $KlassPair

  %handOff = function_ref @handOff : $@convention(thin) (@owned KlassPair) -> ()
  apply %handOff(%0) : $@convention(thin) (@owned KlassPair) -> ()

  return %4 : $State
}

  • Given an @none operand, the result value must have @none ownership.

  • Given an @unowned operand, the result value will have @unowned ownership. It will be validated just like any other @unowned value, namely that it must be copied before use.

An additional wrinkle here is that even though the vast majority of forwarding instructions forward all types of ownership, this is not true in general. To see why this is necessary, lets compare/contrast struct_extract (which does not forward @owned ownership) and unchecked_enum_data (which can forward /all/ ownership kinds). The reason for this difference is that struct_extract inherently can only extract out a single field of a larger object implying that the instruction could only represent consuming a sub-field of a value instead of the entire value at once. This violates our constraint that owned values can never be partially consumed: a value is either completely alive or completely dead. In contrast, enums always represent their payloads as elements in a single tuple value. This means that unchecked_enum_data when it extracts that payload from an enum, can consume the entire enum+payload.

To handle cases where we want to use struct_extract in a consuming way, we instead are able to use the destructure_struct instruction that consumes the entire struct at once and gives one back the structs individual constituent parts:

struct KlassPair {
  var fieldOne: Klass
  var fieldTwo: Klass
}

sil @getFirstPairElt : $@convention(thin) (@owned KlassPair) -> @owned Klass {
bb0(%0 : @owned $KlassPair):
  // If we were to just do this directly and consume KlassPair to access
  // fieldOne... what would happen to fieldTwo? Would it be consumed?
  //
  // %1 = struct_extract %0 : $KlassPair, #KlassPair.fieldOne
  //
  // Instead we need to destructure to ensure we consume the entire owned value at once.
  (%1, %2) = destructure_struct $KlassPair

  // We only want to return %1, so we need to cleanup %2.
  destroy_value %2 : $Klass

  // Then return %1 to our caller
  return %1 : $Klass
}

Forwarding Address-Only Values

Address-only values are potentially unmovable when borrowed. This means that they cannot be forwarded with guaranteed ownership unless the forwarded value has the same representation as in the original value and can reuse the same storage. Non-destructive projection is allowed, such as struct_extract. Aggregation, such as struct, and destructive disaggregation, such as switch_enum is not allowed. This is an invariant for OSSA with opaque SIL values for these reasons:

1. To avoid implicit semantic copies. For move-only values, this allows complete diagnostics. And in general, it makes it impossible for SIL passes to "accidentally" create copies.

2. To reuse borrowed storage. This allows the optimizer to share the same storage for multiple exclusive reads of the same variable, avoiding copies. It may also be necessary to support native Swift atomics, which will be unmovable-when-borrowed.

Borrowed Object based Safe Interior Pointers

What is an "Unsafe Interior Pointer"

An unsafe interior pointer is a bare pointer into the innards of an object. A simple example of this in C++ would be using the method std::vector::data() to get to the innards of a std::vector. In general interior pointers are unsafe to use since languages do not provide any guarantees that the interior pointer will not be used after the underlying object has been deallocated. To see this, consider the following C++ example:

int unfortunateFunction() {
  int *unsafeInteriorPointer = nullptr;
  {
    std::vector<int> vector;
    vector.push_back(5);
    unsafeInteriorPointer = vector.data();
    printf("%d\n", *unsafeInteriorPointer); // Prints "5".
  } // vector deallocated here
  return *unsafeInteriorPointer; // Kaboom
}

In words, C++ allows for us to get the interior pointer into the vector, but then lets us do whatever we want with the pointer, including use it after the underlying memory has been invalidated.

From a user's perspective, interior pointers are really useful since one can use it to pass data to other APIs that are only expecting a pointer and also since one can use it to sometimes get better performance. But from a language designer perspective, this sort of API verboten and leads to bugs, crashes, and security vulnerabilities. That being said, clearly users have a need for such functionality, so we, as language designers, should figure out manners to express these sorts of patterns in our various languages in a safe way that prevents user’s from foot-gunning themselves. In SIL, we have solved this problem via the direct modeling of interior pointer instructions as a high level concept in our IR.

Safe Interior Pointers in SIL

In contrast to LLVM-IR, SIL provides mechanisms that language designers can use to express concepts like the above in a manner that allows the language to define away compiler generated unsafe interior pointer usage using "Safe Interior Pointers". This is implemented in SIL by:

  1. Classifying a set of instructions as being "interior pointer" instructions.
  2. Enforcing in the SILVerifier that all "interior pointer" instructions can only have operands with Guaranteed ownership.
  3. Enforcing in the SILVerifier that any transitive address use of the interior pointer to be a liveness requirement of the "interior pointer"'s operand.

Note that the transitive address use verifier from (3) does not attempt to classify uses directly. Instead the verifier:

  1. Has an explicit list of instructions that it understands as requiring liveness of the base object.
  2. Has a second list of instructions that require liveness and produce a address whose transitive uses need to be recursively processed.
  3. Asserts on any instructions that are not known to the verifier. This ensures that the verifier is kept up to date with new instructions.

Note that typically instructions in category (1) are instructions whose uses do not propagate the pointer value, so they are safe. In contrast, some other instructions in category (1) are escaping uses of the address such as pointer_to_address. Those uses are unsafe--the user is responsible for managing unsafe pointer lifetimes and the compiler must not extend those pointer lifetimes.

These rules ensure statically that any uses of the address that are not escaped explicitly by an instruction like pointer_to_address are within the guaranteed pointers scope where the guaranteed value is statically known to be live. As a result, in SIL it is impossible to express such a bug in compiler generated code. As an example, consider the following unsafe interior pointer SIL:

class Klass { var k: KlassField }
struct KlassWrapper { var k: Klass }

// ...

// Today SIL restricts interior pointer instructions to only have operands
// with guaranteed ownership.
%1 = begin_borrow %0 : $Klass

// %2 is an interior pointer into %1. Since %2 is an address, it's uses are
// not treated as uses of underlying borrowed object %1 in the ownership
// system. This is because at the ownership level objects with None
// ownership are not verified and do not have any constraints on how they
// are used from the ownership system.
//
// Instead the ownership verifier gathers up all such uses and treats them
// as uses of the object from which the interior pointer was projected from
// transitively. This means that this is a constraint on the guaranteed
// objects use, not on the trivial values.
%2 = ref_element_addr %1 : $Klass, #Klass.k // %2 is a $*KlassWrapper
%3 = struct_element_addr %2 : $*KlassWrapper, #KlassWrapper.k // %3 is a $*Klass

// So if we end the borrow %1 at this point, invalidating the addresses
// ``%2`` and ``%3``.
end_borrow %1 : $Klass

// We would here be loading from an invalidated address. This would cause a
// verifier error since %3's use here is a regular use that is inferred up
// on %1.
%4 = load [copy] %3 : $*KlassWrapper

// ...

Notice how due to a possible bug in the compiler, we are loading from potentially uninitialized memory %4. This would have caused a verifier error stating that %4 was an interior pointer based use-after-free of %1 implying this is mal-formed SIL.

NOTE: This is a constraint on the base object, not on the addresses themselves which are viewed as outside of the ownership system since they have None ownership.

In contrast to the previous example, the following example follows ownership invariants and is valid SIL:

class Klass { var k: KlassField }
struct KlassWrapper { var k: Klass }

// ...

%1 = begin_borrow %0 : $Klass
// %2 is an interior pointer into the Klass k. Since %2 is an address and
// addresses have None ownership, it's uses are not treated as uses of the
// underlying object %1.
%2 = ref_element_addr %1 : $Klass, #Klass.k // %2 is a $*KlassWrapper

// Destroying %1 at this location would result in a verifier error since
// %2's uses are considered to be uses of %1.
//
// end_lifetime %1 : $Klass

// We are statically not loading from an invalidated address here since we
// are within the lifetime of ``%1``.
%3 = struct_element_addr %2 : $*KlassWrapper, #KlassWrapper.k
%4 = load [copy] %3 : $*Klass // %1 must be live here transitively

// ``%1``'s lifetime ends. Importantly we know that within the lifetime of
// ``%1``, ``%0``'s lifetime can not shrink past this point, implying
// transitive static safety.
end_borrow %1 : $Klass

In the second example, we show a well-formed SIL program showing off SIL's Safe Interior Pointers. All of the uses of %2, the interior pointer, are transitively uses of the base underlying object, %0.

The current list of interior pointer SIL instructions are:

  • project_box - projects a pointer out of a reference counted box. (*)
  • ref_element_addr - projects a field out of a reference counted class.
  • ref_tail_addr - projects out a pointer to a class’s tail allocated array memory (assuming the class was initialized to have such an array).
  • open_existential_box - projects the address of the value out of a boxed existential container using the current function context/protocol conformance to create an "opened archetype".
  • project_existential_box - projects a pointer to the value inside a boxed existential container. Must be the type for which the box was initially allocated for and not for an "opened" archetype.

(*) We still need to finish adding support for project_box, but all other interior pointers are guarded already.

Variable Lifetimes

In order for programmer intended lifetimes to be maintained under optimization, the lifetimes of SIL values which correspond to named source-level values can only be modified in limited ways. Generally, the behavior is that the lifetime of a named source-level value is anchored to the variable's lexical scope and confined by deinit barriers. Specifically, code motion may not move the ends of these lifetimes across a deinit barrier.

Source level variables (lets, vars, ...) and function arguments will result in SIL-level lexical lifetimes if either of the two sets of circumstances apply: (1) Inferred lexicality. - the type is non-trivial - the type is not eager-move - the variable or argument is not annotated to be eager-move OR (2) Explicit lexicality. - the type, variable, or argument is annotated @_lexical

A type is eager-move by satisfying one of two conditions: (1) Inferred: An aggregate is inferred to be eager-move if all of its fields are

eager-move.
  1. Annotated: Any type can be eager-move if it is annotated with an attribute that explicitly specifies it to be: @_eagerMove, @_noImplicitCopy.

A variable or argument is eager-move by satisfying one of two conditions: (1) Inferred: Its type is eager-move. (2) Annotated: The variable or argument is annotated with an attribute that

specifies it to be: @_eagerMove, @_noImplicitCopy.

These source-level rules result in a few sorts of SIL value whose destroys must not be moved across deinit barriers:

1: begin_borrow [lexical] 2: move_value [lexical] 3: function arguments 4: alloc_stack [lexical]

To translate from the source-level representation of lexicality to the SIL-level representation, for source-level variables (vars, lets, ...) SILGen generates begin_borrow [lexical], move_value [lexical], alloc_stack [lexical] . For function arguments, there is no work to do: a SILFunctionArgument itself can be lexical (SILFunctionArgument::isLexical).

That the first three have constrained lifetimes is encoded in ValueBase::isLexical, which should be checked before changing the lifetime of a value.

When a function is inlined into its caller, a lexical borrow scope is added for each of its @guaranteed arguments, and a lexical move is added for each of its @owned arguments, (unless the values being passed are already lexical themselves) ensuring that the lifetimes of the corresponding source-level values are not shortened in a way that doesn't respect deinit barriers.

Unlike the other sorts, alloc_stack [lexical] isn't a SILValue. Instead, it constrains the lifetime of an addressable variable. Since the constraint is applied to the in-memory representation, no additional lexical SILValue is required.

Deinit Barriers

Deinit barriers (see Instruction.isDeinitBarrier(_:)) are instructions which would be affected by the side effects of deinitializers. To maintain the order of effects that is visible to the programmer, destroys of lexical values cannot be reordered with respect to them. There are three kinds:

  1. synchronization points (locks, memory barriers, syscalls, etc.)
  2. loads of weak or unowned values
  3. accesses of pointers

Examples:

  1. Given an instance of a class which owns a file handle and closes the file handle on deinit, writing to the file handle and then deallocating the instance would result in changes being written. If the destroy of the instance were hoisted above the call to write to the file handle, an error would be raised instead.
  2. Given an instance c of a class C which weakly references an instance d of a second class D, if d is referenced via a local variable v, then loading that weak reference from c within the variable scope should return a non-nil reference to d. Hoisting the destroy of v above the weak load from c, however, would result in the destruction of d before that load and a nil weak reference to D.
  3. Given an instance of a class which owns a buffer and deallocates it on deinitialization, accessing the pointer and then deallocating the instance is defined behavior. Hoisting the destroy of the instance above the access to the memory would result in accessing a freed pointer.

Memory Lifetime

Similar to Ownership SSA, there are also lifetime rules for values in memory. With "memory" we refer to memory which is addressed by SIL instruction with address-type operands, like load, store, switch_enum_addr, etc.

Each memory location which holds a non-trivial value is either uninitialized or initialized. A memory location gets initialized by storing values into it (except assignment, which expects a location to be already initialized). A memory location gets de-initialized by "taking" from it or destroying it, e.g. with destroy_addr. It is illegal to re-initialize a memory location or to use a location after it was de-initialized.

If a memory location holds a trivial value (e.g. an Int), it is not required to de-initialize the location.

The SIL verifier checks this rule for memory locations which can be uniquely identified, for example and alloc_stack or an indirect parameter. The verifier cannot check memory locations which are potentially aliased, e.g. a ref_element_addr (a stored class property).

Lifetime of Enums in Memory

The situation is a bit more complicated with enums, because an enum can have both, cases with non-trivial payloads and cases with no payload or trivial payloads.

Even if an enum itself is not trivial (because it has at least on case with a non-trivial payload), it is not required to de-initialize such an enum memory location on paths where it's statically provable that the enum contains a trivial or non-payload case.

That's the case if the destroy point is jointly dominated by:

  • a store [trivial] to the enum memory location.

or

  • an inject_enum_addr to the enum memory location with a non-trivial or non-payload case.

or

  • a successor of a switch_enum or switch_enum_addr for a non-trivial or non-payload case.

Dead End Blocks

In SIL, one can express that a program is semantically expected to exit at the end of a block by terminating the block with an unreachable. Such a block is called a program terminating block and all blocks that are post-dominated by blocks of the aforementioned kind are called dead end blocks. Intuitively, any path through a dead end block is known to result in program termination, so resources that normally would need to be released back to the system will instead be returned to the system by process tear down.

Since we rely on the system at these points to perform resource cleanup, we are able to loosen our lifetime requirements by allowing for values to not have their lifetimes ended along paths that end in program terminating blocks. Operationally, this implies that:

  • All SIL values must have exactly one lifetime ending use on all paths that terminate in a return or throw. In contrast, a SIL value does not need to have a lifetime ending use along paths that end in an unreachable.
  • end_borrow and destroy_value are redundant, albeit legal, in blocks where all paths through the block end in an unreachable.

Consider the following legal SIL where we leak %0 in blocks prefixed with bbDeadEndBlock and consume it in bb2:

sil @user : $@convention(thin) (@owned Klass) -> @owned Klass {
bb0(%0 : @owned $Klass):
  cond_br ..., bb1, bb2

bb1:
  // This is a dead end block since it is post-dominated by two dead end
  // blocks. It is not a program terminating block though since the program
  // does not end in this block.
  cond_br ..., bbDeadEndBlock1, bbDeadEndBlock2

bbDeadEndBlock1:
  // This is a dead end block and a program terminating block.
  //
  // We are exiting the program here causing the operating system to clean up
  // all resources associated with our process, so there is no need for a
  // destroy_value. That memory will be cleaned up anyways.
  unreachable

bbDeadEndBlock2:
  // This is a dead end block and a program terminating block.
  //
  // Even though we do not need to insert destroy_value along these paths, we
  // can if we want to. It is just necessary and the optimizer can eliminate
  // such a destroy_value if it wishes.
  //
  // NOTE: The author arbitrarily chose just to destroy %0: we could legally
  // destroy either value (or both!).
  destroy_value %0 : $Klass
  unreachable

bb2:
  cond_br ..., bb3, bb4

bb3:
  // This block is live, so we need to ensure that %0 is consumed within the
  // block. In this case, %0 is consumed by returning %0 to our caller.
  return %0 : $Klass

bb4:
  // This block is also live, but since we do not return %0, we must insert a
  // destroy_value to cleanup %0.
  //
  // NOTE: The copy_value/destroy_value here is redundant and can be removed by
  // the optimizer. The author left it in for illustrative purposes.
  %1 = copy_value %0 : $Klass
  destroy_value %0 : $Klass
  return %1 : $Klass
}

Move Only Wrapped Types

Semantics

Today SIL supports values of move only wrapped type. Given a copyable type $T, the type $@moveOnly T is the move only wrapped variant of the type. This type is always non-trivial even if $T itself is trivial (see discussion below). All move only wrapped types obey the invariant that they can be copied in Raw SIL but cannot be copied in Canonical SIL and later SIL stages. This ensures that once we are in Canonical SIL such values are "move only values" that are guaranteed to never be copied. This is enforced by:

  • Having SILGen emit copies as it normally does.
  • Use OSSA canonicalization to eliminate copies that aren't needed semantically due to consuming uses of the value. This is implemented by the guaranteed passes "MoveOnlyObjectChecker" and "MoveOnlyAddressChecker". We will emit errors on any of the consuming uses that we found in said pass.

Assuming that no errors are emitted, we can then conclude before we reach canonical SIL that the value was never copied and thus is a "move only value" even though the actual underlying wrapped type is copyable. As an example of this, consider the following Swift:

func doSomething(@_noImplicitCopy _ x: Klass) -> () { // expected-error {{'x' is borrowed and cannot be consumed}}
  x.doSomething()
  let x2 = x // expected-note {{consuming use}}
  x2.doSomething()
}

which codegens to the following SIL:

sil hidden [ossa] @doSomething : $@convention(thin) (@guaranteed Klass) -> () {
bb0(%0 : @noImplicitCopy $Klass):
  %1 = copyable_to_moveonlywrapper [guaranteed] %0 : $@moveOnly Klass
  %2 = copy_value %1 : $@moveOnly Klass
  %3 = mark_must_check [no_consume_or_assign] %2 : $@moveOnly Klass
  debug_value %3 : $@moveOnly Klass, let, name "x", argno 1
  %4 = begin_borrow %3 : $@moveOnly Klass
  %5 = function_ref @$s4test5KlassC11doSomethingyyF : $@convention(method) (@guaranteed Klass) -> ()
  %6 = moveonlywrapper_to_copyable [guaranteed] %4 : $@moveOnly Klass
  %7 = apply %5(%6) : $@convention(method) (@guaranteed Klass) -> ()
  end_borrow %4 : $@moveOnly Klass
  %9 = begin_borrow %3 : $@moveOnly Klass
  %10 = copy_value %9 : $@moveOnly Klass
  %11 = moveonlywrapper_to_copyable [owned] %10 : $@moveOnly Klass
  %12 = begin_borrow [lexical] %11 : $Klass
  debug_value %12 : $Klass, let, name "x2"
  end_borrow %9 : $@moveOnly Klass
  %15 = function_ref @$s4test5KlassC11doSomethingyyF : $@convention(method) (@guaranteed Klass) -> ()
  %16 = apply %15(%12) : $@convention(method) (@guaranteed Klass) -> ()
  end_borrow %12 : $Klass
  destroy_value %11 : $Klass
  destroy_value %3 : $@moveOnly Klass
  %20 = tuple ()
  return %20 : $()
} // end sil function '$s4test11doSomethingyyAA5KlassCF'

When the move only checker runs upon this SIL, it will see that the moveonlywrapped_to_copyable [owned] is a transitive consuming use of %2 (ignoring copies) and that we have a non-consuming use later. So it will emit an error that we have a guaranteed parameter that is being consumed. If we remove the assignment to x2, then after the move checker runs successfully we would get the following SIL:

sil hidden [ossa] @doSomething : $@convention(thin) (@guaranteed Klass) -> () {
bb0(%0 : @noImplicitCopy $Klass):
  %1 = copyable_to_moveonlywrapper [guaranteed] %0 : $Klass
  debug_value %1 : $@moveOnly Klass, let, name "x", argno 1
  %4 = begin_borrow %1 : $@moveOnly Klass
  %5 = function_ref @$s4test5KlassC11doSomethingyyF : $@convention(method) (@guaranteed Klass) -> ()
  %6 = moveonlywrapper_to_copyable [guaranteed] %4 : $@moveOnly Klass
  %7 = apply %5(%6) : $@convention(method) (@guaranteed Klass) -> ()
  end_borrow %4 : $@moveOnly Klass
  %20 = tuple ()
  return %20 : $()
} // end sil function '$s4test11doSomethingyyAA5KlassCF'

yielding SIL without any copies just as we wanted.

At the ABI level, @moveOnly does not exist and thus never shows up in a SILFunctionType's parameters. Instead, we represent it only in the body of functions and on a function's internal SILArgument representation. This is since move only wrapped is intended to be a local power tool for controlling lifetimes rather than a viral type level annotation that would constrain the type system.

As mentioned above trivial move only wrapped types are actually non-trivial. This is because in SIL ownership is tied directly to non-trivialness so unless we did that we could not track ownership accurately. This loss of triviality is not an issue for most of the pipeline since we eliminate all move only wrapper types for trivial types during the guaranteed optimizations after we have run various ownership checkers but before we have run diagnostics for trivial types (e.x.: DiagnosticConstantPropagation).

As an example in practice, consider the following Swift:

func doSomethingWithInt(@_noImplicitCopy _ x: Int) -> Int {
  x + x
}

Today this codegens to the following Swift:

sil hidden [ossa] @doSomethingWithInt : $@convention(thin) (Int) -> Int {
bb0(%0 : @noImplicitCopy $Int):
  %1 = copyable_to_moveonlywrapper [owned] %0 : $Int
  %2 = move_value [lexical] %1 : $@moveOnly Int
  %3 = mark_must_check [consumable_and_assignable] %2 : $@moveOnly Int
  %5 = begin_borrow %3 : $@moveOnly Int
  %6 = begin_borrow %3 : $@moveOnly Int
  %7 = function_ref @addIntegers : $@convention(method) (Int, Int Int.Type) -> Int
  %8 = moveonlywrapper_to_copyable [guaranteed] %5 : $@moveOnly Int
  %9 = moveonlywrapper_to_copyable [guaranteed] %6 : $@moveOnly Int
  %10 = apply %7(%8, %9) : $@convention(method) (Int, Int) -> Int
  end_borrow %6 : $@moveOnly Int
  end_borrow %5 : $@moveOnly Int
  destroy_value %3 : $@moveOnly Int
  return %10 : $Int
}

once the checker has run, this becomes:

sil hidden [ossa] @doSomethingWithInt : $@convention(thin) (Int) -> Int {
bb0(%0 : @noImplicitCopy $Int):
  %1 = copyable_to_moveonlywrapper [owned] %0 : $Int
  %2 = move_value [lexical] %1 : $@moveOnly Int
  %5 = begin_borrow %2 : $@moveOnly Int
  %6 = begin_borrow %2 : $@moveOnly Int
  %7 = function_ref @addIntegers : $@convention(method) (Int, Int Int.Type) -> Int
  %8 = moveonlywrapper_to_copyable [guaranteed] %5 : $@moveOnly Int
  %9 = moveonlywrapper_to_copyable [guaranteed] %6 : $@moveOnly Int
  %10 = apply %7(%8, %9) : $@convention(method) (Int, Int) -> Int
  end_borrow %6 : $@moveOnly Int
  end_borrow %5 : $@moveOnly Int
  return %10 : $Int
}

and once we have run the move only wrapped type lowerer, we get the following SIL:

sil hidden [ossa] @doSomethingWithInt : $@convention(thin) (Int) -> Int {
bb0(%0 : @noImplicitCopy $Int):
  %1 = function_ref @addIntegers : $@convention(method) (Int, Int) -> Int
  %2 = apply %1(%0, %0) : $@convention(method) (Int, Int) -> Int
  return %2 : $Int
}

exactly what we wanted in the end.

If we are given an owned argument or a let binding, we use a similar approach. Consider the following Swift:

func doSomethingWithKlass(_ x: Klass) -> Klass {
  @_noImplicitCopy let value = x
  let value2 = value
  return value
}

A hypothetical SILGen for this code is as follows:

sil hidden [ossa] @$s4test20doSomethingWithKlassyAA0E0CADF : $@convention(thin) (@guaranteed Klass) -> @owned Klass {
bb0(%0 : @guaranteed $Klass):
  debug_value %0 : $Klass, let, name "x", argno 1
  %3 = begin_borrow [lexical] %0 : $Klass
  %4 = copy_value %3 : $Klass
  %5 = copyable_to_moveonlywrapper [owned] %4 : $Klass
  %6 = mark_must_check [consumable_and_assignable] %5 : $@moveOnly Klass
  debug_value %6 : $@moveOnly Klass, let, name "value"
  %8 = begin_borrow %6 : $@moveOnly Klass
  %9 = copy_value %8 : $@moveOnly Klass
  %10 = moveonlywrapper_to_copyable [owned] %9 : $@moveOnly Klass
  %11 = begin_borrow [lexical] %10 : $Klass
  debug_value %11 : $Klass, let, name "value2"
  end_borrow %8 : $@moveOnly Klass
  %14 = begin_borrow %6 : $@moveOnly Klass
  %15 = copy_value %14 : $@moveOnly Klass
  end_borrow %14 : $@moveOnly Klass
  end_borrow %11 : $Klass
  destroy_value %10 : $Klass
  destroy_value %6 : $@moveOnly Klass
  end_borrow %3 : $Klass
  %22 = moveonlywrapper_to_copyable [owned] %15 : $@moveOnly Klass
  return %22 : $Klass
} // end sil function '$s4test20doSomethingWithKlassyAA0E0CADF'

Notice above how the moveonlywrapper_to_copyable [owned] is used to escape the no implicit copy value. In fact, if one runs the following SILGen through sil-opt, one will see that we actually have an ownership violation due to the two uses of "value", one for initializing value2 and the other for the return value.

Move Only Types

NOTE: This is experimental and is just an attempt to describe where the design is currently for others reading SIL today. It should not be interpreted as final.

Currently there are two kinds of "move only types" in SIL: pure move only types that are always move only and move only wrapped types that are move only versions of copyable types. The invariant that values of Move Only type obey is that they can only be copied (e.x.: operand to a copy_value, copy_addr [init]) during the guaranteed passes when we are in Raw SIL. Once we are in non-Raw SIL though (i.e. Canonical and later SIL stages), a program is ill formed if one copies a move only type.

The reason why we have this special rule for move only types is that this allows for SIL code generators to insert copies and then have a later guaranteed checker optimization pass recover the underlying move only semantics by reconstructing needed copies and removing unneeded copies using Ownership SSA. If any such copies are actually needed according to Ownership SSA, the checker pass emits a diagnostic stating that move semantics have been violated. If such a diagnostic is emitted then the checker pass transforms all copies on move only types to their explicit copy forms to ensure that once we leave the diagnostic passes and enter canonical SIL, our "copy" invariant is maintained.

Runtime Failure

Some operations, such as failed unconditional checked conversions or the Builtin.trap compiler builtin, cause a runtime failure, which unconditionally terminates the current actor. If it can be proven that a runtime failure will occur or did occur, runtime failures may be reordered so long as they remain well-ordered relative to operations external to the actor or the program as a whole. For instance, with overflow checking on integer arithmetic enabled, a simple for loop that reads inputs in from one or more arrays and writes outputs to another array, all local to the current actor, may cause runtime failure in the update operations:

// Given unknown start and end values, this loop may overflow
for var i = unknownStartValue; i != unknownEndValue; ++i {
  ...
}

It is permitted to hoist the overflow check and associated runtime failure out of the loop itself and check the bounds of the loop prior to entering it, so long as the loop body has no observable effect outside of the current actor.

Undefined Behavior

Incorrect use of some operations is undefined behavior, such as invalid unchecked casts involving Builtin.RawPointer types, or use of compiler builtins that lower to LLVM instructions with undefined behavior at the LLVM level. A SIL program with undefined behavior is meaningless, much like undefined behavior in C, and has no predictable semantics. Undefined behavior should not be triggered by valid SIL emitted by a correct Swift program using a correct standard library, but cannot in all cases be diagnosed or verified at the SIL level.

Calling Convention

This section describes how Swift functions are emitted in SIL.

Swift Calling Convention @convention(swift)

The Swift calling convention is the one used by default for native Swift functions.

Tuples in the input type of the function are recursively destructured into separate arguments, both in the entry point basic block of the callee, and in the apply instructions used by callers:

func foo(_ x:Int, y:Int)

sil @foo : $(x:Int, y:Int) -> () {
entry(%x : $Int, %y : $Int):
  ...
}

func bar(_ x:Int, y:(Int, Int))

sil @bar : $(x:Int, y:(Int, Int)) -> () {
entry(%x : $Int, %y0 : $Int, %y1 : $Int):
  ...
}

func call_foo_and_bar() {
  foo(1, 2)
  bar(4, (5, 6))
}

sil @call_foo_and_bar : $() -> () {
entry:
  ...
  %foo = function_ref @foo : $(x:Int, y:Int) -> ()
  %foo_result = apply %foo(%1, %2) : $(x:Int, y:Int) -> ()
  ...
  %bar = function_ref @bar : $(x:Int, y:(Int, Int)) -> ()
  %bar_result = apply %bar(%4, %5, %6) : $(x:Int, y:(Int, Int)) -> ()
}

Calling a function with trivial value types as inputs and outputs simply passes the arguments by value. This Swift function:

func foo(_ x:Int, y:Float) -> UnicodeScalar

foo(x, y)

gets called in SIL as:

%foo = constant_ref $(Int, Float) -> UnicodeScalar, @foo
%z = apply %foo(%x, %y) : $(Int, Float) -> UnicodeScalar

Reference Counts

NOTE This section only is speaking in terms of rules of thumb. The actual behavior of arguments with respect to arguments is defined by the argument's convention attribute (e.g. @owned), not the calling convention itself.

Reference type arguments are passed in at +1 retain count and consumed by the callee. A reference type return value is returned at +1 and consumed by the caller. Value types with reference type components have their reference type components each retained and released the same way. This Swift function:

class A {}

func bar(_ x:A) -> (Int, A) { ... }

bar(x)

gets called in SIL as:

%bar = function_ref @bar : $(A) -> (Int, A)
strong_retain %x : $A
%z = apply %bar(%x) : $(A) -> (Int, A)
// ... use %z ...
%z_1 = tuple_extract %z : $(Int, A), 1
strong_release %z_1

When applying a thick function value as a callee, the function value is also consumed at +1 retain count.

Address-Only Types

For address-only arguments, the caller allocates a copy and passes the address of the copy to the callee. The callee takes ownership of the copy and is responsible for destroying or consuming the value, though the caller must still deallocate the memory. For address-only return values, the caller allocates an uninitialized buffer and passes its address as the first argument to the callee. The callee must initialize this buffer before returning. This Swift function:

 @API struct A {}

func bas(_ x:A, y:Int) -> A { return x }

var z = bas(x, y)
// ... use z ...

gets called in SIL as:

%bas = function_ref @bas : $(A, Int) -> A
%z = alloc_stack $A
%x_arg = alloc_stack $A
copy_addr %x to [initialize] %x_arg : $*A
apply %bas(%z, %x_arg, %y) : $(A, Int) -> A
dealloc_stack %x_arg : $*A // callee consumes %x.arg, caller deallocs
// ... use %z ...
destroy_addr %z : $*A
dealloc_stack stack %z : $*A

The implementation of @bas is then responsible for consuming %x_arg and initializing %z.

Tuple arguments are destructured regardless of the address-only-ness of the tuple type. The destructured fields are passed individually according to the above convention. This Swift function:

@API struct A {}

func zim(_ x:Int, y:A, (z:Int, w:(A, Int)))

zim(x, y, (z, w))

gets called in SIL as:

%zim = function_ref @zim : $(x:Int, y:A, (z:Int, w:(A, Int))) -> ()
%y_arg = alloc_stack $A
copy_addr %y to [initialize] %y_arg : $*A
%w_0_addr = element_addr %w : $*(A, Int), 0
%w_0_arg = alloc_stack $A
copy_addr %w_0_addr to [initialize] %w_0_arg : $*A
%w_1_addr = element_addr %w : $*(A, Int), 1
%w_1 = load %w_1_addr : $*Int
apply %zim(%x, %y_arg, %z, %w_0_arg, %w_1) : $(x:Int, y:A, (z:Int, w:(A, Int))) -> ()
dealloc_stack %w_0_arg
dealloc_stack %y_arg

Variadic Arguments

Variadic arguments and tuple elements are packaged into an array and passed as a single array argument. This Swift function:

func zang(_ x:Int, (y:Int, z:Int...), v:Int, w:Int...)

zang(x, (y, z0, z1), v, w0, w1, w2)

gets called in SIL as:

%zang = function_ref @zang : $(x:Int, (y:Int, z:Int...), v:Int, w:Int...) -> ()
%zs = <<make array from %z1, %z2>>
%ws = <<make array from %w0, %w1, %w2>>
apply %zang(%x, %y, %zs, %v, %ws)  : $(x:Int, (y:Int, z:Int...), v:Int, w:Int...) -> ()

@inout Arguments

@inout arguments are passed into the entry point by address. The callee does not take ownership of the referenced memory. The referenced memory must be initialized upon function entry and exit. If the @inout argument refers to a fragile physical variable, then the argument is the address of that variable. If the @inout argument refers to a logical property, then the argument is the address of a caller-owned writeback buffer. It is the caller's responsibility to initialize the buffer by storing the result of the property getter prior to calling the function and to write back to the property on return by loading from the buffer and invoking the setter with the final value. This Swift function:

func inout(_ x: inout Int) {
  x = 1
}

gets lowered to SIL as:

sil @inout : $(@inout Int) -> () {
entry(%x : $*Int):
  %1 = integer_literal $Int, 1
  store %1 to %x
  return
}

Swift Method Calling Convention @convention(method)

The method calling convention is currently identical to the freestanding function convention. Methods are considered to be curried functions, taking the "self" argument as their outer argument clause, and the method arguments as the inner argument clause(s). The "self" argument is thus passed last:

struct Foo {
  func method(_ x:Int) -> Int {}
}

sil @Foo_method_1 : $((x : Int), @inout Foo) -> Int { ... }

Witness Method Calling Convention @convention(witness_method)

The witness method calling convention is used by protocol witness methods in witness tables. It is identical to the method calling convention except that its handling of generic type parameters. For non-witness methods, the machine-level convention for passing type parameter metadata may be arbitrarily dependent on static aspects of the function signature, but because witnesses must be polymorphically dispatchable on their Self type, the Self-related metadata for a witness must be passed in a maximally abstracted manner.

C Calling Convention @convention(c)

In Swift's C module importer, C types are always mapped to Swift types considered trivial by SIL. SIL does not concern itself with platform ABI requirements for indirect return, register vs. stack passing, etc.; C function arguments and returns in SIL are always by value regardless of the platform calling convention.

SIL (and therefore Swift) cannot currently invoke variadic C functions.

Objective-C Calling Convention @convention(objc_method)

Reference Counts

Objective-C methods use the same argument and return value ownership rules as ARC Objective-C. Selector families and the ns_consumed, ns_returns_retained, etc. attributes from imported Objective-C definitions are honored.

Applying a @convention(block) value does not consume the block.

Method Currying

In SIL, the "self" argument of an Objective-C method is uncurried to the last argument of the uncurried type, just like a native Swift method:

@objc class NSString {
  func stringByPaddingToLength(Int) withString(NSString) startingAtIndex(Int)
}

sil @NSString_stringByPaddingToLength_withString_startingAtIndex \
  : $((Int, NSString, Int), NSString)

That self is passed as the first argument at the IR level is abstracted away in SIL, as is the existence of the _cmd selector argument.

Type Based Alias Analysis

SIL supports two types of Type Based Alias Analysis (TBAA): Class TBAA and Typed Access TBAA.

Class TBAA

Class instances and other heap object references are pointers at the implementation level, but unlike SIL addresses, they are first class values and can be capture-d and aliased. Swift, however, is memory-safe and statically typed, so aliasing of classes is constrained by the type system as follows:

  • A Builtin.NativeObject may alias any native Swift heap object, including a Swift class instance, a box allocated by alloc_box, or a thick function's closure context. It may not alias natively Objective-C class instances.
  • An AnyObject or Builtin.BridgeObject may alias any class instance, whether Swift or Objective-C, but may not alias non-class-instance heap objects.
  • Two values of the same class type $C may alias. Two values of related class type $B and $D, where there is a subclass relationship between $B and $D, may alias. Two values of unrelated class types may not alias. This includes different instantiations of a generic class type, such as $C<Int> and $C<Float>, which currently may never alias.
  • Without whole-program visibility, values of archetype or protocol type must be assumed to potentially alias any class instance. Even if it is locally apparent that a class does not conform to that protocol, another component may introduce a conformance by an extension. Similarly, a generic class instance, such as $C<T> for archetype T, must be assumed to potentially alias concrete instances of the generic type, such as $C<Int>, because Int is a potential substitution for T.

A violation of the above aliasing rules only results in undefined behavior if the aliasing references are dereferenced within Swift code. For example, __SwiftNativeNS[Array|Dictionary|String] classes alias with NS[Array|Dictionary|String] classes even though they are not statically related. Since Swift never directly accesses stored properties on the Foundation classes, this aliasing does not pose a danger.

Typed Access TBAA

Define a typed access of an address or reference as one of the following:

  • Any instruction that performs a typed read or write operation upon the memory at the given location (e.x. load, store).
  • Any instruction that yields a typed offset of the pointer by performing a typed projection operation (e.x. ref_element_addr, tuple_element_addr).

With limited exceptions, it is undefined behavior to perform a typed access to an address or reference addressed memory is not bound to the relevant type.

This allows the optimizer to assume that two addresses cannot alias if there does not exist a substitution of archetypes that could cause one of the types to be the type of a subobject of the other. Additionally, this applies to the types of the values from which the addresses were derived via a typed projection.

Consider the following SIL:

struct Element {
  var i: Int
}
struct S1 {
  var elt: Element
}
struct S2 {
  var elt: Element
}
%adr1 = struct_element_addr %ptr1 : $*S1, #S.elt
%adr2 = struct_element_addr %ptr2 : $*S2, #S.elt

The optimizer may assume that %adr1 does not alias with %adr2 because the values that the addresses are derived from (%ptr1 and %ptr2) have unrelated types. However, in the following example, the optimizer cannot assume that %adr1 does not alias with %adr2 because %adr2 is derived from a cast, and any subsequent typed operations on the address will refer to the common Element type:

%adr1 = struct_element_addr %ptr1 : $*S1, #S.elt
%adr2 = pointer_to_address %ptr2 : $Builtin.RawPointer to $*Element

Exceptions to typed access TBAA rules are only allowed for blessed alias-introducing operations. This permits limited type-punning. The only current exception is the non-struct pointer_to_address variant. The optimizer must be able to defensively determine that none of the roots of an address are alias-introducing operations. An address root is the operation that produces the address prior to applying any typed projections, indexing, or casts. The following are valid address roots:

  • Object allocation that generates an address, such as alloc_stack and alloc_box.
  • Address-type function arguments. These are crucially not considered alias-introducing operations. It is illegal for the SIL optimizer to form a new function argument from an arbitrary address-type value. Doing so would require the optimizer to guarantee that the new argument is both has a non-alias-introducing address root and can be properly represented by the calling convention (address types do not have a fixed representation).
  • A strict cast from an untyped pointer, pointer_to_address [strict]. It is illegal for pointer_to_address [strict] to derive its address from an alias-introducing operation's value. A type punned address may only be produced from an opaque pointer via a non-strict pointer_to_address at the point of conversion.

Address-to-address casts, via unchecked_addr_cast, transparently forward their source's address root, just like typed projections.

Address-type basic block arguments can be conservatively considered aliasing-introducing operations; they are uncommon enough not to matter and may eventually be prohibited altogether.

Although some pointer producing intrinsics exist, they do not need to be considered alias-introducing exceptions to TBAA rules. Builtin.inttoptr produces a Builtin.RawPointer which is not interesting because by definition it may alias with everything. Similarly, the LLVM builtins Builtin.bitcast and Builtin.trunc|sext|zextBitCast cannot produce typed pointers. These pointer values must be converted to an address via pointer_to_address before typed access can occur. Whether the pointer_to_address is strict determines whether aliasing may occur.

Memory may be rebound to an unrelated type. Addresses to unrelated types may alias as long as typed access only occurs while memory is bound to the relevant type. Consequently, the optimizer cannot outright assume that addresses accessed as unrelated types are nonaliasing. For example, pointer comparison cannot be eliminated simply because the two addresses derived from those pointers are accessed as unrelated types at different program points.

Value Dependence

In general, analyses can assume that independent values are independently assured of validity. For example, a class method may return a class reference:

bb0(%0 : $MyClass):
  %1 = class_method %0 : $MyClass, #MyClass.foo
  %2 = apply %1(%0) : $@convention(method) (@guaranteed MyClass) -> @owned MyOtherClass
  // use of %2 goes here; no use of %1
  strong_release %2 : $MyOtherClass
  strong_release %1 : $MyClass

The optimizer is free to move the release of %1 to immediately after the call here, because %2 can be assumed to be an independently-managed value, and because Swift generally permits the reordering of destructors.

However, some instructions do create values that are intrinsically dependent on their operands. For example, the result of ref_element_addr will become a dangling pointer if the base is released too soon. This is captured by the concept of value dependence, and any transformation which can reorder of destruction of a value around another operation must remain conscious of it.

A value %1 is said to be value-dependent on a value %0 if:

  • %1 is the result and %0 is the first operand of one of the following instructions:
    • ref_element_addr
    • struct_element_addr
    • tuple_element_addr
    • unchecked_take_enum_data_addr
    • pointer_to_address
    • address_to_pointer
    • index_addr
    • index_raw_pointer
    • possibly some other conversions
  • %1 is the result of mark_dependence and %0 is either of the operands.
  • %1 is the value address of a box allocation instruction of which %0 is the box reference.
  • %1 is the result of a struct, tuple, or enum instruction and %0 is an operand.
  • %1 is the result of projecting out a subobject of %0 with tuple_extract, struct_extract, unchecked_enum_data, select_enum, or select_enum_addr.
  • %1 is the result of select_value and %0 is one of the cases.
  • %1 is a basic block parameter and %0 is the corresponding argument from a branch to that block.
  • %1 is the result of a load from %0. However, the value dependence is cut after the first attempt to manage the value of %1, e.g. by retaining it.
  • Transitivity: there exists a value %2 which %1 depends on and which depends on %0. However, transitivity does not apply to different subobjects of a struct, tuple, or enum.

Note, however, that an analysis is not required to track dependence through memory. Nor is it required to consider the possibility of dependence being established "behind the scenes" by opaque code, such as by a method returning an unsafe pointer to a class property. The dependence is required to be locally obvious in a function's SIL instructions. Precautions must be taken against this either by SIL generators (by using mark_dependence appropriately) or by the user (by using the appropriate intrinsics and attributes with unsafe language or library features).

Only certain types of SIL value can carry value-dependence:

  • SIL address types
  • unmanaged pointer types:
    • @sil_unmanaged types
    • Builtin.RawPointer
    • aggregates containing such a type, such as UnsafePointer, possibly recursively
  • non-trivial types (but they can be independently managed)

This rule means that casting a pointer to an integer type breaks value-dependence. This restriction is necessary so that reading an Int from a class doesn't force the class to be kept around! A class holding an unsafe reference to an object must use some sort of unmanaged pointer type to do so.

This rule does not include generic or resilient value types which might contain unmanaged pointer types. Analyses are free to assume that e.g. a copy_addr of a generic or resilient value type yields an independently-managed value. The extension of value dependence to types containing obvious unmanaged pointer types is an affordance to make the use of such types more convenient; it does not shift the ultimate responsibility for assuring the safety of unsafe language/library features away from the user.

Copy-on-Write Representation

Copy-on-Write (COW) data structures are implemented by a reference to an object which is copied on mutation in case it's not uniquely referenced.

A COW mutation sequence in SIL typically looks like:

  (%uniq, %buffer) = begin_cow_mutation %immutable_buffer : $BufferClass
  cond_br %uniq, bb_uniq, bb_not_unique
bb_uniq:
  br bb_mutate(%buffer : $BufferClass)
bb_not_unique:
  %copied_buffer = apply %copy_buffer_function(%buffer) : ...
  br bb_mutate(%copied_buffer : $BufferClass)
bb_mutate(%mutable_buffer : $BufferClass):
  %field = ref_element_addr %mutable_buffer : $BufferClass, #BufferClass.Field
  store %value to %field : $ValueType
  %new_immutable_buffer = end_cow_mutation %buffer : $BufferClass

Loading from a COW data structure looks like:

%field1 = ref_element_addr [immutable] %immutable_buffer : $BufferClass, #BufferClass.Field
%value1 = load %field1 : $*FieldType
...
%field2 = ref_element_addr [immutable] %immutable_buffer : $BufferClass, #BufferClass.Field
%value2 = load %field2 : $*FieldType

The immutable attribute means that loading values from ref_element_addr and ref_tail_addr instructions, which have the same operand, are equivalent. In other words, it's guaranteed that a buffer's properties are not mutated between two ref_element/tail_addr [immutable] as long as they have the same buffer reference as operand. This is even true if e.g. the buffer 'escapes' to an unknown function.

In the example above, %value2 is equal to %value1 because the operand of both ref_element_addr instructions is the same %immutable_buffer. Conceptually, the content of a COW buffer object can be seen as part of the same static (immutable) SSA value as the buffer reference.

The lifetime of a COW value is strictly separated into mutable and immutable regions by begin_cow_mutation and end_cow_mutation instructions:

%b1 = alloc_ref $BufferClass
// The buffer %b1 is mutable
%b2 = end_cow_mutation %b1 : $BufferClass
// The buffer %b2 is immutable
(%u1, %b3) = begin_cow_mutation %b1 : $BufferClass
// The buffer %b3 is mutable
%b4 = end_cow_mutation %b3 : $BufferClass
// The buffer %b4 is immutable
...

Both, begin_cow_mutation and end_cow_mutation, consume their operand and return the new buffer as an owned value. The begin_cow_mutation will compile down to a uniqueness check and end_cow_mutation will compile to a no-op.

Although the physical pointer value of the returned buffer reference is the same as the operand, it's important to generate a new buffer reference in SIL. It prevents the optimizer from moving buffer accesses from a mutable into a immutable region and vice versa.

Because the buffer content is conceptually part of the buffer reference SSA value, there must be a new buffer reference every time the buffer content is mutated.

To illustrate this, let's look at an example, where a COW value is mutated in a loop. As with a scalar SSA value, also mutating a COW buffer will enforce a phi-argument in the loop header block (for simplicity the code for copying a non-unique buffer is not shown):

header_block(%b_phi : $BufferClass):
  (%u, %b_mutate) = begin_cow_mutation %b_phi : $BufferClass
  // Store something to %b_mutate
  %b_immutable = end_cow_mutation %b_mutate : $BufferClass
  cond_br %loop_cond, exit_block, backedge_block
backedge_block:
  br header_block(b_immutable : $BufferClass)
exit_block:

Two adjacent begin_cow_mutation and end_cow_mutation instructions don't need to be in the same function.

Stack discipline

Certain instructions in the SIL instruction set are identified as stack allocation instructions or stack deallocation instructions. These instructions jointly participate in a set of rules called the stack allocation discipline, designed to allow SIL functions to easily and safely dynamically allocate and deallocate memory in a scoped fashion on the stack.

All stack deallocation instructions have an operand which identifies their paired stack allocation instruction. This operand must always be exactly the result of a stack allocation instruction, with no intervening conversions, basic block arguments, or other abstractions. A single stack allocation instruction may be paired with any number of stack deallocation instructions. It can even be paired with no instructions at all; by the rules below, this can only happen in non-terminating functions.

  • At any point in a SIL function, there is an ordered list of stack allocation instructions called the active allocations list.

  • The active allocations list is defined to be empty at the initial point of the entry block of the function.

  • The active allocations list is required to be the same at the initial point of any successor block as it is at the final point of any predecessor block. Note that this also requires all predecessors/successors of a given block to have the same final/initial active allocations lists.

    In other words, the set of active stack allocations must be the same at a given place in the function no matter how it was reached.

  • The active allocations list for the point following a stack allocation instruction is defined to be the result of adding that instruction to the end of the active allocations list for the point preceding the instruction.

  • The active allocations list for the point following a stack deallocation instruction is defined to be the result of removing the instruction from the end of the active allocations list for the point preceding the instruction. The active allocations list for the preceding point is required to be non-empty, and the last instruction in it must be paired with the deallocation instruction.

    In other words, all stack allocations must be deallocated in last-in, first-out order, aka stack order.

  • The active allocations list for the point following any other instruction is defined to be the same as the active allocations list for the point preceding the instruction.

  • The active allocations list is required to be empty prior to return or throw instructions.

    In other words, all stack allocations must be deallocated prior to exiting the function.

Note that these rules implicitly prevent an allocation instruction from still being active when it is reached.

The control-flow rule forbids certain patterns that would theoretically be useful, such as conditionally performing an allocation around an operation. SIL generally makes this sort of pattern somewhat difficult to use, however, as it is illegal to locally abstract over addresses, and therefore a conditional allocation cannot be used in the intermediate operation anyway.

Structural type matching for pack indices

In order to catch type errors in applying pack indices, SIL requires the projected element types of pack-indexing operations to be structurally well-typed for the given pack type and index.

First, the projected element type must match the direct-ness of the indexed pack type: if the pack is indirect, the project element type must be an address type, and otherwise it must be an object type.

Second, the pack index must be a pack indexing instruction (one of scalar_pack_index, pack_pack_index, or dynamic_pack_index), and it must index into a pack type with the same shape as the indexed pack type.

Third, additional restrictions must be satisifed depending on which pack indexing instruction the pack index is:

  • For scalar_pack_index, the projected element type must be the same type as the scalar type at the given index in the pack type. (It must be a scalar type because of the shape restriction above.)

  • For pack_pack_index, the projected element type must be structurally well-typed for a slice of the pack type (as specified by the instruction) at the pack sub-index operand.

  • For dynamic_pack_index, consider each opened pack element archetype in the projected element type that is opened by an open_pack_element instruction whose pack index operand is the same dynamic_pack_index instruction. Because the pack substitutions in open_pack_element must have the same shape as the indexed pack type of its pack index operand, by transitivity they must have the same shape as the indexed pack type of the pack-indexing operation. Then for each component of this shape, the corresponding element component (or the pattern type for a projection component) of the indexed pack type must equal the result of applying a substitution to the projected element type which replaces any opened pack element archetype with the corresponding element component (pattern type for a projection component) of the pack substitution for that archetype in the open_pack_element which introduced it.

    For example, if the indexed pack type is Pack{Optional<Int>, Optional<Float>, repeat Optional<each T>}, a projected element type is $*Optional<@pack_element("1234") U> is structurally well-typed for a dynamic_pack_index pack index if 1234 is the UUID of an open_pack_element indexed by the same dynamic_pack_index instruction and the pack substitution corresponding to U in that open_pack_element is Pack{Int, Float, repeat each T}.

Instruction Set

Allocation and Deallocation

These instructions allocate and deallocate memory.

alloc_stack

sil-instruction ::= 'alloc_stack' alloc-stack-option* sil-type (',' debug-var-attr)*
alloc-stack-option ::= '[dynamic_lifetime]'
alloc-stack-option ::= '[lexical]'
alloc-stack-option ::= '[moveable_value_debuginfo]'

%1 = alloc_stack $T
// %1 has type $*T

Allocates uninitialized memory that is sufficiently aligned on the stack to contain a value of type T. The result of the instruction is the address of the allocated memory.

alloc_stack always allocates memory on the stack even for runtime-sized type.

alloc_stack is a stack allocation instruction. See the section above on stack discipline. The corresponding stack deallocation instruction is dealloc_stack.

The dynamic_lifetime attribute specifies that the initialization and destruction of the stored value cannot be verified at compile time. This is the case, e.g. for conditionally initialized objects.

The optional lexical attribute specifies that the storage corresponds to a local variable in the Swift source.

The optional moveable_value_debuginfo attribute specifies that when emitting debug info, the code generator can not assume that the value in the alloc_stack can be semantically valid over the entire function frame when emitting debug info. NOTE: This is implicitly set to true if the alloc_stack's type is noncopyable. This is just done to make SIL less verbose.

The memory is not retainable. To allocate a retainable box for a value type, use alloc_box.

T must not be a pack type. To allocate a pack, use alloc_pack.

alloc_pack

sil-instruction ::= 'alloc_pack' sil-type

%1 = alloc_pack $Pack{Int, Float, repeat each T}
// %1 has type $*Pack{Int, Float, repeat each T}

Allocates uninitialized memory on the stack for a value pack of the given type, which must be a pack type. The result of the instruction is the address of the allocated memory.

alloc_pack is a stack allocation instruction. See the section above on stack discipline. The corresponding stack deallocation instruction is dealloc_pack.

alloc_pack_metadata

sil-instruction ::= 'alloc_pack_metadata' $()

Inserted as the last SIL lowering pass of IRGen, indicates that the next instruction may have on-stack pack metadata allocated on its behalf.

Notionally, alloc_pack_metadata is a stack allocation instruction. See the section above on stack discipline. The corresponding stack deallocation instruction is dealloc_pack_metadata.

alloc_ref

sil-instruction ::= 'alloc_ref'
                      ('[' 'objc' ']')?
                      ('[' 'stack' ']')?
                      ('[' 'tail_elems' sil-type '*' sil-operand ']')*
                      sil-type

%1 = alloc_ref [stack] $T
%1 = alloc_ref [tail_elems $E * %2 : Builtin.Word] $T
// $T must be a reference type
// %1 has type $T
// $E is the type of the tail-allocated elements
// %2 must be of a builtin integer type

Allocates an object of reference type T. The object will be initialized with retain count 1; its state will be otherwise uninitialized. The optional objc attribute indicates that the object should be allocated using Objective-C's allocation methods (+allocWithZone:).

The optional stack attribute indicates that the object can be allocated on the stack instead on the heap. In this case the instruction must be balanced with a dealloc_stack_ref instruction to mark the end of the object's lifetime. Note that the stack attribute only specifies that stack allocation is possible. The final decision on stack allocation is done during llvm IR generation. This is because the decision also depends on the object size, which is not necessarily known at SIL level.

The optional tail_elems attributes specifies the amount of space to be reserved for tail-allocated arrays of given element types and element counts. If there are more than one tail_elems attributes then the tail arrays are allocated in the specified order. The count-operand must be of a builtin integer type. The instructions ref_tail_addr and tail_addr can be used to project the tail elements. The objc attribute cannot be used together with tail_elems.

alloc_ref_dynamic

sil-instruction ::= 'alloc_ref_dynamic'
                      ('[' 'objc' ']')?
                      ('[' 'tail_elems' sil-type '*' sil-operand ']')*
                      sil-operand ',' sil-type

%1 = alloc_ref_dynamic %0 : $@thick T.Type, $T
%1 = alloc_ref_dynamic [objc] %0 : $@objc_metatype T.Type, $T
%1 = alloc_ref_dynamic [tail_elems $E * %2 : Builtin.Word] %0 : $@thick T.Type, $T
// $T must be a class type
// %1 has type $T
// $E is the type of the tail-allocated elements
// %2 must be of a builtin integer type

Allocates an object of class type T or a subclass thereof. The dynamic type of the resulting object is specified via the metatype value %0. The object will be initialized with retain count 1; its state will be otherwise uninitialized.

The optional tail_elems and objc attributes have the same effect as for alloc_ref. See alloc_ref for details.

alloc_box

sil-instruction ::= 'alloc_box' alloc-box-option* sil-type (',' debug-var-attr)*
alloc-box-option ::= moveable_value_debuginfo

%1 = alloc_box $T
//   %1 has type $@box T

Allocates a reference-counted @box on the heap large enough to hold a value of type T, along with a retain count and any other metadata required by the runtime. The result of the instruction is the reference-counted @box reference that owns the box. The project_box instruction is used to retrieve the address of the value inside the box.

The box will be initialized with a retain count of 1; the storage will be uninitialized. The box owns the contained value, and releasing it to a retain count of zero destroys the contained value as if by destroy_addr. Releasing a box is undefined behavior if the box's value is uninitialized. To deallocate a box whose value has not been initialized, dealloc_box should be used.

The optional moveable_value_debuginfo attribute specifies that when emitting debug info, the code generator can not assume that the value in the alloc_stack can be semantically valid over the entire function frame when emitting debug info. NOTE: This is implicitly set to true if the alloc_stack's type is noncopyable. This is just done to make SIL less verbose.

alloc_global

sil-instruction ::= 'alloc_global' sil-global-name

alloc_global @foo

Initialize the storage for a global variable. This instruction has undefined behavior if the global variable has already been initialized.

The type operand must be a lowered object type.

get_async_continuation

sil-instruction ::= 'get_async_continuation' '[throws]'? sil-type

%0 = get_async_continuation $T
%0 = get_async_continuation [throws] $U

Begins a suspension of an @async function. This instruction can only be used inside an @async function. The result of the instruction is an UnsafeContinuation<T> value, where T is the formal type argument to the instruction, or an UnsafeThrowingContinuation<T> if the instruction carries the [throws] attribute. T must be a loadable type. The continuation must be consumed by a await_async_continuation terminator on all paths. Between get_async_continuation and await_async_continuation, the following restrictions apply:

  • The function cannot return, throw, yield, or unwind.
  • There cannot be nested suspend points; namely, the function cannot call another @async function, nor can it initiate another suspend point with get_async_continuation.

The function suspends execution when the matching await_async_continuation terminator is reached, and resumes execution when the continuation is resumed. The continuation resumption operation takes a value of type T which is passed back into the function when it resumes execution in the await_async_continuation instruction's resume successor block. If the instruction has the [throws] attribute, it can also be resumed in an error state, in which case the matching await_async_continuation instruction must also have an error successor.

Within the enclosing SIL function, the result continuation is consumed by the await_async_continuation, and cannot be referenced after the await_async_continuation executes. Dynamically, the continuation value must be resumed exactly once in the course of the program's execution; it is undefined behavior to resume the continuation more than once. Conversely, failing to resume the continuation will leave the suspended async coroutine hung in its suspended state, leaking any resources it may be holding.

get_async_continuation_addr

sil-instruction ::= 'get_async_continuation_addr' '[throws]'? sil-type ',' sil-operand

%1 = get_async_continuation_addr $T, %0 : $*T
%1 = get_async_continuation_addr [throws] $U, %0 : $*U

Begins a suspension of an @async function, like get_async_continuation, additionally binding a specific memory location for receiving the value when the result continuation is resumed. The operand must be an address whose type is the maximally-abstracted lowered type of the formal resume type. The memory must be uninitialized, and must remain allocated until the matching await_async_continuation instruction(s) consuming the result continuation have executed. The behavior is otherwise the same as get_async_continuation, and the same restrictions apply on code appearing between get_async_continuation_addr and await_async_continuation as apply between get_async_continuation and await_async_continuation. Additionally, the state of the memory referenced by the operand is indefinite between the execution of get_async_continuation_addr and await_async_continuation, and it is undefined behavior to read or modify the memory during this time. After the await_async_continuation resumes normally to its resume successor, the memory referenced by the operand is initialized with the resume value, and that value is then owned by the current function. If await_async_continuation instead resumes to its error successor, then the memory remains uninitialized.

hop_to_executor

sil-instruction ::= 'hop_to_executor' sil-operand

hop_to_executor %0 : $T

// $T must be Builtin.Executor or conform to the Actor protocol

Ensures that all instructions, which need to run on the actor's executor actually run on that executor. This instruction can only be used inside an @async function.

Checks if the current executor is the one which is bound to the operand actor. If not, begins a suspension point and enqueues the continuation to the executor which is bound to the operand actor.

SIL generation emits this instruction with operands of actor type as well as of type Builtin.Executor. The former are expected to be lowered by the SIL pipeline, so that IR generation only operands of type Builtin.Executor remain.

The operand is a guaranteed operand, i.e. not consumed.

extract_executor

sil-instruction ::= 'extract_executor' sil-operand

%1 = extract_executor %0 : $T
// $T must be Builtin.Executor or conform to the Actor protocol
// %1 will be of type Builtin.Executor

Extracts the executor from the executor or actor operand. SIL generation emits this instruction to produce executor values when needed (e.g., to provide to a runtime function). It will be lowered away by the SIL pipeline.

The operand is a guaranteed operand, i.e. not consumed.

dealloc_stack

sil-instruction ::= 'dealloc_stack' sil-operand

dealloc_stack %0 : $*T
// %0 must be of $*T type

Deallocates memory previously allocated by alloc_stack. The allocated value in memory must be uninitialized or destroyed prior to being deallocated.

dealloc_stack is a stack deallocation instruction. See the section on Stack Discipline above. The operand must be an alloc_stack instruction.

dealloc_pack

sil-instruction ::= 'dealloc_pack' sil-operand

dealloc_pack %0 : $*Pack{Int, Float, repeat each T}
// %0 must be the result of `alloc_pack $Pack{Int, Float, repeat each T}`

Deallocates memory for a pack value previously allocated by alloc_pack. If the pack elements are direct, they must be uninitialized or destroyed prior to being deallocated.

dealloc_pack is a stack deallocation instruction. See the section on Stack Discipline above. The operand must be an alloc_pack instruction.

dealloc_pack_metadata

sil-instruction ::= 'dealloc_pack_metadata' sil-operand

dealloc_pack_metadata $0 : $*()

Inserted as the last SIL lowering pass of IRGen, indicates that the on-stack pack metadata emitted on behalf of its operand (actually on behalf of the instruction after its operand) must be cleaned up here.

dealloc_pack_metadata is a stack deallocation instruction. See the section on Stack Discipline above. The operand must be an alloc_pack_metadata instruction.

dealloc_box

sil-instruction ::= 'dealloc_box' sil-operand

dealloc_box %0 : $@box T

Deallocates a box, bypassing the reference counting mechanism. The box variable must have a retain count of one. The boxed type must match the type passed to the corresponding alloc_box exactly, or else undefined behavior results.

This does not destroy the boxed value. The contents of the value must have been fully uninitialized or destroyed before dealloc_box is applied.

project_box

sil-instruction ::= 'project_box' sil-operand

%1 = project_box %0 : $@box T

// %1 has type $*T

Given a @box T reference, produces the address of the value inside the box.

dealloc_stack_ref

sil-instruction ::= 'dealloc_stack_ref' sil-operand

dealloc_stack_ref %0 : $T
// $T must be a class type
// %0 must be an 'alloc_ref [stack]' instruction

Marks the deallocation of the stack space for an alloc_ref [stack].

dealloc_ref

sil-instruction ::= 'dealloc_ref' sil-operand

dealloc_ref %0 : $T
// $T must be a class type

Deallocates an uninitialized class type instance, bypassing the reference counting mechanism.

The type of the operand must match the allocated type exactly, or else undefined behavior results.

The instance must have a retain count of one.

This does not destroy stored properties of the instance. The contents of stored properties must be fully uninitialized at the time dealloc_ref is applied.

The stack attribute indicates that the instruction is the balanced deallocation of its operand which must be a alloc_ref [stack]. In this case the instruction marks the end of the object's lifetime but has no other effect.

dealloc_partial_ref

sil-instruction ::= 'dealloc_partial_ref' sil-operand sil-metatype

dealloc_partial_ref %0 : $T, %1 : $U.Type
// $T must be a class type
// $T must be a subclass of U

Deallocates a partially-initialized class type instance, bypassing the reference counting mechanism.

The type of the operand must be a supertype of the allocated type, or else undefined behavior results.

The instance must have a retain count of one.

All stored properties in classes more derived than the given metatype value must be initialized, and all other stored properties must be uninitialized. The initialized stored properties are destroyed before deallocating the memory for the instance.

This does not destroy the reference type instance. The contents of the heap object must have been fully uninitialized or destroyed before dealloc_ref is applied.

Debug Information

Debug information is generally associated with allocations (alloc_stack or alloc_box) by having a Decl node attached to the allocation with a SILLocation. For declarations that have no allocation we have explicit instructions for doing this. This is used by 'let' declarations, which bind a value to a name and for var decls who are promoted into registers. The decl they refer to is attached to the instruction with a SILLocation.

debug_value

sil-instruction ::= debug_value sil-debug-value-option* sil-operand (',' debug-var-attr)* advanced-debug-var-attr* (',' 'expr' debug-info-expr)?
sil-debug-value-option ::= [poison]
sil-debug-value-option ::= [moveable_value_debuginfo]
sil-debug-value-option ::= [trace]

debug_value %1 : $Int

This indicates that the value of a declaration has changed value to the specified operand. The declaration in question is identified by either the SILLocation attached to the debug_value instruction or the SILLocation specified in the advanced debug variable attributes.

If the moveable_value_debuginfo flag is set, then one knows that the debug_value's operand is moved at some point of the program, so one can not model the debug_value using constructs that assume that the value is live for the entire function (e.x.: llvm.dbg.declare). NOTE: This is implicitly set to true if the alloc_stack's type is noncopyable. This is just done to make SIL less verbose.

debug-var-attr ::= 'var'
debug-var-attr ::= 'let'
debug-var-attr ::= 'name' string-literal
debug-var-attr ::= 'argno' integer-literal
debug-var-attr ::= 'implicit'

There are a number of attributes that provide details about the source variable that is being described, including the name of the variable. For function and closure arguments argno is the number of the function argument starting with 1. A compiler-generated source variable will be marked implicit and optimizers are free to remove it even in -Onone.

If the '[poison]' flag is set, then all references within this debug value will be overwritten with a sentinel at this point in the program. This is used in debug builds when shortening non-trivial value lifetimes to ensure the debugger cannot inspect invalid memory. debug_value instructions with the poison flag are not generated until OSSA is lowered. They are not expected to be serialized within the module, and the pipeline is not expected to do any significant code motion after lowering.

advanced-debug-var-attr ::= '(' 'name' string-literal (',' sil-instruction-source-info)? ')'
advanced-debug-var-attr ::= 'type' sil-type

Advanced debug variable attributes represent source locations and the type of the source variable when it was originally declared. It is useful when we're indirectly associating the SSA value with the source variable (via SIL DIExpression, for example) in which case SSA value's type is different from that of source variable.

debug-info-expr   ::= di-expr-operand (':' di-expr-operand)*
di-expr-operand   ::= di-expr-operator (':' sil-operand)*
di-expr-operator  ::= 'op_fragment'
di-expr-operator  ::= 'op_deref'

SIL debug info expression (SIL DIExpression) is a powerful method to connect SSA value with the source variable in an indirect fashion. Di-expression in SIL uses a stack based execution model to evaluate the expression and apply on the associated (SIL) SSA value before connecting it with the debug variable. For instance, given the following SIL code:

debug_value %a : $*Int, name "x", expr op_deref

It means: "You can get the value of source variable 'x' by dereferencing SSA value %a". The op_deref is a SIL DIExpression operator that represents "dereference". If there are multiple SIL DIExpression operators (or arguments), they are evaluated from left to right:

debug_value %b : $**Int, name "y", expr op_deref:op_deref

In the snippet above, two op_deref operators will be applied on SSA value %b sequentially.

Note that normally when the SSA value has an address type, there will be a op_deref in the SIL DIExpression. Because there is no pointer in Swift so you always need to dereference an address-type SSA value to get the value of a source variable. However, if the SSA value is a alloc_stack, the debug_value is used to indicate the declaration of a source variable. Or, you can say, used to specify the location (memory address) of the source variable. Therefore, we don't need to add a op_deref in this case:

%a = alloc_stack $Int, ...
debug_value %a : $*Int, name "my_var"

The op_fragment operator is used to specify the SSA value of a specific field in an aggregate-type source variable. This SIL DIExpression operator takes a field declaration -- which references the desired sub-field in source variable -- as its argument. Here is an example:

struct MyStruct {
  var x: Int
  var y: Int
}
...
debug_value %1 : $Int, var, (name "the_struct", loc "file.swift":8:7), type $MyStruct, expr op_fragment:#MyStruct.y, loc "file.swift":9:4

In the snippet above, source variable "the_struct" has an aggregate type $MyStruct and we use a SIL DIExpression with op_fragment operator to associate %1 to the y member variable (via the #MyStruct.y directive) inside "the_struct". Note that the extra source location directive follows right after name "the_struct" indicate that "the_struct" was originally declared in line 8, but not until line 9 -- the current debug_value instruction's source location -- does member y got updated with SSA value %1.

It is worth noting that a SIL DIExpression is similar to !DIExpression in LLVM debug info metadata. While LLVM represents !DIExpression are a list of 64-bit integers, SIL DIExpression can have elements with various types, like AST nodes or strings.

The [trace] flag is available for compiler unit testing. It is not produced during normal compilation. It is used combination with internal logging and optimization controls to select specific values to trace or to transform. For example, liveness analysis combines all "traced" values into a single live range with multiple definitions. This exposes corner cases that cannot be represented by passing valid SIL through the pipeline.

debug_step

sil-instruction ::= debug_step

debug_step

This instruction is inserted by Onone optimizations as a replacement for deleted instructions to ensure that it's possible to set a breakpoint on its location.

It is code-generated to a NOP instruction.

Testing

test_specification

sil-instruction ::= 'test_specification' string-literal

test_specification "parsing @trace[3] @function[other].block[2].instruction[1]"

Exists only for writing FileCheck tests. Specifies a list of test arguments which should be used in order to run a particular test "in the context" of the function containing the instruction.

Parsing of these test arguments is done via parseTestArgumentsFromSpecification.

The following types of test arguments are supported:

  • boolean: true false
  • unsigned integer: 0...ULONG_MAX
  • string
  • function: @function <-- the current function
    @function[uint] <-- function at index uint @function[name] <-- function named name
  • block: @block <-- the block containing the test_specification instruction
    @block[+uint] <-- the block uint blocks after the containing block @block[-uint] <-- the block uint blocks before the containing block @block[uint] <-- the block at index uint @{function}.{block} <-- the indicated block in the indicated function Example: @function[foo].block[2]
  • trace: @trace <-- the first debug_value [trace] in the current function
    @trace[uint] <-- the debug_value [trace] at index uint @{function}.{trace} <-- the indicated trace in the indicated function Example: @function[bar].trace
  • argument: @argument <-_ the first argument of the current block
    @argument[uint] <-- the argument at index uint of the current block @{block}.{argument} <-- the indicated argument in the indicated block @{function}.{argument} <-- the indicated argument in the entry block of the indicated function
  • instruction: @instruction <-- the instruction after* the test_specification instruction
    @instruction[+uint] <-- the instruction uint instructions after* the test_specification instruction @instruction[-uint] <-- the instruction uint instructions before* the test_specification instruction @instruction[uint] <-- the instruction at index uint @{function}.{instruction} <-- the indicated instruction in the indicated function Example: @function[baz].instruction[19] @{block}.{instruction} <-- the indicated instruction in the indicated block Example: @function[bam].block.instruction
  • operand: @operand <-- the first operand
    @operand[uint] <-- the operand at index uint @{instruction}.{operand} <-- the indicated operand of the indicated instruction Example: @block[19].instruction[2].operand[3] Example: @function[2].instruction.operand

  • Not counting instructions that are deleted when processing functions for tests. The following instructions currently are deleted:

    test_specification debug_value [trace]

Profiling

increment_profiler_counter

sil-instruction ::= 'increment_profiler_counter' int-literal ',' string-literal ',' 'num_counters' int-literal ',' 'hash' int-literal

increment_profiler_counter 1, "$foo", num_counters 3, hash 0

Increments a given profiler counter for a given PGO function name. This is lowered to the llvm.instrprof.increment LLVM intrinsic. This instruction is emitted when profiling is enabled, and enables features such as code coverage and profile-guided optimization.

Accessing Memory

load

sil-instruction ::= 'load' sil-operand

%1 = load %0 : $*T
// %0 must be of a $*T address type for loadable type $T
// %1 will be of type $T

Loads the value at address %0 from memory. T must be a loadable type. This does not affect the reference count, if any, of the loaded value; the value must be retained explicitly if necessary. It is undefined behavior to load from uninitialized memory or to load from an address that points to deallocated storage.

store

sil-instruction ::= 'store' sil-value 'to' sil-operand

store %0 to %1 : $*T
// $T must be a loadable type

Stores the value %0 to memory at address %1. The type of %1 is *T and the type of %0 is T, which must be a loadable type. This will overwrite the memory at %1. If %1 already references a value that requires release or other cleanup, that value must be loaded before being stored over and cleaned up. It is undefined behavior to store to an address that points to deallocated storage.

load_borrow

sil-instruction ::= 'load_borrow' sil-value

%1 = load_borrow %0 : $*T
// $T must be a loadable type

Loads the value %1 from the memory location %0. The load_borrow instruction creates a borrowed scope in which a read-only borrow value %1 can be used to read the value stored in %0. The end of scope is delimited by an end_borrow instruction. All load_borrow instructions must be paired with exactly one end_borrow instruction along any path through the program. Until end_borrow, it is illegal to invalidate or store to %0.

store_borrow

sil-instruction ::= 'store_borrow' sil-value 'to' sil-operand

%2 = store_borrow %0 to %1 : $*T
// $T must be a loadable type
// %1 must be an alloc_stack $T
// %2 is the return address

Stores the value %0 to a stack location %1, which must be an alloc_stack $T. The stack location must not be modified by other instructions than store_borrow. All uses of the store_borrow destination `%1 should be via the store_borrow return address %2 except dealloc_stack. The stored value is alive until the end_borrow. During its lifetime, the stored value must not be modified or destroyed. The source value %0 is borrowed (i.e. not copied) and its borrow scope must outlive the lifetime of the stored value.

Notionally, the outer borrow scope ensures that there's something to be addressed. The inner borrow scope provides the address to work with.

begin_borrow

sil-instruction ::= 'begin_borrow' '[lexical]'? sil-operand

%1 = begin_borrow %0 : $T

Given a value %0 with Owned or Guaranteed ownership, produces a new same typed value with Guaranteed ownership: %1. %1 is guaranteed to have a lifetime ending use (e.x.: end_borrow) along all paths that do not end in Dead End Blocks. This begin_borrow and the lifetime ending uses of %1 are considered to be liveness requiring uses of %0 and as such in the region in between this borrow and its lifetime ending use, %0 must be live. This makes sense semantically since %1 is modeling a new value with a dependent lifetime on %0.

The optional lexical attribute specifies that the operand corresponds to a local variable in the Swift source, so special care must be taken when moving the end_borrow.

This instruction is only valid in functions in Ownership SSA form.

end_borrow

sil-instruction ::= 'end_borrow' sil-operand

// somewhere earlier
// %1 = begin_borrow %0
end_borrow %1 : $T

Ends the scope for which the Guaranteed ownership possessing SILValue %1 is borrowed from the SILValue %0. Must be paired with at most 1 borrowing instruction (like load_borrow, begin_borrow) along any path through the program. In the region in between the borrow instruction and the end_borrow, the original SILValue can not be modified. This means that:

  1. If %0 is an address, %0 can not be written to.
  2. If %0 is a non-trivial value, %0 can not be destroyed.

We require that %1 and %0 have the same type ignoring SILValueCategory.

This instruction is only valid in functions in Ownership SSA form.

end_lifetime

sil-instruction ::= 'end_lifetime' sil-operand

This instruction signifies the end of it's operand's lifetime to the ownership verifier. It is inserted by the compiler in instances where it could be illegal to insert a destroy operation. Ex: if the sil-operand had an undef value.

This instruction is valid only in OSSA and is lowered to a no-op when lowering to non-OSSA.

assign

sil-instruction ::= 'assign' sil-value 'to' sil-operand

assign %0 to %1 : $*T
// $T must be a loadable type

Represents an abstract assignment of the value %0 to memory at address %1 without specifying whether it is an initialization or a normal store. The type of %1 is *T and the type of %0 is T, which must be a loadable type. This will overwrite the memory at %1 and destroy the value currently held there.

The purpose of the assign instruction is to simplify the definitive initialization analysis on loadable variables by removing what would otherwise appear to be a load and use of the current value. It is produced by SILGen, which cannot know which assignments are meant to be initializations. If it is deemed to be an initialization, it can be replaced with a store; otherwise, it must be replaced with a sequence that also correctly destroys the current value.

This instruction is only valid in Raw SIL and is rewritten as appropriate by the definitive initialization pass.

assign_by_wrapper

sil-instruction ::= 'assign_by_wrapper' sil-operand 'to' mode? sil-operand ',' 'init' sil-operand ',' 'set' sil-operand

mode ::= '[init]' | '[assign]' | '[assign_wrapped_value]'

assign_by_wrapper %0 : $S to %1 : $*T, init %2 : $F, set %3 : $G
// $S can be a value or address type
// $T must be the type of a property wrapper.
// $F must be a function type, taking $S as a single argument (or multiple arguments in case of a tuple) and returning $T
// $G must be a function type, taking $S as a single argument (or multiple arguments in case of a tuple) and without a return value

Similar to the assign instruction, but the assignment is done via a delegate.

Initially the instruction is created with no mode. Once the mode is decided (by the definitive initialization pass), the instruction is lowered as follows:

If the mode is initialization, the function %2 is called with %0 as argument. The result is stored to %1. In case of an address type, %1 is simply passed as a first out-argument to %2.

The assign mode works similar to initialization, except that the destination is "assigned" rather than "initialized". This means that the existing value in the destination is destroyed before the new value is stored.

If the mode is assign_wrapped_value, the function %3 is called with %0 as argument. As %3 is a setter (e.g. for the property in the containing nominal type), the destination address %1 is not used in this case.

This instruction is only valid in Raw SIL and is rewritten as appropriate by the definitive initialization pass.

mark_uninitialized

sil-instruction ::= 'mark_uninitialized' '[' mu_kind ']' sil-operand
mu_kind ::= 'var'
mu_kind ::= 'rootself'
mu_kind ::= 'crossmodulerootself'
mu_kind ::= 'derivedself'
mu_kind ::= 'derivedselfonly'
mu_kind ::= 'delegatingself'
mu_kind ::= 'delegatingselfallocated'

%2 = mark_uninitialized [var] %1 : $*T
// $T must be an address

Indicates that a symbolic memory location is uninitialized, and must be explicitly initialized before it escapes or before the current function returns. This instruction returns its operands, and all accesses within the function must be performed against the return value of the mark_uninitialized instruction.

The kind of mark_uninitialized instruction specifies the type of data the mark_uninitialized instruction refers to:

  • var: designates the start of a normal variable live range
  • rootself: designates self in a struct, enum, or root class
  • crossmodulerootself: same as rootself, but in a case where it's not
    really safe to treat self as a root because the original module might add more stored properties. This is only used for Swift 4 compatibility.
  • derivedself: designates self in a derived (non-root) class
  • derivedselfonly: designates self in a derived (non-root) class whose stored properties have already been initialized
  • delegatingself: designates self on a struct, enum, or class in a delegating constructor (one that calls self.init)
  • delegatingselfallocated: designates self on a class convenience initializer's initializing entry point

The purpose of the mark_uninitialized instruction is to enable definitive initialization analysis for global variables (when marked as 'globalvar') and instance variables (when marked as 'rootinit'), which need to be distinguished from simple allocations.

It is produced by SILGen, and is only valid in Raw SIL. It is rewritten as appropriate by the definitive initialization pass.

mark_function_escape

sil-instruction ::= 'mark_function_escape' sil-operand (',' sil-operand)

mark_function_escape %1 : $*T

Indicates that a function definition closes over a symbolic memory location. This instruction is variadic, and all of its operands must be addresses.

The purpose of the mark_function_escape instruction is to enable definitive initialization analysis for global variables and instance variables, which are not represented as box allocations.

It is produced by SILGen, and is only valid in Raw SIL. It is rewritten as appropriate by the definitive initialization pass.

mark_uninitialized_behavior

init-case ::= sil-value sil-apply-substitution-list? '(' sil-value ')' ':' sil-type
set-case ::= sil-value sil-apply-substitution-list? '(' sil-value ')' ':' sil-type
sil-instruction ::= 'mark_uninitialized_behavior' init-case set-case

mark_uninitialized_behavior %init<Subs>(%storage) : $T -> U,
                            %set<Subs>(%self) : $V -> W

Indicates that a logical property is uninitialized at this point and needs to be initialized by the end of the function and before any escape point for this instruction. Assignments to the property trigger the behavior's init or set logic based on the logical initialization state of the property.

It is expected that the init-case is passed some sort of storage and the set case is passed self.

This is only valid in Raw SIL.

copy_addr

sil-instruction ::= 'copy_addr' '[take]'? sil-value
                      'to' '[init]'? sil-operand

copy_addr [take] %0 to [init] %1 : $*T
// %0 and %1 must be of the same $*T address type

Loads the value at address %0 from memory and assigns a copy of it back into memory at address %1. A bare copy_addr instruction when T is a non-trivial type:

copy_addr %0 to %1 : $*T

is equivalent to:

%new = load %0 : $*T        // Load the new value from the source
%old = load %1 : $*T        // Load the old value from the destination
strong_retain %new : $T            // Retain the new value
strong_release %old : $T           // Release the old
store %new to %1 : $*T      // Store the new value to the destination

except that copy_addr may be used even if %0 is of an address-only type. The copy_addr may be given one or both of the [take] or [init] attributes:

  • [take] destroys the value at the source address in the course of the copy.
  • [init] indicates that the destination address is uninitialized. Without the attribute, the destination address is treated as already initialized, and the existing value will be destroyed before the new value is stored.

The three attributed forms thus behave like the following loadable type operations:

// take-assignment
  copy_addr [take] %0 to %1 : $*T
// is equivalent to:
  %new = load %0 : $*T
  %old = load %1 : $*T
  // no retain of %new!
  strong_release %old : $T
  store %new to %1 : $*T

// copy-initialization
  copy_addr %0 to [init] %1 : $*T
// is equivalent to:
  %new = load %0 : $*T
  strong_retain %new : $T
  // no load/release of %old!
  store %new to %1 : $*T

// take-initialization
  copy_addr [take] %0 to [init] %1 : $*T
// is equivalent to:
  %new = load %0 : $*T
  // no retain of %new!
  // no load/release of %old!
  store %new to %1 : $*T

If T is a trivial type, then copy_addr is always equivalent to its take-initialization form.

It is illegal in non-Raw SIL to apply copy_addr [init] to a value that is move only.

explicit_copy_addr

sil-instruction ::= 'explicit_copy_addr' '[take]'? sil-value
                      'to' '[init]'? sil-operand

explicit_copy_addr [take] %0 to [init] %1 : $*T
// %0 and %1 must be of the same $*T address type

This instruction is exactly the same as copy_addr except that it has special behavior for move only types. Specifically, an explicit_copy_addr is viewed as a copy_addr that is allowed on values that are move only. This is only used by a move checker after it has emitted an error diagnostic to preserve the general copy_addr [init] ban in Canonical SIL on move only types.

destroy_addr

sil-instruction ::= 'destroy_addr' sil-operand

destroy_addr %0 : $*T
// %0 must be of an address $*T type

Destroys the value in memory at address %0. If T is a non-trivial type, This is equivalent to:

%1 = load %0
strong_release %1

except that destroy_addr may be used even if %0 is of an address-only type. This does not deallocate memory; it only destroys the pointed-to value, leaving the memory uninitialized.

If T is a trivial type, then destroy_addr can be safely eliminated. However, a memory location %a must not be accessed after destroy_addr %a (which has not yet been eliminated) regardless of its type.

index_addr

sil-instruction ::= 'index_addr' ('[' 'stack_protection' ']')? sil-operand ',' sil-operand

%2 = index_addr %0 : $*T, %1 : $Builtin.Int<n>
// %0 must be of an address type $*T
// %1 must be of a builtin integer type
// %2 will be of type $*T

Given an address that references into an array of values, returns the address of the %1-th element relative to %0. The address must reference into a contiguous array. It is undefined to try to reference offsets within a non-array value, such as fields within a homogeneous struct or tuple type, or bytes within a value, using index_addr. (Int8 address types have no special behavior in this regard, unlike char* or void* in C.) It is also undefined behavior to index out of bounds of an array, except to index the "past-the-end" address of the array.

The stack_protection flag indicates that stack protection is done for the pointer origin.

tail_addr

sil-instruction ::= 'tail_addr' sil-operand ',' sil-operand ',' sil-type

%2 = tail_addr %0 : $*T, %1 : $Builtin.Int<n>, $E
// %0 must be of an address type $*T
// %1 must be of a builtin integer type
// %2 will be of type $*E

Given an address of an array of %1 values, returns the address of an element which is tail-allocated after the array. This instruction is equivalent to index_addr except that the resulting address is aligned-up to the tail-element type $E.

This instruction is used to project the N-th tail-allocated array from an object which is created by an alloc_ref with multiple tail_elems. The first operand is the address of an element of the (N-1)-th array, usually the first element. The second operand is the number of elements until the end of that array. The result is the address of the first element of the N-th array.

It is undefined behavior if the provided address, count and type do not match the actual layout of tail-allocated arrays of the underlying object.

index_raw_pointer

sil-instruction ::= 'index_raw_pointer' sil-operand ',' sil-operand

%2 = index_raw_pointer %0 : $Builtin.RawPointer, %1 : $Builtin.Int<n>
// %0 must be of $Builtin.RawPointer type
// %1 must be of a builtin integer type
// %2 will be of type $Builtin.RawPointer

Given a Builtin.RawPointer value %0, returns a pointer value at the byte offset %1 relative to %0.

bind_memory

sil-instruction ::= 'bind_memory' sil-operand ',' sil-operand 'to' sil-type

%token = bind_memory %0 : $Builtin.RawPointer, %1 : $Builtin.Word to $T
// %0 must be of $Builtin.RawPointer type
// %1 must be of $Builtin.Word type
// %token is an opaque $Builtin.Word representing the previously bound types
// for this memory region.

Binds memory at Builtin.RawPointer value %0 to type $T with enough capacity to hold %1 values. See SE-0107: UnsafeRawPointer.

Produces a opaque token representing the previous memory state for memory binding semantics. This abstract state includes the type that the memory was previously bound to along with the size of the affected memory region, which can be derived from %1. The token cannot, for example, be used to retrieve a metatype. It only serves a purpose when used by rebind_memory, which has no static type information. The token dynamically passes type information from the first bind_memory into a chain of rebind_memory operations.

Example:

%_      = bind_memory %0   : $Builtin.RawPointer, %numT : $Builtin.Word to $T // holds type 'T'
%token0 = bind_memory %0   : $Builtin.RawPointer, %numU : $Builtin.Word to $U // holds type 'U'
%token1 = rebind_memory %0 : $Builtin.RawPointer, %token0 : $Builtin.Word  // holds type 'T'
%token2 = rebind_memory %0 : $Builtin.RawPointer, %token1 : $Builtin.Word  // holds type 'U'

rebind_memory

sil-instruction ::= 'rebind_memory' sil-operand ' 'to' sil-value

%out_token = rebind_memory %0 : $Builtin.RawPointer to %in_token
// %0 must be of $Builtin.RawPointer type
// %in_token represents a cached set of bound types from a prior memory state.
// %out_token is an opaque $Builtin.Word representing the previously bound
// types for this memory region.

This instruction's semantics are identical to bind_memory, except that the types to which memory will be bound, and the extent of the memory region is unknown at compile time. Instead, the bound-types are represented by a token that was produced by a prior memory binding operation. %in_token must be the result of bind_memory or rebind_memory.

begin_access

sil-instruction ::= 'begin_access' '[' sil-access ']' '[' sil-enforcement ']' '[no_nested_conflict]'? '[builtin]'? sil-operand ':' sil-type
sil-access ::= init
sil-access ::= read
sil-access ::= modify
sil-access ::= deinit
sil-enforcement ::= unknown
sil-enforcement ::= static
sil-enforcement ::= dynamic
sil-enforcement ::= unsafe
sil-enforcement ::= signed
%1 = begin_access [read] [unknown] %0 : $*T
// %0 must be of $*T type.

Begins an access to the target memory.

The operand must be a root address derivation:

  • a function argument,
  • an alloc_stack instruction,
  • a project_box instruction,
  • a global_addr instruction,
  • a ref_element_addr instruction, or
  • another begin_access instruction.

It will eventually become a basic structural rule of SIL that no memory access instructions can be directly applied to the result of one of these instructions; they can only be applied to the result of a begin_access on them. For now, this rule will be conditional based on compiler settings and the SIL stage.

An access is ended with a corresponding end_access. Accesses must be uniquely ended on every control flow path which leads to either a function exit or back to the begin_access instruction. The set of active accesses must be the same on every edge into a basic block.

An init access takes uninitialized memory and initializes it. It must always use static enforcement.

An deinit access takes initialized memory and leaves it uninitialized. It must always use static enforcement.

read and modify accesses take initialized memory and leave it initialized. They may use unknown enforcement only in the raw SIL stage.

A no_nested_conflict access has no potentially conflicting access within its scope (on any control flow path between it and its corresponding end_access). Consequently, the access will not need to be tracked by the runtime for the duration of its scope. This access may still conflict with an outer access scope; therefore may still require dynamic enforcement at a single point.

A signed access is for pointers that are signed in architectures that support pointer signing.

A builtin access was emitted for a user-controlled Builtin (e.g. the standard library's KeyPath access). Non-builtin accesses are auto-generated by the compiler to enforce formal access that derives from the language. A builtin access is always fully enforced regardless of the compilation mode because it may be used to enforce access outside of the current module.

end_access

sil-instruction ::= 'end_access' ( '[' 'abort' ']' )? sil-operand

Ends an access. The operand must be a begin_access instruction.

If the begin_access is init or deinit, the end_access may be an abort, indicating that the described transition did not in fact take place.

begin_unpaired_access

sil-instruction ::= 'begin_unpaired_access' '[' sil-access ']' '[' sil-enforcement ']' '[no_nested_conflict]'? '[builtin]'? sil-operand : sil-type, sil-operand : $*Builtin.UnsafeValueBuffer
sil-access ::= init
sil-access ::= read
sil-access ::= modify
sil-access ::= deinit
sil-enforcement ::= unknown
sil-enforcement ::= static
sil-enforcement ::= dynamic
sil-enforcement ::= unsafe
%2 = begin_unpaired_access [read] [dynamic] %0 : $*T, %1 : $*Builtin.UnsafeValueBuffer
// %0 must be of $*T type.

Begins an access to the target memory. This has the same semantics and obeys all the same constraints as begin_access. With the following exceptions:

  • begin_unpaired_access has an additional operand for the scratch buffer used to uniquely identify this access within its scope.
  • An access initiated by begin_unpaired_access must end with end_unpaired_access unless it has the no_nested_conflict flag. A begin_unpaired_access with no_nested_conflict is effectively an instantaneous access with no associated scope.
  • The associated end_unpaired_access must use the same scratch buffer.

end_unpaired_access

sil-instruction ::= 'end_unpaired_access' ( '[' 'abort' ']' )? '[' sil-enforcement ']' sil-operand : $*Builtin.UnsafeValueBuffer
sil-enforcement ::= unknown
sil-enforcement ::= static
sil-enforcement ::= dynamic
sil-enforcement ::= unsafe
%1 = end_unpaired_access [dynamic] %0 : $*Builtin.UnsafeValueBuffer

Ends an access. This has the same semantics and constraints as end_access with the following exceptions:

  • The single operand refers to the scratch buffer that uniquely identified the access with this scope.
  • The enforcement level is reiterated, since the corresponding begin_unpaired_access may not be statically discoverable. It must be identical to the begin_unpaired_access enforcement.

Reference Counting

These instructions handle reference counting of heap objects. Values of strong reference type have ownership semantics for the referenced heap object. Retain and release operations, however, are never implicit in SIL and always must be explicitly performed where needed. Retains and releases on the value may be freely moved, and balancing retains and releases may be deleted, so long as an owning retain count is maintained for the uses of the value.

All reference-counting operations are defined to work correctly on null references (whether strong, unowned, or weak). A non-null reference must actually refer to a valid object of the indicated type (or a subtype). Address operands are required to be valid and non-null.

While SIL makes reference-counting operations explicit, the SIL type system also fully represents strength of reference. This is useful for several reasons:

  1. Type-safety: it is impossible to erroneously emit SIL that naively uses a @weak or @unowned reference as if it were a strong reference.
  2. Consistency: when a reference is kept in memory, instructions like copy_addr and destroy_addr implicitly carry the right semantics in the type of the address, rather than needing special variants or flags.
  3. Ease of tooling: SIL directly stores the user's intended strength of reference, making it straightforward to generate instrumentation that would convey this to a memory profiler. In principle, with only a modest number of additions and restrictions on SIL, it would even be possible to drop all reference-counting instructions and use the type information to feed a garbage collector.

strong_retain

sil-instruction ::= 'strong_retain' sil-operand

strong_retain %0 : $T
// $T must be a reference type

Increases the strong retain count of the heap object referenced by %0.

strong_release

strong_release %0 : $T
// $T must be a reference type.

Decrements the strong reference count of the heap object referenced by %0. If the release operation brings the strong reference count of the object to zero, the object is destroyed and @weak references are cleared. When both its strong and unowned reference counts reach zero, the object's memory is deallocated.

set_deallocating

set_deallocating %0 : $T
// $T must be a reference type.

Explicitly sets the state of the object referenced by %0 to deallocated. This is the same operation what's done by a strong_release immediately before it calls the deallocator of the object.

It is expected that the strong reference count of the object is one. Furthermore, no other thread may increment the strong reference count during execution of this instruction.

strong_copy_unowned_value

sil-instruction ::= 'strong_copy_unowned_value' sil-operand

%1 = strong_copy_unowned_value %0 : $@unowned T
// %1 will be a strong @owned value of type $T.
// $T must be a reference type

Asserts that the strong reference count of the heap object referenced by %0 is still positive, then increments the reference count and returns a new strong reference to %0. The intention is that this instruction is used as a "safe ownership conversion" from unowned to strong.

strong_retain_unowned

sil-instruction ::= 'strong_retain_unowned' sil-operand

strong_retain_unowned %0 : $@unowned T
// $T must be a reference type

Asserts that the strong reference count of the heap object referenced by %0 is still positive, then increases it by one.

unowned_retain

sil-instruction ::= 'unowned_retain' sil-operand

unowned_retain %0 : $@unowned T
// $T must be a reference type

Increments the unowned reference count of the heap object underlying %0.

unowned_release

sil-instruction ::= 'unowned_release' sil-operand

unowned_release %0 : $@unowned T
// $T must be a reference type

Decrements the unowned reference count of the heap object referenced by %0. When both its strong and unowned reference counts reach zero, the object's memory is deallocated.

load_weak

sil-instruction ::= 'load_weak' '[take]'? sil-operand

load_weak [take] %0 : $*@sil_weak Optional<T>
// $T must be an optional wrapping a reference type

Increments the strong reference count of the heap object held in the operand, which must be an initialized weak reference. The result is value of type $Optional<T>, except that it is null if the heap object has begun deallocation.

If [take] is specified then the underlying weak reference is invalidated implying that the weak reference count of the loaded value is decremented. If [take] is not specified then the underlying weak reference count is not affected by this operation (i.e. it is a +0 weak ref count operation). In either case, the strong reference count will be incremented before any changes to the weak reference count.

This operation must be atomic with respect to the final strong_release on the operand heap object. It need not be atomic with respect to store_weak operations on the same address.

store_weak

sil-instruction ::= 'store_weak' sil-value 'to' '[init]'? sil-operand

store_weak %0 to [init] %1 : $*@sil_weak Optional<T>
// $T must be an optional wrapping a reference type

Initializes or reassigns a weak reference. The operand may be nil.

If [init] is given, the weak reference must currently either be uninitialized or destroyed. If it is not given, the weak reference must currently be initialized. After the evaluation:

  • The value that was originally referenced by the weak reference will have its weak reference count decremented by 1.
  • If the optionally typed operand is non-nil, the strong reference wrapped in the optional has its weak reference count incremented by 1. In contrast, the reference's strong reference count is not touched.

This operation must be atomic with respect to the final strong_release on the operand (source) heap object. It need not be atomic with respect to store_weak or load_weak operations on the same address.

load_unowned

TODO: Fill this in

store_unowned

TODO: Fill this in

fix_lifetime

sil-instruction :: 'fix_lifetime' sil-operand

fix_lifetime %0 : $T
// Fix the lifetime of a value %0
fix_lifetime %1 : $*T
// Fix the lifetime of the memory object referenced by %1

Acts as a use of a value operand, or of the value in memory referenced by an address operand. Optimizations may not move operations that would destroy the value, such as release_value, strong_release, copy_addr [take], or destroy_addr, past this instruction.

mark_dependence

sil-instruction :: 'mark_dependence' sil-operand 'on' sil-operand

%2 = mark_dependence %0 : $*T on %1 : $Builtin.NativeObject

Indicates that the validity of the first operand depends on the value of the second operand. Operations that would destroy the second value must not be moved before any instructions which depend on the result of this instruction, exactly as if the address had been obviously derived from that operand (e.g. using ref_element_addr).

The result is always equal to the first operand. The first operand will typically be an address, but it could be an address in a non-obvious form, such as a Builtin.RawPointer or a struct containing the same. Transformations should be somewhat forgiving here.

The second operand may have either object or address type. In the latter case, the dependency is on the current value stored in the address.

is_unique

sil-instruction ::= 'is_unique' sil-operand

%1 = is_unique %0 : $*T
// $T must be a reference-counted type
// %1 will be of type Builtin.Int1

Checks whether %0 is the address of a unique reference to a memory object. Returns 1 if the strong reference count is 1, and 0 if the strong reference count is greater than 1.

A discussion of the semantics can be found here: is_unique instruction

begin_cow_mutation

sil-instruction ::= 'begin_cow_mutation' '[native]'? sil-operand

(%1, %2) = begin_cow_mutation %0 : $C
// $C must be a reference-counted type
// %1 will be of type Builtin.Int1
// %2 will be of type C

Checks whether %0 is a unique reference to a memory object. Returns 1 in the first result if the strong reference count is 1, and 0 if the strong reference count is greater than 1.

Returns the reference operand in the second result. The returned reference can be used to mutate the object. Technically, the returned reference is the same as the operand. But it's important that optimizations see the result as a different SSA value than the operand. This is important to ensure the correctness of ref_element_addr [immutable].

The operand is consumed and the second result is returned as owned.

The optional native attribute specifies that the operand has native Swift reference counting.

end_cow_mutation

sil-instruction ::= 'end_cow_mutation' '[keep_unique]'? sil-operand

%1 = end_cow_mutation %0 : $C
// $C must be a reference-counted type
// %1 will be of type C

Marks the end of the mutation of a reference counted object. Returns the reference operand. Technically, the returned reference is the same as the operand. But it's important that optimizations see the result as a different SSA value than the operand. This is important to ensure the correctness of ref_element_addr [immutable].

The operand is consumed and the result is returned as owned. The result is guaranteed to be uniquely referenced.

The optional keep_unique attribute indicates that the optimizer must not replace this reference with a not uniquely reference object.

is_escaping_closure

sil-instruction ::= 'is_escaping_closure' sil-operand

%1 = is_escaping_closure %0 : $@callee_guaranteed () -> ()
// %0 must be an escaping swift closure.
// %1 will be of type Builtin.Int1

Checks whether the context reference is not nil and bigger than one and returns true if it is.

copy_block

sil-instruction :: 'copy_block' sil-operand

%1 = copy_block %0 : $@convention(block) T -> U

Performs a copy of an Objective-C block. Unlike retains of other reference-counted types, this can produce a different value from the operand if the block is copied from the stack to the heap.

copy_block_without_escaping

sil-instruction :: 'copy_block_without_escaping' sil-operand 'withoutEscaping' sil-operand

%1 = copy_block %0 : $@convention(block) T -> U withoutEscaping %1 : $T -> U

Performs a copy of an Objective-C block. Unlike retains of other reference-counted types, this can produce a different value from the operand if the block is copied from the stack to the heap.

Additionally, consumes the withoutEscaping operand %1 which is the closure sentinel. SILGen emits these instructions when it passes @noescape swift closures to Objective C. A mandatory SIL pass will lower this instruction into a copy_block and a is_escaping/cond_fail/destroy_value at the end of the lifetime of the objective c closure parameter to check whether the sentinel closure was escaped.

Literals

These instructions bind SIL values to literal constants or to global entities.

function_ref

sil-instruction ::= 'function_ref' sil-function-name ':' sil-type

%1 = function_ref @function : $@convention(thin) T -> U
// $@convention(thin) T -> U must be a thin function type
// %1 has type $T -> U

Creates a reference to a SIL function.

dynamic_function_ref

sil-instruction ::= 'dynamic_function_ref' sil-function-name ':' sil-type

%1 = dynamic_function_ref @function : $@convention(thin) T -> U
// $@convention(thin) T -> U must be a thin function type
// %1 has type $T -> U

Creates a reference to a dynamically_replacable SIL function. A dynamically_replacable SIL function can be replaced at runtime.

For the following Swift code:

dynamic func test_dynamically_replaceable() {}

func test_dynamic_call() {
  test_dynamically_replaceable()
}

We will generate:

sil [dynamically_replacable] @test_dynamically_replaceable : $@convention(thin) () -> () {
bb0:
  %0 = tuple ()
  return %0 : $()
}

sil @test_dynamic_call : $@convention(thin) () -> () {
bb0:
  %0 = dynamic_function_ref @test_dynamically_replaceable : $@convention(thin) () -> ()
  %1 = apply %0() : $@convention(thin) () -> ()
  %2 = tuple ()
  return %2 : $()
}

prev_dynamic_function_ref

sil-instruction ::= 'prev_dynamic_function_ref' sil-function-name ':' sil-type

%1 = prev_dynamic_function_ref @function : $@convention(thin) T -> U
// $@convention(thin) T -> U must be a thin function type
// %1 has type $T -> U

Creates a reference to a previous implementation of a dynamic_replacement SIL function.

For the following Swift code:

@_dynamicReplacement(for: test_dynamically_replaceable())
func test_replacement() {
  test_dynamically_replaceable() // calls previous implementation
}

We will generate:

sil [dynamic_replacement_for "test_dynamically_replaceable"] @test_replacement : $@convention(thin) () -> () {
bb0:
  %0 = prev_dynamic_function_ref @test_replacement : $@convention(thin) () -> ()
  %1 = apply %0() : $@convention(thin) () -> ()
  %2 = tuple ()
  return %2 : $()
}

global_addr

sil-instruction ::= 'global_addr' sil-global-name ':' sil-type

%1 = global_addr @foo : $*Builtin.Word

Creates a reference to the address of a global variable which has been previously initialized by alloc_global. It is undefined behavior to perform this operation on a global variable which has not been initialized, except the global variable has a static initializer.

global_value

sil-instruction ::= 'global_value' sil-global-name ':' sil-type

%1 = global_value @v : $T

Returns the value of a global variable which has been previously initialized by alloc_global. It is undefined behavior to perform this operation on a global variable which has not been initialized, except the global variable has a static initializer.

integer_literal

sil-instruction ::= 'integer_literal' sil-type ',' int-literal

%1 = integer_literal $Builtin.Int<n>, 123
// $Builtin.Int<n> must be a builtin integer type
// %1 has type $Builtin.Int<n>

Creates an integer literal value. The result will be of type Builtin.Int<n>, which must be a builtin integer type. The literal value is specified using Swift's integer literal syntax.

float_literal

sil-instruction ::= 'float_literal' sil-type ',' int-literal

%1 = float_literal $Builtin.FP<n>, 0x3F800000
// $Builtin.FP<n> must be a builtin floating-point type
// %1 has type $Builtin.FP<n>

Creates a floating-point literal value. The result will be of type Builtin.FP<n>, which must be a builtin floating-point type. The literal value is specified as the bitwise representation of the floating point value, using Swift's hexadecimal integer literal syntax.

string_literal

sil-instruction ::= 'string_literal' encoding string-literal
encoding ::= 'utf8'
encoding ::= 'utf16'
encoding ::= 'objc_selector'

%1 = string_literal "asdf"
// %1 has type $Builtin.RawPointer

Creates a reference to a string in the global string table. The result is a pointer to the data. The referenced string is always null-terminated. The string literal value is specified using Swift's string literal syntax (though \() interpolations are not allowed). When the encoding is objc_selector, the string literal produces a reference to a UTF-8-encoded Objective-C selector in the Objective-C method name segment.

base_addr_for_offset

sil-instruction ::= 'base_addr_for_offset' sil-type

%1 = base_addr_for_offset $*S
// %1 has type $*S

Creates a base address for offset calculations. The result can be used by address projections, like struct_element_addr, which themselves return the offset of the projected fields. IR generation simply creates a null pointer for base_addr_for_offset.

Dynamic Dispatch

These instructions perform dynamic lookup of class and generic methods.

The class_method and super_method instructions must reference Swift native methods and always use vtable dispatch.

The objc_method and objc_super_method instructions must reference Objective-C methods (indicated by the foreign marker on a method reference, as in #NSObject.description!foreign).

Note that objc_msgSend invocations can only be used as the callee of an apply instruction or partial_apply instruction. They cannot be stored or used as apply or partial_apply arguments.

class_method

sil-instruction ::= 'class_method' sil-method-attributes?
                      sil-operand ',' sil-decl-ref ':' sil-type

%1 = class_method %0 : $T, #T.method : $@convention(class_method) U -> V
// %0 must be of a class type or class metatype $T
// #T.method must be a reference to a Swift native method of T or
// of one of its superclasses
// %1 will be of type $U -> V

Looks up a method based on the dynamic type of a class or class metatype instance. It is undefined behavior if the class value is null.

If the static type of the class instance is known, or the method is known to be final, then the instruction is a candidate for devirtualization optimization. A devirtualization pass can consult the module's VTables to find the SIL function that implements the method and promote the instruction to a static function_ref.

objc_method

sil-instruction ::= 'objc_method' sil-method-attributes?
                      sil-operand ',' sil-decl-ref ':' sil-type

%1 = objc_method %0 : $T, #T.method!foreign : $@convention(objc_method) U -> V
// %0 must be of a class type or class metatype $T
// #T.method must be a reference to an Objective-C method of T or
// of one of its superclasses
// %1 will be of type $U -> V

Performs Objective-C method dispatch using objc_msgSend().

Objective-C method calls are never candidates for devirtualization.

super_method

sil-instruction ::= 'super_method' sil-method-attributes?
                      sil-operand ',' sil-decl-ref ':' sil-type

%1 = super_method %0 : $T, #Super.method : $@convention(thin) U -> V
// %0 must be of a non-root class type or class metatype $T
// #Super.method must be a reference to a native Swift method of T's
// superclass or of one of its ancestor classes
// %1 will be of type $@convention(thin) U -> V

Looks up a method in the superclass of a class or class metatype instance.

objc_super_method

sil-instruction ::= 'super_method' sil-method-attributes?
                      sil-operand ',' sil-decl-ref ':' sil-type

%1 = super_method %0 : $T, #Super.method!foreign : $@convention(thin) U -> V
// %0 must be of a non-root class type or class metatype $T
// #Super.method!foreign must be a reference to an ObjC method of T's
// superclass or of one of its ancestor classes
// %1 will be of type $@convention(thin) U -> V

This instruction performs an Objective-C message send using objc_msgSuper().

witness_method

sil-instruction ::= 'witness_method' sil-method-attributes?
                      sil-type ',' sil-decl-ref ':' sil-type

%1 = witness_method $T, #Proto.method \
  : $@convention(witness_method) <Self: Proto> U -> V
// $T must be an archetype
// #Proto.method must be a reference to a method of one of the protocol
//   constraints on T
// <Self: Proto> U -> V must be the type of the referenced method,
//   generic on Self
// %1 will be of type $@convention(thin) <Self: Proto> U -> V

Looks up the implementation of a protocol method for a generic type variable constrained by that protocol. The result will be generic on the Self archetype of the original protocol and have the witness_method calling convention. If the referenced protocol is an @objc protocol, the resulting type has the objc calling convention.

Function Application

These instructions call functions or wrap them in partial application or specialization thunks.

apply

sil-instruction ::= 'apply' '[nothrow]'? sil-value
                      sil-apply-substitution-list?
                      '(' (sil-value (',' sil-value)*)? ')'
                      ':' sil-type

sil-apply-substitution-list ::= '<' sil-substitution
                                    (',' sil-substitution)* '>'
sil-substitution ::= type '=' type

%r = apply %0(%1, %2, ...) : $(A, B, ...) -> R
// Note that the type of the callee '%0' is specified *after* the arguments
// %0 must be of a concrete function type $(A, B, ...) -> R
// %1, %2, etc. must be of the argument types $A, $B, etc.
// %r will be of the return type $R

%r = apply %0<A, B>(%1, %2, ...) : $<T, U>(T, U, ...) -> R
// %0 must be of a polymorphic function type $<T, U>(T, U, ...) -> R
// %1, %2, etc. must be of the argument types after substitution $A, $B, etc.
// %r will be of the substituted return type $R'

Transfers control to function %0, passing it the given arguments. In the instruction syntax, the type of the callee is specified after the argument list; the types of the argument and of the defined value are derived from the function type of the callee. The input argument tuple type is destructured, and each element is passed as an individual argument. The apply instruction does no retaining or releasing of its arguments by itself; the calling convention's retain/release policy must be handled by separate explicit retain and release instructions. The return value will likewise not be implicitly retained or released.

The callee value must have function type. That function type may not have an error result, except the instruction has the nothrow attribute set. The nothrow attribute specifies that the callee has an error result but does not actually throw. For the regular case of calling a function with error result, use try_apply.

NB: If the callee value is of a thick function type, apply currently consumes the callee value at +1 strong retain count.

If the callee is generic, all of its generic parameters must be bound by the given substitution list. The arguments and return value is given with these generic substitutions applied.

begin_apply

sil-instruction ::= 'begin_apply' '[nothrow]'? sil-value
                      sil-apply-substitution-list?
                      '(' (sil-value (',' sil-value)*)? ')'
                      ':' sil-type

(%anyAddr, %float, %token) = begin_apply %0() : $@yield_once () -> (@yields @inout %Any, @yields Float)
// %anyAddr : $*Any
// %float : $Float
// %token is a token

Transfers control to coroutine %0, passing it the given arguments. The rules for the application generally follow the rules for apply, except:

  • the callee value must have a yield_once coroutine type,
  • control returns to this function not when the coroutine performs a return, but when it performs a yield, and
  • the instruction results are derived from the yields of the coroutine instead of its normal results.

The final result of a begin_apply is a "token", a special value which can only be used as the operand of an end_apply or abort_apply instruction. Before this second instruction is executed, the coroutine is said to be "suspended", and the token represents a reference to its suspended activation record.

The other results of the instruction correspond to the yields in the coroutine type. In general, the rules of a yield are similar to the rules for a parameter, interpreted as if the coroutine caller (the one executing the begin_apply) were being "called" by the yield:

  • If a yield has an indirect convention, the corresponding result will have an address type; otherwise it has an object type. For example, a result corresponding to an @in Any yield will have type $Any.
  • The convention attributes are the same as the parameter convention attributes, interpreted as if the yield were the "call" and the begin_apply marked the entry to the "callee". For example, an @in Any yield transfers ownership of the Any value reference from the coroutine to the caller, which must destroy or move the value from that position before ending or aborting the coroutine.

A begin_apply must be uniquely either ended or aborted before exiting the function or looping to an earlier portion of the function.

When throwing coroutines are supported, there will need to be a try_begin_apply instruction.

abort_apply

sil-instruction ::= 'abort_apply' sil-value

abort_apply %token

Aborts the given coroutine activation, which is currently suspended at a yield instruction. Transfers control to the coroutine and takes the unwind path from the yield. Control is transferred back when the coroutine reaches an unwind instruction.

The operand must always be the token result of a begin_apply instruction, which is why it need not specify a type.

Throwing coroutines will not require a new instruction for aborting a coroutine; a coroutine is not allowed to throw when it is being aborted.

end_apply

sil-instruction ::= 'end_apply' sil-value

end_apply %token

Ends the given coroutine activation, which is currently suspended at a yield instruction. Transfers control to the coroutine and takes the resume path from the yield. Control is transferred back when the coroutine reaches a return instruction.

The operand must always be the token result of a begin_apply instruction, which is why it need not specify a type.

end_apply currently has no instruction results. If coroutines were allowed to have normal results, they would be producted by end_apply.

When throwing coroutines are supported, there will need to be a try_end_apply instruction.

partial_apply

sil-instruction ::= 'partial_apply' callee-ownership-attr? on-stack-attr? sil-value
                      sil-apply-substitution-list?
                      '(' (sil-value (',' sil-value)*)? ')'
                      ':' sil-type
callee-ownership-attr ::= '[callee_guaranteed]'
on-stack-attr ::= '[on_stack]'

%c = partial_apply %0(%1, %2, ...) : $(Z..., A, B, ...) -> R
// Note that the type of the callee '%0' is specified *after* the arguments
// %0 must be of a concrete function type $(Z..., A, B, ...) -> R
// %1, %2, etc. must be of the argument types $A, $B, etc.,
//   of the tail part of the argument tuple of %0
// %c will be of the partially-applied thick function type (Z...) -> R

%c = partial_apply %0<A, B>(%1, %2, ...) : $(Z..., T, U, ...) -> R
// %0 must be of a polymorphic function type $<T, U>(T, U, ...) -> R
// %1, %2, etc. must be of the argument types after substitution $A, $B, etc.
//   of the tail part of the argument tuple of %0
// %r will be of the substituted thick function type $(Z'...) -> R'

Creates a closure by partially applying the function %0 to a partial sequence of its arguments. This instruction is used to implement closures.

A local function in Swift that captures context, such as bar in the following example:

func foo(_ x:Int) -> Int {
  func bar(_ y:Int) -> Int {
    return x + y
  }
  return bar(1)
}

lowers to an uncurried entry point and is curried in the enclosing function:

func @bar : $@convention(thin) (Int, @box Int, *Int) -> Int {
entry(%y : $Int, %x_box : $@box Int, %x_address : $*Int):
  // ... body of bar ...
}

func @foo : $@convention(thin) Int -> Int {
entry(%x : $Int):
  // Create a box for the 'x' variable
  %x_box = alloc_box $Int
  %x_addr = project_box %x_box : $@box Int
  store %x to %x_addr : $*Int

  // Create the bar closure
  %bar_uncurried = function_ref @bar : $(Int, Int) -> Int
  %bar = partial_apply %bar_uncurried(%x_box, %x_addr) \
    : $(Int, Builtin.NativeObject, *Int) -> Int

  // Apply it
  %1 = integer_literal $Int, 1
  %ret = apply %bar(%1) : $(Int) -> Int

  // Clean up
  release %bar : $(Int) -> Int
  return %ret : $Int
}

Ownership Semantics of Closure Context during Invocation: By default, an escaping partial_apply (partial_apply without [on_stack]] creates a closure whose invocation takes ownership of the context, meaning that a call implicitly releases the closure. If the partial_apply is marked with the flag [callee_guaranteed] the invocation instead uses a caller-guaranteed model, where the caller promises not to release the closure while the function is being called.

Captured Value Ownership Semantics: In the instruction syntax, the type of the callee is specified after the argument list; the types of the argument and of the defined value are derived from the function type of the callee. Even so, the ownership requirements of the partial apply are not the same as that of the callee function (and thus said signature). Instead:

  1. If the partial_apply has a @noescape function type (partial_apply [on_stack]) the closure context is allocated on the stack and is initialized to contain the closed-over values without taking ownership of those values. The closed-over values are not retained and the lifetime of the closed-over values must be managed by other instruction independently of the partial_apply. The lifetime of the stack context of a partial_apply [on_stack] must be terminated with a dealloc_stack.
  2. If the partial_apply has an escaping function type (not [on_stack]) then the closure context will be heap allocated with a retain count of 1. Any closed over parameters (except for @inout parameters) will be consumed by the partial_apply. This ensures that no matter when the partial_apply is called, the captured arguments are alive. When the closure context's reference count reaches zero, the contained values are destroyed. If the callee requires an owned parameter, then the implicit partial_apply forwarder created by IRGen will copy the underlying argument and pass it to the callee.
  3. If an address argument has @inout_aliasable convention, the closure obtained from partial_apply will not own its underlying value. The @inout_aliasable parameter convention is used when a @noescape closure captures an inout argument.

NOTE: If the callee is generic, all of its generic parameters must be bound by the given substitution list. The arguments are given with these generic substitutions applied, and the resulting closure is of concrete function type with the given substitutions applied. The generic parameters themselves cannot be partially applied; all of them must be bound. The result is always a concrete function.

TODO: The instruction, when applied to a generic function, currently implicitly performs abstraction difference transformations enabled by the given substitutions, such as promoting address-only arguments and returns to register arguments. This should be fixed.

builtin

sil-instruction ::= 'builtin' string-literal
                      sil-apply-substitution-list?
                      '(' (sil-operand (',' sil-operand)*)? ')'
                      ':' sil-type

%1 = builtin "foo"(%1 : $T, %2 : $U) : $V
// "foo" must name a function in the Builtin module

Invokes functionality built into the backend code generator, such as LLVM- level instructions and intrinsics.

Metatypes

These instructions access metatypes, either statically by type name or dynamically by introspecting class or generic values.

metatype

sil-instruction ::= 'metatype' sil-type

%1 = metatype $T.Type
// %1 has type $T.Type

Creates a reference to the metatype object for type T.

value_metatype

sil-instruction ::= 'value_metatype' sil-type ',' sil-operand

%1 = value_metatype $T.Type, %0 : $T
// %0 must be a value or address of type $T
// %1 will be of type $T.Type

Obtains a reference to the dynamic metatype of the value %0.

existential_metatype

sil-instruction ::= 'existential_metatype' sil-type ',' sil-operand

%1 = existential_metatype $P.Type, %0 : $P
// %0 must be a value of class protocol or protocol composition
//   type $P, or an address of address-only protocol type $*P
// %1 will be a $P.Type value referencing the metatype of the
//   concrete value inside %0

Obtains the metatype of the concrete value referenced by the existential container referenced by %0.

objc_protocol

sil-instruction ::= 'objc_protocol' protocol-decl : sil-type

%0 = objc_protocol #ObjCProto : $Protocol

TODO Fill this in.

Aggregate Types

These instructions construct and project elements from structs, tuples, and class instances.

retain_value

sil-instruction ::= 'retain_value' sil-operand

retain_value %0 : $A

Retains a loadable value, which simply retains any references it holds.

For trivial types, this is a no-op. For reference types, this is equivalent to a strong_retain. For @unowned types, this is equivalent to an unowned_retain. In each of these cases, those are the preferred forms.

For aggregate types, especially enums, it is typically both easier and more efficient to reason about aggregate copies than it is to reason about copies of the subobjects.

retain_value_addr

sil-instruction ::= 'retain_value_addr' sil-operand

retain_value_addr %0 : $*A

Retains a loadable value inside given address, which simply retains any references it holds.

unmanaged_retain_value

sil-instruction ::= 'unmanaged_retain_value' sil-value

unmanaged_retain_value %0 : $A

This instruction has the same local semantics as retain_value but:

  • Is valid in ownership qualified SIL.
  • Is not intended to be statically paired at compile time by the compiler.

The intention is that this instruction is used to implement unmanaged constructs.

strong_copy_unmanaged_value

sil-instruction ::= 'strong_copy_unmanaged_value' sil-value

%1 = strong_copy_unmanaged_value %0 : $@sil_unmanaged A
// %1 will be a strong @owned $A.

This instruction has the same semantics as copy_value except that its input is a trivial @sil_unmanaged type that doesn't require ref counting. This is intended to be used semantically as a "conversion" like instruction from unmanaged to strong and thus should never be removed by the optimizer. Since the returned value is a strong owned value, this instruction semantically should be treated as performing a strong copy of the underlying value as if by the value's type lowering.

copy_value

sil-instruction ::= 'copy_value' sil-operand

%1 = copy_value %0 : $A

Performs a copy of a loadable value as if by the value's type lowering and returns the copy. The returned copy semantically is a value that is completely independent of the operand. In terms of specific types:

  1. For trivial types, this is equivalent to just propagating through the trivial value.
  2. For reference types, this is equivalent to performing a strong_retain operation and returning the reference.
  3. For @unowned types, this is equivalent to performing an unowned_retain and returning the operand.
  4. For aggregate types, this is equivalent to recursively performing a copy_value on its components, forming a new aggregate from the copied components, and then returning the new aggregate.

In ownership qualified functions, a copy_value produces a +1 value that must be consumed at most once along any path through the program.

It is illegal in non-Raw SIL to copy_value a value that is "move only".

explicit_copy_value

sil-instruction ::= 'explicit_copy_value' sil-operand

%1 = explicit_copy_value %0 : $A

This is exactly the same instruction semantically as copy_value with the exception that when move only checking is performed, explicit_copy_value is treated as an explicit copy asked for by the user that should not be rewritten and should be treated as a non-consuming use.

This is used for two things:

  1. Implementing a copy builtin for no implicit copy types.
  2. To enable the move checker, once it has emitted an error diagnostic, to still produce valid Ownership SSA SIL at the end of the guaranteed optimization pipeline when we enter the Canonical SIL stage.

move_value

sil-instruction ::= 'move_value' '[lexical]'? sil-operand

%1 = move_value %0 : $@_moveOnly A

Performs a move of the operand, ending its lifetime. When ownership is enabled, it always takes in an @owned T and produces a new @owned @_moveOnly T.

  1. For trivial types, this is equivalent to just propagating through the trivial value.
  2. For reference types, this is equivalent to ending the lifetime of the operand, beginning a new lifetime for the result and setting the result to the value of the operand.
  3. For aggregates, the operation is equivalent to performing a move_value on each of its fields recursively.

After ownership is lowered, we leave in the move_value to provide a place for IRGenSIL to know to store a potentially new variable (in case the move was associated with a let binding).

NOTE: This instruction is used in an experimental feature called 'move only values'. A move_value instruction is an instruction that introduces (or injects) a type T into the move only value space.

The lexical attribute specifies that the value corresponds to a local variable in the Swift source.

drop_deinit

sil-instruction ::= 'drop_deinit' sil-operand

%1 = drop_deinit %0 : $T
// T must be a move-only type
// %1 is an @owned T
%3 = drop_deinit %2 : $*T
// T must be a move-only type
// %2 has type *T

This instruction is a marker for a following destroy instruction to suppress the call of the move-only type's deinitializer. The instruction accepts an object or address type. If its argument is an object type it takes in an @owned T and produces a new @owned T. If its argument is an address type, it's an identity projection.

The instruction is only valid in ownership SIL.

release_value

sil-instruction ::= 'release_value' sil-operand

release_value %0 : $A

Destroys a loadable value, by releasing any retainable pointers within it.

This is defined to be equivalent to storing the operand into a stack allocation and using 'destroy_addr' to destroy the object there.

For trivial types, this is a no-op. For reference types, this is equivalent to a strong_release. For @unowned types, this is equivalent to an unowned_release. In each of these cases, those are the preferred forms.

For aggregate types, especially enums, it is typically both easier and more efficient to reason about aggregate destroys than it is to reason about destroys of the subobjects.

release_value_addr

sil-instruction ::= 'release_value_addr' sil-operand

release_value_addr %0 : $*A

Destroys a loadable value inside given address, by releasing any retainable pointers within it.

unmanaged_release_value

sil-instruction ::= 'unmanaged_release_value' sil-value

unmanaged_release_value %0 : $A

This instruction has the same local semantics as release_value but:

  • Is valid in ownership qualified SIL.
  • Is not intended to be statically paired at compile time by the compiler.

The intention is that this instruction is used to implement unmanaged constructs.

destroy_value

sil-instruction ::= 'destroy_value' '[poison]'? sil-operand

destroy_value %0 : $A

Destroys a loadable value, by releasing any retainable pointers within it.

This is defined to be equivalent to storing the operand into a stack allocation and using 'destroy_addr' to destroy the object there.

For trivial types, this is a no-op. For reference types, this is equivalent to a strong_release. For @unowned types, this is equivalent to an unowned_release. In each of these cases, those are the preferred forms.

For aggregate types, especially enums, it is typically both easier and more efficient to reason about aggregate destroys than it is to reason about destroys of the subobjects.

autorelease_value

sil-instruction ::= 'autorelease_value' sil-operand

autorelease_value %0 : $A

TODO Complete this section.

tuple

sil-instruction ::= 'tuple' sil-tuple-elements
sil-tuple-elements ::= '(' (sil-operand (',' sil-operand)*)? ')'
sil-tuple-elements ::= sil-type '(' (sil-value (',' sil-value)*)? ')'

%1 = tuple (%a : $A, %b : $B, ...)
// $A, $B, etc. must be loadable non-address types
// %1 will be of the "simple" tuple type $(A, B, ...)

%1 = tuple $(a:A, b:B, ...) (%a, %b, ...)
// (a:A, b:B, ...) must be a loadable tuple type
// %1 will be of the type $(a:A, b:B, ...)

Creates a loadable tuple value by aggregating multiple loadable values.

If the destination type is a "simple" tuple type, that is, it has no keyword argument labels or variadic arguments, then the first notation can be used, which interleaves the element values and types. If keyword names or variadic fields are specified, then the second notation must be used, which spells out the tuple type before the fields.

tuple_extract

sil-instruction ::= 'tuple_extract' sil-operand ',' int-literal

%1 = tuple_extract %0 : $(T...), 123
// %0 must be of a loadable tuple type $(T...)
// %1 will be of the type of the selected element of %0

Extracts an element from a loadable tuple value.

tuple_element_addr

sil-instruction ::= 'tuple_element_addr' sil-operand ',' int-literal

%1 = tuple_element_addr %0 : $*(T...), 123
// %0 must of a $*(T...) address-of-tuple type
// %1 will be of address type $*U where U is the type of the 123rd
//   element of T

Given the address of a tuple in memory, derives the address of an element within that value.

tuple_pack_element_addr

sil-instruction ::= 'tuple_pack_element_addr' sil-value 'of' sil-operand 'as' sil-type

%addr = tuple_pack_element_addr %index of %tuple : $*(repeat each T) as $*@pack_element("01234567-89AB-CDEF-0123-000000000000") U
// %index must be of $Builtin.PackIndex type
// %tuple must be of address-of-tuple type
// %addr will be of the result type specified by the 'as' clause

Given the address of a tuple in memory, derives the address of a dynamic element within that value.

The induced pack type for the tuple operand is the indirect pack type corresponding to the types of the tuple elements and tuple element expansions, exactly as if the labels were removed and the parentheses were replaced with Pack{...`}`. For example, for the tuple type (repeat Optional<each T>, Float), the induced pack type is Pack{repeat Optional<each T>, Float}.

The pack index operand must be a pack indexing instruction. The result type (given by the as clause) must be structurally well-typed for the pack index and the induced pack type; see the structural type matching rules for pack indices.

destructure_tuple

sil-instruction ::= 'destructure_tuple' sil-operand

(%elt1, ..., %eltn) = destructure_tuple %0 : $(Elt1Ty, ..., EltNTy)
// %0 must be a tuple of type $(Elt1Ty, ..., EltNTy)
// %eltN must have the type $EltNTy

Given a tuple value, split the value into its constituent elements.

struct

sil-instruction ::= 'struct' sil-type '(' (sil-operand (',' sil-operand)*)? ')'

%1 = struct $S (%a : $A, %b : $B, ...)
// $S must be a loadable struct type
// $A, $B, ... must be the types of the physical 'var' fields of $S in order
// %1 will be of type $S

Creates a value of a loadable struct type by aggregating multiple loadable values.

struct_extract

sil-instruction ::= 'struct_extract' sil-operand ',' sil-decl-ref

%1 = struct_extract %0 : $S, #S.field
// %0 must be of a loadable struct type $S
// #S.field must be a physical 'var' field of $S
// %1 will be of the type of the selected field of %0

Extracts a physical field from a loadable struct value.

struct_element_addr

sil-instruction ::= 'struct_element_addr' sil-operand ',' sil-decl-ref

%1 = struct_element_addr %0 : $*S, #S.field
// %0 must be of a struct type $S
// #S.field must be a physical 'var' field of $S
// %1 will be the address of the selected field of %0

Given the address of a struct value in memory, derives the address of a physical field within the value.

destructure_struct

sil-instruction ::= 'destructure_struct' sil-operand

(%elt1, ..., %eltn) = destructure_struct %0 : $S
// %0 must be a struct of type $S
// %eltN must have the same type as the Nth field of $S

Given a struct, split the struct into its constituent fields.

object

sil-instruction ::= 'object' sil-type '(' (sil-operand (',' sil-operand)*)? ')'

object $T (%a : $A, %b : $B, ...)
// $T must be a non-generic or bound generic reference type
// The first operands must match the stored properties of T
// Optionally there may be more elements, which are tail-allocated to T

Constructs a statically initialized object. This instruction can only appear as final instruction in a global variable static initializer list.

ref_element_addr

sil-instruction ::= 'ref_element_addr' '[immutable]'? sil-operand ',' sil-decl-ref

%1 = ref_element_addr %0 : $C, #C.field
// %0 must be a value of class type $C
// #C.field must be a non-static physical field of $C
// %1 will be of type $*U where U is the type of the selected field
//   of C

Given an instance of a class, derives the address of a physical instance variable inside the instance. It is undefined behavior if the class value is null.

The immutable attribute specifies that all loads of the same instance variable from the same class reference operand are guaranteed to yield the same value.

ref_tail_addr

sil-instruction ::= 'ref_tail_addr' '[immutable]'? sil-operand ',' sil-type

%1 = ref_tail_addr %0 : $C, $E
// %0 must be a value of class type $C with tail-allocated elements $E
// %1 will be of type $*E

Given an instance of a class, which is created with tail-allocated array(s), derives the address of the first element of the first tail-allocated array. This instruction is used to project the first tail-allocated element from an object which is created by an alloc_ref with tail_elems. It is undefined behavior if the class instance does not have tail-allocated arrays or if the element-types do not match.

The immutable attribute specifies that all loads of the same instance variable from the same class reference operand are guaranteed to yield the same value.

Enums

These instructions construct and manipulate values of enum type. Loadable enum values are created with the enum instruction. Address-only enums require two-step initialization. First, if the case requires data, that data is stored into the enum at the address projected by init_enum_data_addr. This step is skipped for cases without data. Finally, the tag for the enum is injected with an inject_enum_addr instruction:

enum AddressOnlyEnum {
  case HasData(AddressOnlyType)
  case NoData
}

sil @init_with_data : $(AddressOnlyType) -> AddressOnlyEnum {
entry(%0 : $*AddressOnlyEnum, %1 : $*AddressOnlyType):
  // Store the data argument for the case.
  %2 = init_enum_data_addr %0 : $*AddressOnlyEnum, #AddressOnlyEnum.HasData!enumelt
  copy_addr [take] %1 to [init] %2 : $*AddressOnlyType
  // Inject the tag.
  inject_enum_addr %0 : $*AddressOnlyEnum, #AddressOnlyEnum.HasData!enumelt
  return
}

sil @init_without_data : $() -> AddressOnlyEnum {
  // No data. We only need to inject the tag.
  inject_enum_addr %0 : $*AddressOnlyEnum, #AddressOnlyEnum.NoData!enumelt
  return
}

Accessing the value of a loadable enum is inseparable from dispatching on its discriminator and is done with the switch_enum terminator:

enum Foo { case A(Int), B(String) }

sil @switch_foo : $(Foo) -> () {
entry(%foo : $Foo):
  switch_enum %foo : $Foo, case #Foo.A!enumelt: a_dest, case #Foo.B!enumelt: b_dest

a_dest(%a : $Int):
  /* use %a */

b_dest(%b : $String):
  /* use %b */
}

An address-only enum can be tested by branching on it using the switch_enum_addr terminator. Its value can then be taken by destructively projecting the enum value with unchecked_take_enum_data_addr:

enum Foo<T> { case A(T), B(String) }

sil @switch_foo : $<T> (Foo<T>) -> () {
entry(%foo : $*Foo<T>):
  switch_enum_addr %foo : $*Foo<T>, case #Foo.A!enumelt: a_dest, \
    case #Foo.B!enumelt: b_dest

a_dest:
  %a = unchecked_take_enum_data_addr %foo : $*Foo<T>, #Foo.A!enumelt
  /* use %a */

b_dest:
  %b = unchecked_take_enum_data_addr %foo : $*Foo<T>, #Foo.B!enumelt
  /* use %b */
}

Both switch_enum and switch_enum_addr must include a default case unless the enum can be exhaustively switched in the current function, i.e. when the compiler can be sure that it knows all possible present and future values of the enum in question. This is generally true for enums defined in Swift, but there are two exceptions: non-frozen enums declared in libraries compiled with the -enable-library-evolution flag, which may grow new cases in the future in an ABI-compatible way; and enums marked with the objc attribute, for which other bit patterns are permitted for compatibility with C. All enums imported from C are treated as "non-exhaustive" for the same reason, regardless of the presence or value of the enum_extensibility Clang attribute.

(See SE-0192 for more information about non-frozen enums.)

enum

sil-instruction ::= 'enum' sil-type ',' sil-decl-ref (',' sil-operand)?

%1 = enum $U, #U.EmptyCase!enumelt
%1 = enum $U, #U.DataCase!enumelt, %0 : $T
// $U must be an enum type
// #U.DataCase or #U.EmptyCase must be a case of enum $U
// If #U.Case has a data type $T, %0 must be a value of type $T
// If #U.Case has no data type, the operand must be omitted
// %1 will be of type $U

Creates a loadable enum value in the given case. If the case has a data type, the enum value will contain the operand value.

unchecked_enum_data

sil-instruction ::= 'unchecked_enum_data' sil-operand ',' sil-decl-ref

%1 = unchecked_enum_data %0 : $U, #U.DataCase!enumelt
// $U must be an enum type
// #U.DataCase must be a case of enum $U with data
// %1 will be of object type $T for the data type of case U.DataCase

Unsafely extracts the payload data for an enum case from an enum value. It is undefined behavior if the enum does not contain a value of the given case.

init_enum_data_addr

sil-instruction ::= 'init_enum_data_addr' sil-operand ',' sil-decl-ref

%1 = init_enum_data_addr %0 : $*U, #U.DataCase!enumelt
// $U must be an enum type
// #U.DataCase must be a case of enum $U with data
// %1 will be of address type $*T for the data type of case U.DataCase

Projects the address of the data for an enum case inside an enum. This does not modify the enum or check its value. It is intended to be used as part of the initialization sequence for an address-only enum. Storing to the init_enum_data_addr for a case followed by inject_enum_addr with that same case is guaranteed to result in a fully-initialized enum value of that case being stored. Loading from the init_enum_data_addr of an initialized enum value or injecting a mismatched case tag is undefined behavior.

The address is invalidated as soon as the operand enum is fully initialized by an inject_enum_addr.

inject_enum_addr

sil-instruction ::= 'inject_enum_addr' sil-operand ',' sil-decl-ref

inject_enum_addr %0 : $*U, #U.Case!enumelt
// $U must be an enum type
// #U.Case must be a case of enum $U
// %0 will be overlaid with the tag for #U.Case

Initializes the enum value referenced by the given address by overlaying the tag for the given case. If the case has no data, this instruction is sufficient to initialize the enum value. If the case has data, the data must be stored into the enum at the init_enum_data_addr address for the case before inject_enum_addr is applied. It is undefined behavior if inject_enum_addr is applied for a case with data to an uninitialized enum, or if inject_enum_addr is applied for a case with data when data for a mismatched case has been stored to the enum.

unchecked_take_enum_data_addr

sil-instruction ::= 'unchecked_take_enum_data_addr' sil-operand ',' sil-decl-ref

%1 = unchecked_take_enum_data_addr %0 : $*U, #U.DataCase!enumelt
// $U must be an enum type
// #U.DataCase must be a case of enum $U with data
// %1 will be of address type $*T for the data type of case U.DataCase

Invalidates an enum value, and takes the address of the payload for the given enum case in-place in memory. The referenced enum value is no longer valid, but the payload value referenced by the result address is valid and must be destroyed. It is undefined behavior if the referenced enum does not contain a value of the given case. The result shares memory with the original enum value; the enum memory cannot be reinitialized as an enum until the payload has also been invalidated.

(1.0 only)

For the first payloaded case of an enum, unchecked_take_enum_data_addr is guaranteed to have no side effects; the enum value will not be invalidated.

select_enum

sil-instruction ::= 'select_enum' sil-operand sil-select-case*
                    (',' 'default' sil-value)?
                    ':' sil-type

%n = select_enum %0 : $U,      \
  case #U.Case1!enumelt: %1,           \
  case #U.Case2!enumelt: %2, /* ... */ \
  default %3 : $T

// $U must be an enum type
// #U.Case1, Case2, etc. must be cases of enum $U
// %1, %2, %3, etc. must have type $T
// %n has type $T

Selects one of the "case" or "default" operands based on the case of an enum value. This is equivalent to a trivial switch_enum branch sequence:

entry:
  switch_enum %0 : $U,            \
    case #U.Case1!enumelt: bb1,           \
    case #U.Case2!enumelt: bb2, /* ... */ \
    default bb_default
bb1:
  br cont(%1 : $T) // value for #U.Case1
bb2:
  br cont(%2 : $T) // value for #U.Case2
bb_default:
  br cont(%3 : $T) // value for default
cont(%n : $T):
  // use argument %n

but turns the control flow dependency into a data flow dependency. For address-only enums, select_enum_addr offers the same functionality for an indirectly referenced enum value in memory.

Like switch_enum, select_enum must have a default case unless the enum can be exhaustively switched in the current function.

select_enum_addr

sil-instruction ::= 'select_enum_addr' sil-operand sil-select-case*
                    (',' 'default' sil-value)?
                    ':' sil-type

%n = select_enum_addr %0 : $*U,      \
  case #U.Case1!enumelt: %1,           \
  case #U.Case2!enumelt: %2, /* ... */ \
  default %3 : $T

// %0 must be the address of an enum type $*U
// #U.Case1, Case2, etc. must be cases of enum $U
// %1, %2, %3, etc. must have type $T
// %n has type $T

Selects one of the "case" or "default" operands based on the case of the referenced enum value. This is the address-only counterpart to select_enum.

Like switch_enum_addr, select_enum_addr must have a default case unless the enum can be exhaustively switched in the current function.

Protocol and Protocol Composition Types

These instructions create and manipulate values of protocol and protocol composition type. From SIL's perspective, protocol and protocol composition types consist of an existential container, which is a generic container for a value of unknown runtime type, referred to as an "existential type" in type theory. The existential container consists of a reference to the witness table(s) for the protocol(s) referred to by the protocol type and a reference to the underlying concrete value, which may be either stored in-line inside the existential container for small values or allocated separately into a buffer owned and managed by the existential container for larger values.

Depending on the constraints applied to an existential type, an existential container may use one of several representations:

  • Opaque existential containers: If none of the protocols in a protocol type are class protocols, then the existential container for that type is address-only and referred to in the implementation as an opaque existential container. The value semantics of the existential container propagate to the contained concrete value. Applying copy_addr to an opaque existential container copies the contained concrete value, deallocating or reallocating the destination container's owned buffer if necessary. Applying destroy_addr to an opaque existential container destroys the concrete value and deallocates any buffers owned by the existential container. The following instructions manipulate opaque existential containers:
  • Opaque existential containers loadable types: In the SIL Opaque Values mode of operation, we take an opaque value as-is. Said value might be replaced with one of the _addr instructions above before IR generation. The following instructions manipulate "loadable" opaque existential containers:
  • Class existential containers: If a protocol type is constrained by one or more class protocols, then the existential container for that type is loadable and referred to in the implementation as a class existential container. Class existential containers have reference semantics and can be retain-ed and release-d. The following instructions manipulate class existential containers:
  • Metatype existential containers: Existential metatypes use a container consisting of the type metadata for the conforming type along with the protocol conformances. Metatype existential containers are trivial types. The following instructions manipulate metatype existential containers:
  • Boxed existential containers: The standard library Error protocol uses a size-optimized reference-counted container, which indirectly stores the conforming value. Boxed existential containers can be retain-ed and release-d. The following instructions manipulate boxed existential containers:

Some existential types may additionally support specialized representations when they contain certain known concrete types. For example, when Objective-C interop is available, the Error protocol existential supports a class existential container representation for NSError objects, so it can be initialized from one using init_existential_ref instead of the more expensive alloc_existential_box:

bb(%nserror: $NSError):
  // The slow general way to form an Error, allocating a box and
  // storing to its value buffer:
  %error1 = alloc_existential_box $Error, $NSError
  %addr = project_existential_box $NSError in %error1 : $Error
  strong_retain %nserror: $NSError
  store %nserror to %addr : $NSError

  // The fast path supported for NSError:
  strong_retain %nserror: $NSError
  %error2 = init_existential_ref %nserror: $NSError, $Error

init_existential_addr

sil-instruction ::= 'init_existential_addr' sil-operand ',' sil-type

%1 = init_existential_addr %0 : $*P, $T
// %0 must be of a $*P address type for non-class protocol or protocol
//   composition type P
// $T must be an AST type that fulfills protocol(s) P
// %1 will be of type $*T', where T' is the maximally abstract lowering
//    of type T

Partially initializes the memory referenced by %0 with an existential container prepared to contain a value of type $T. The result of the instruction is an address referencing the storage for the contained value, which remains uninitialized. The contained value must be store-d or copy_addr-ed to in order for the existential value to be fully initialized. If the existential container needs to be destroyed while the contained value is uninitialized, deinit_existential_addr must be used to do so. A fully initialized existential container can be destroyed with destroy_addr as usual. It is undefined behavior to destroy_addr a partially-initialized existential container.

init_existential_value

sil-instruction ::= 'init_existential_value' sil-operand ',' sil-type ','
                                             sil-type

%1 = init_existential_value %0 : $L, $C, $P
// %0 must be of loadable type $L, lowered from AST type $C, conforming to
//    protocol(s) $P
// %1 will be of type $P

Loadable version of the above: Inits-up the existential container prepared to contain a value of type $P.

deinit_existential_addr

sil-instruction ::= 'deinit_existential_addr' sil-operand

deinit_existential_addr %0 : $*P
// %0 must be of a $*P address type for non-class protocol or protocol
// composition type P

Undoes the partial initialization performed by init_existential_addr. deinit_existential_addr is only valid for existential containers that have been partially initialized by init_existential_addr but haven't had their contained value initialized. A fully initialized existential must be destroyed with destroy_addr.

deinit_existential_value

sil-instruction ::= 'deinit_existential_value' sil-operand

deinit_existential_value %0 : $P
// %0 must be of a $P opaque type for non-class protocol or protocol
// composition type P

Undoes the partial initialization performed by init_existential_value. deinit_existential_value is only valid for existential containers that have been partially initialized by init_existential_value but haven't had their contained value initialized. A fully initialized existential must be destroyed with destroy_value.

open_existential_addr

sil-instruction ::= 'open_existential_addr' sil-allowed-access sil-operand 'to' sil-type
sil-allowed-access ::= 'immutable_access'
sil-allowed-access ::= 'mutable_access'

%1 = open_existential_addr immutable_access %0 : $*P to $*@opened P
// %0 must be of a $*P type for non-class protocol or protocol composition
//   type P
// $*@opened P must be a unique archetype that refers to an opened
// existential type P.
// %1 will be of type $*@opened P

Obtains the address of the concrete value inside the existential container referenced by %0. The protocol conformances associated with this existential container are associated directly with the archetype $*@opened P. This pointer can be used with any operation on archetypes, such as witness_method assuming this operation obeys the access constraint: The returned address can either allow mutable_access or immutable_access. Users of the returned address may only consume (e.g destroy_addr or copy_addr [take]) or mutate the value at the address if they have mutable_access.

open_existential_value

sil-instruction ::= 'open_existential_value' sil-operand 'to' sil-type

%1 = open_existential_value %0 : $P to $@opened P
// %0 must be of a $P type for non-class protocol or protocol composition
//   type P
// $@opened P must be a unique archetype that refers to an opened
// existential type P.
// %1 will be of type $@opened P

Loadable version of the above: Opens-up the existential container associated with %0. The protocol conformances associated with this existential container are associated directly with the archetype $@opened P.

init_existential_ref

sil-instruction ::= 'init_existential_ref' sil-operand ':' sil-type ','
                                           sil-type

%1 = init_existential_ref %0 : $C' : $C, $P
// %0 must be of class type $C', lowered from AST type $C, conforming to
//    protocol(s) $P
// $P must be a class protocol or protocol composition type
// %1 will be of type $P

Creates a class existential container of type $P containing a reference to the class instance %0.

open_existential_ref

sil-instruction ::= 'open_existential_ref' sil-operand 'to' sil-type

%1 = open_existential_ref %0 : $P to $@opened P
// %0 must be of a $P type for a class protocol or protocol composition
// $@opened P must be a unique archetype that refers to an opened
//   existential type P
// %1 will be of type $@opened P

Extracts the class instance reference from a class existential container. The protocol conformances associated with this existential container are associated directly with the archetype @opened P. This pointer can be used with any operation on archetypes, such as witness_method. When the operand is of metatype type, the result will be the metatype of the opened archetype.

init_existential_metatype

sil-instruction ::= 'init_existential_metatype' sil-operand ',' sil-type

%1 = init_existential_metatype $0 : $@<rep> T.Type, $@<rep> P.Type
// %0 must be of a metatype type $@<rep> T.Type where T: P
// %@<rep> P.Type must be the existential metatype of a protocol or protocol
//    composition, with the same metatype representation <rep>
// %1 will be of type $@<rep> P.Type

Creates a metatype existential container of type $P.Type containing the conforming metatype of $T.

open_existential_metatype

sil-instruction ::= 'open_existential_metatype' sil-operand 'to' sil-type

%1 = open_existential_metatype %0 : $@<rep> P.Type to $@<rep> (@opened P).Type
// %0 must be of a $P.Type existential metatype for a protocol or protocol
//    composition
// $@<rep> (@opened P).Type must be the metatype of a unique archetype that
//   refers to an opened existential type P, with the same metatype
//   representation <rep>
// %1 will be of type $@<rep> (@opened P).Type

Extracts the metatype from an existential metatype. The protocol conformances associated with this existential container are associated directly with the archetype @opened P.

alloc_existential_box

sil-instruction ::= 'alloc_existential_box' sil-type ',' sil-type

%1 = alloc_existential_box $P, $T
// $P must be a protocol or protocol composition type with boxed
//   representation
// $T must be an AST type that conforms to P
// %1 will be of type $P

Allocates a boxed existential container of type $P with space to hold a value of type $T'. The box is not fully initialized until a valid value has been stored into the box. If the box must be deallocated before it is fully initialized, dealloc_existential_box must be used. A fully initialized box can be retain-ed and release-d like any reference-counted type. The project_existential_box instruction is used to retrieve the address of the value inside the container.

project_existential_box

sil-instruction ::= 'project_existential_box' sil-type 'in' sil-operand

%1 = project_existential_box $T in %0 : $P
// %0 must be a value of boxed protocol or protocol composition type $P
// $T must be the most abstracted lowering of the AST type for which the box
// was allocated
// %1 will be of type $*T

Projects the address of the value inside a boxed existential container. The address is dependent on the lifetime of the owner reference %0. It is undefined behavior if the concrete type $T is not the same type for which the box was allocated with alloc_existential_box.

open_existential_box

sil-instruction ::= 'open_existential_box' sil-operand 'to' sil-type

%1 = open_existential_box %0 : $P to $*@opened P
// %0 must be a value of boxed protocol or protocol composition type $P
// %@opened P must be the address type of a unique archetype that refers to
///   an opened existential type P
// %1 will be of type $*@opened P

Projects the address of the value inside a boxed existential container, and uses the enclosed type and protocol conformance metadata to bind the opened archetype $@opened P. The result address is dependent on both the owning box and the enclosing function; in order to "open" a boxed existential that has directly adopted a class reference, temporary scratch space may need to have been allocated.

open_existential_box_value

sil-instruction ::= 'open_existential_box_value' sil-operand 'to' sil-type

%1 = open_existential_box_value %0 : $P to $@opened P
// %0 must be a value of boxed protocol or protocol composition type $P
// %@opened P must be a unique archetype that refers to an opened
//   existential type P
// %1 will be of type $@opened P

Projects the value inside a boxed existential container, and uses the enclosed type and protocol conformance metadata to bind the opened archetype $@opened P.

dealloc_existential_box

sil-instruction ::= 'dealloc_existential_box' sil-operand, sil-type

dealloc_existential_box %0 : $P, $T
// %0 must be an uninitialized box of boxed existential container type $P
// $T must be the AST type for which the box was allocated

Deallocates a boxed existential container. The value inside the existential buffer is not destroyed; either the box must be uninitialized, or the value must have been projected out and destroyed beforehand. It is undefined behavior if the concrete type $T is not the same type for which the box was allocated with alloc_existential_box.

Blocks

project_block_storage

sil-instruction ::= 'project_block_storage' sil-operand ':' sil-type

init_block_storage_header

TODO Fill this in. The printing of this instruction looks incomplete on trunk currently.

Pack Indexing

These instructions are collectively called the pack indexing instructions. Each of them produces a single value of type Builtin.PackIndex. Instructions that consume pack indices generally provide a projected element type which is required to be structurally well-typed for the given pack index and the actual pack type they index into. This rule depends on the exact pack indexing instruction used and is described in a section above.

All pack indexing instructions carry an indexed pack type, which is a formal type that must be a pack type. Pack indexing instructions can be used to index into any pack with the same shape as the indexed pack type. The components of the actual indexed pack do not need to be exactly the same as the components of the indexing instruction's indexed pack type as long as they contain expansions in the same places and those expansions expand pack parameters with the same shape.

scalar_pack_index

sil-instruction ::= 'scalar_pack_index' int-literal 'of' sil-type

Produce the dynamic pack index of a scalar (non-pack-expansion) component of a pack. The type operand is the indexed pack type. The integer operand is an index into the components of this pack type; it must be in range and resolve to a component that is not a pack expansion.

Substitution must adjust the component index appropriately so that it still refers to the same component. For example, if the pack type is Pack{repeat each T, Int}, and substitution replaces T with Pack{Float, repeat each U}, a component index of 1 must be adjusted to 2 so that it still refers to the Int element.

pack_pack_index

sil-instruction ::= 'pack_pack_index' int-literal, sil-value 'of' sil-type

Produce the dynamic pack index of an element of a slice of a pack. The type operand is the indexed pack type. The integer operand is an index into the components of this pack type and must be in range. The value operand is the index in the pack slice and must be another pack indexing instruction. The pack slice starts at the given index and extends for a number of components equal to the number of components in the indexed pack type of the operand. The pack type induced from the indexed pack type by this slice must have the same shape as the indexed pack type of the operand.

Substitution must adjust the component index appropriately so that it still refers to the same component. For example, if the pack type is Pack{repeat each T, Int}, and substitution replaces T with Pack{Float, repeat each U}, a component index of 1 must be adjusted to 2 so that the slice will continue to begin at the Int element.

Note how, in the example above, the slice does not contain any pack expansions. (It is either empty or the singleton pack Pack{Int}.) This is not typically how this instruction is used but can easily occur after inlining or other type substitution.

dynamic_pack_index

sil-instruction ::= 'dynamic_pack_index' sil-value 'of' sil-type

Produce the dynamic pack index of an unknown element of a pack. The type operand is the indexed pack type. The value operand is a dynamic index into the dynamic elements of the pack and must have type Builtin.Word. The instruction has undefined behavior if the index is not in range for the pack.

Variadic Generics

pack_length

sil-instruction ::= 'pack_length' sil-type

Produce the dynamic length of the given pack, which must be a formal pack type. The value of the instruction has type Builtin.Word.

open_pack_element

sil-instruction ::= 'open_pack_element' sil-value 'of' generic-parameter-list+ 'at' sil-apply-substitution-list ',' 'shape' sil-type ',' 'uuid' string-literal

Binds one or more opened pack element archetypes in the local type environment.

The generic signature is the generalization signature of the pack elements. This signature need not be related in any way to the generic signature (if any) of the enclosing SIL function.

The shape type operand is resolved in the context of the generalization signature. It must name a pack parameter. Archetypes will be bound for all pack parameters with the same shape as this parameter.

The uuid operand must be an RFC 4122 UUID string, which is composed of 32 hex digits separated by hyphens in the pattern 8-4-4-4-12. There must not be any other open_pack_element instruction with this UUID in the SIL function. Opened pack element archetypes are identified by this UUID and are different from any other opened pack element archetypes in the function, even if the operands otherwise match exactly.

The value operand is the pack index and must be the result of a pack indexing instruction.

The substitution list matches the generalization signature and provides contextual bindings for all of the type information there. As usual, the substitutions for any pack parameters must be pack types. For pack parameters with the same shape as the shape operand, these pack substitutions must have the same shape as the indexed pack type of the pack index operand (and therefore the same shape as each other).

The cost of this instruction is proportionate to the sum of the number of pack parameters in the generalization signature with the same shape as the shape type and the number of protocol conformance requirements the generalization signature imposes on those parameters and their associated types. If any of this information is not required for the correct execution of the SIL function, simplifying the generalization signature used by the``open_pack_element`` can be a significant optimization.

pack_element_get

sil-instruction ::= 'pack_element_get' sil-value 'of' sil-operand 'as' sil-type

Extracts the value previously stored in a pack at a particular index. If the pack element is uninitialized, this has undefined behavior.

Ownership is unclear for direct packs.

The first operand is the pack index and must be a pack indexing instruction. The second operand is the pack and must be the address of a pack value. The type operand is the projected element type of the pack element and must be structurally well-typed for the given index and pack type; see the structural type matching rules for pack indices.

pack_element_set

sil-instruction ::= 'pack_element_set' sil-operand 'into' sil-value 'of' sil-operand

Places a value in a pack at a particular index.

Ownership is unclear for direct packs.

The first operand is the new element value. The second operand is the pack index and must be a pack indexing instruction. The third operand is the pack and must be the address of a pack value. The type of the element value operand is the projected element type of the pack element and must be structurally well-typed for the given index and pack type; see the structural type matching rules for pack indices.

Unchecked Conversions

These instructions implement type conversions which are not checked. These are either user-level conversions that are always safe and do not need to be checked, or implementation detail conversions that are unchecked for performance or flexibility.

upcast

sil-instruction ::= 'upcast' sil-operand 'to' sil-type

%1 = upcast %0 : $D to $B
// $D and $B must be class types or metatypes, with B a superclass of D
// %1 will have type $B

Represents a conversion from a derived class instance or metatype to a superclass, or from a base-class-constrained archetype to its base class.

address_to_pointer

sil-instruction ::= 'address_to_pointer' ('[' 'stack_protection' ']')? sil-operand 'to' sil-type

%1 = address_to_pointer %0 : $*T to $Builtin.RawPointer
// %0 must be of an address type $*T
// %1 will be of type Builtin.RawPointer

Creates a Builtin.RawPointer value corresponding to the address %0. Converting the result pointer back to an address of the same type will give an address equivalent to %0. It is undefined behavior to cast the RawPointer to any address type other than its original address type or any layout compatible types.

The stack_protection flag indicates that stack protection is done for the pointer origin.

pointer_to_address

sil-instruction ::= 'pointer_to_address' sil-operand 'to' ('[' 'strict' ']')? ('[' 'invariant' ']')? ('[' 'alignment' '=' alignment ']')? sil-type
alignment ::= [0-9]+

%1 = pointer_to_address %0 : $Builtin.RawPointer to [strict] $*T
// %1 will be of type $*T

Creates an address value corresponding to the Builtin.RawPointer value %0. Converting a RawPointer back to an address of the same type as its originating address_to_pointer instruction gives back an equivalent address. It is undefined behavior to cast the RawPointer back to any type other than its original address type or layout compatible types. It is also undefined behavior to cast a RawPointer from a heap object to any address type.

The strict flag indicates whether the returned address adheres to strict aliasing. If true, then the type of each memory access dependent on this address must be consistent with the memory's bound type. A memory access from an address that is not strict cannot have its address substituted with a strict address, even if other nearby memory accesses at the same location are strict.

The invariant flag is set if loading from the returned address always produces the same value.

The alignment integer value specifies the byte alignment of the address. alignment=0 is the default, indicating the natural alignment of T.

unchecked_ref_cast

sil-instruction ::= 'unchecked_ref_cast' sil-operand 'to' sil-type

%1 = unchecked_ref_cast %0 : $A to $B
// %0 must be an object of type $A
// $A must be a type with retainable pointer representation
// %1 will be of type $B
// $B must be a type with retainable pointer representation

Converts a heap object reference to another heap object reference type. This conversion is unchecked, and it is undefined behavior if the destination type is not a valid type for the heap object. The heap object reference on either side of the cast may be a class existential, and may be wrapped in one level of Optional.

unchecked_ref_cast_addr

sil-instruction ::= 'unchecked_ref_cast_addr'
                    sil-type 'in' sil-operand 'to'
                    sil-type 'in' sil-operand

unchecked_ref_cast_addr $A in %0 : $*A to $B in %1 : $*B
// %0 must be the address of an object of type $A
// $A must be a type with retainable pointer representation
// %1 must be the address of storage for an object of type $B
// $B must be a retainable pointer representation

Loads a heap object reference from an address and stores it at the address of another uninitialized heap object reference. The loaded reference is always taken, and the stored reference is initialized. This conversion is unchecked, and it is undefined behavior if the destination type is not a valid type for the heap object. The heap object reference on either side of the cast may be a class existential, and may be wrapped in one level of Optional.

unchecked_addr_cast

sil-instruction ::= 'unchecked_addr_cast' sil-operand 'to' sil-type

%1 = unchecked_addr_cast %0 : $*A to $*B
// %0 must be an address
// %1 will be of type $*B

Converts an address to a different address type. Using the resulting address is undefined unless B is layout compatible with A. The layout of B may be smaller than that of A as long as the lower order bytes have identical layout.

unchecked_trivial_bit_cast

sil-instruction ::= 'unchecked_trivial_bit_cast' sil-operand 'to' sil-type

%1 = unchecked_trivial_bit_cast %0 : $Builtin.NativeObject to $Builtin.Word
// %0 must be an object.
// %1 must be an object with trivial type.

Bitcasts an object of type A to be of same sized or smaller type B with the constraint that B must be trivial. This can be used for bitcasting among trivial types, but more importantly is a one way bitcast from non-trivial types to trivial types.

unchecked_bitwise_cast

sil-instruction ::= 'unchecked_bitwise_cast' sil-operand 'to' sil-type

%1 = unchecked_bitwise_cast %0 : $A to $B

Bitwise copies an object of type A into a new object of type B of the same size or smaller.

unchecked_value_cast

sil-instruction ::= 'unchecked_value_cast' sil-operand 'to' sil-type

%1 = unchecked_value_cast %0 : $A to $B

Bitwise copies an object of type A into a new layout-compatible object of type B of the same size.

This instruction is assumed to forward a fixed ownership (set upon its construction) and lowers to 'unchecked_bitwise_cast' in non-ossa code. This causes the cast to lose its guarantee of layout-compatibility.

unchecked_ownership_conversion

sil-instruction ::= 'unchecked_ownership_conversion' sil-operand ',' sil-value-ownership-kind 'to' sil-value-ownership-kind

%1 = unchecked_ownership_conversion %0 : $A, @guaranteed to @owned

Converts its operand to an identical value of the same type but with different ownership without performing any semantic operations normally required by for ownership conversion.

This is used in Objective-C compatible destructors to convert a guaranteed parameter to an owned parameter without performing a semantic copy.

The resulting value must meet the usual ownership requirements; for example, a trivial type must have '.none' ownership.

ref_to_raw_pointer

sil-instruction ::= 'ref_to_raw_pointer' sil-operand 'to' sil-type

%1 = ref_to_raw_pointer %0 : $C to $Builtin.RawPointer
// $C must be a class type, or Builtin.NativeObject, or AnyObject
// %1 will be of type $Builtin.RawPointer

Converts a heap object reference to a Builtin.RawPointer. The RawPointer result can be cast back to the originating class type but does not have ownership semantics. It is undefined behavior to cast a RawPointer from a heap object reference to an address using pointer_to_address.

raw_pointer_to_ref

sil-instruction ::= 'raw_pointer_to_ref' sil-operand 'to' sil-type

%1 = raw_pointer_to_ref %0 : $Builtin.RawPointer to $C
// $C must be a class type, or Builtin.NativeObject, or AnyObject
// %1 will be of type $C

Converts a Builtin.RawPointer back to a heap object reference. Casting a heap object reference to Builtin.RawPointer back to the same type gives an equivalent heap object reference (though the raw pointer has no ownership semantics for the object on its own). It is undefined behavior to cast a RawPointer to a type unrelated to the dynamic type of the heap object. It is also undefined behavior to cast a RawPointer from an address to any heap object type.

ref_to_unowned

sil-instruction ::= 'ref_to_unowned' sil-operand

%1 = unowned_to_ref %0 : T
// $T must be a reference type
// %1 will have type $@unowned T

Adds the @unowned qualifier to the type of a reference to a heap object. No runtime effect.

unowned_to_ref

sil-instruction ::= 'unowned_to_ref' sil-operand

%1 = unowned_to_ref %0 : $@unowned T
// $T must be a reference type
// %1 will have type $T

Strips the @unowned qualifier off the type of a reference to a heap object. No runtime effect.

ref_to_unmanaged

TODO

unmanaged_to_ref

TODO

convert_function

sil-instruction ::= 'convert_function' sil-operand 'to'
                    ('[' 'without_actually_escaping' ']')?
                    sil-type

%1 = convert_function %0 : $T -> U to $T' -> U'
// %0 must be of a function type $T -> U ABI-compatible with $T' -> U'
//   (see below)
// %1 will be of type $T' -> U'

Performs a conversion of the function %0 to type T, which must be ABI- compatible with the type of %0. Function types are ABI-compatible if their input and result types are tuple types that, after destructuring, differ only in the following ways:

  • Corresponding tuple elements may add, remove, or change keyword names. (a:Int, b:Float, UnicodeScalar) -> () and (x:Int, Float, z:UnicodeScalar) -> () are ABI compatible.
  • A class tuple element of the destination type may be a superclass or subclass of the source type's corresponding tuple element.

The function types may also differ in attributes, except that the convention attribute cannot be changed and the @noescape attribute must not change for functions with context.

A convert_function cannot be used to change a thick type's @noescape attribute (@noescape function types with context are not ABI compatible with escaping function types with context) -- however, thin function types with and without @noescape are ABI compatible because they have no context. To convert from an escaping to a @noescape thick function type use convert_escape_to_noescape.

With the without_actually_escaping attribute, the convert_function may be used to convert a non-escaping closure into an escaping function type. This attribute must be present whenever the closure operand has an unboxed capture (via @inout_aliasable) and the resulting function type is escaping. (This only happens as a result of withoutActuallyEscaping()). If the attribute is present then the resulting function type must be escaping, but the operand's function type may or may not be @noescape. Note that a non-escaping closure may have unboxed captured even though its SIL function type is "escaping".

convert_escape_to_noescape

sil-instruction ::= 'convert_escape_to_noescape' sil-operand 'to' sil-type
%1 = convert_escape_to_noescape %0 : $T -> U to $@noescape T' -> U'
// %0 must be of a function type $T -> U ABI-compatible with $T' -> U'
//   (see convert_function)
// %1 will be of the trivial type $@noescape T -> U

Converts an escaping (non-trivial) function type to a @noescape trivial function type. Something must guarantee the lifetime of the input %0 for the duration of the use %1.

A convert_escape_to_noescape [not_guaranteed] %opd indicates that the lifetime of its operand was not guaranteed by SILGen and a mandatory pass must be run to ensure the lifetime of %opd` for the conversion's uses.

A convert_escape_to_noescape [escaped] indicates that the result was passed to a function (materializeForSet) which escapes the closure in a way not expressed by the convert's users. The mandatory pass must ensure the lifetime in a conservative way.

classify_bridge_object

sil-instruction ::= 'classify_bridge_object' sil-operand

%1 = classify_bridge_object %0 : $Builtin.BridgeObject
// %1 will be of type (Builtin.Int1, Builtin.Int1)

Decodes the bit representation of the specified Builtin.BridgeObject value, returning two bits: the first indicates whether the object is an Objective-C object, the second indicates whether it is an Objective-C tagged pointer value.

value_to_bridge_object

sil-instruction ::= 'value_to_bridge_object' sil-operand

%1 = value_to_bridge_object %0 : $T
// %1 will be of type Builtin.BridgeObject

Sets the BridgeObject to a tagged pointer representation holding its operands by tagging and shifting the operand if needed:

value_to_bridge_object %x ===
(x << _swift_abi_ObjCReservedLowBits) | _swift_BridgeObject_TaggedPointerBits

%x thus must not be using any high bits shifted away or the tag bits post-shift. ARC operations on such tagged values are NOPs.

ref_to_bridge_object

sil-instruction ::= 'ref_to_bridge_object' sil-operand, sil-operand

%2 = ref_to_bridge_object %0 : $C, %1 : $Builtin.Word
// %1 must be of reference type $C
// %2 will be of type Builtin.BridgeObject

Creates a Builtin.BridgeObject that references %0, with spare bits in the pointer representation populated by bitwise-OR-ing in the value of %1. It is undefined behavior if this bitwise OR operation affects the reference identity of %0; in other words, after the following instruction sequence:

%b = ref_to_bridge_object %r : $C, %w : $Builtin.Word
%r2 = bridge_object_to_ref %b : $Builtin.BridgeObject to $C

%r and %r2 must be equivalent. In particular, it is assumed that retaining or releasing the BridgeObject is equivalent to retaining or releasing the original reference, and that the above ref_to_bridge_object / bridge_object_to_ref round-trip can be folded away to a no-op.

On platforms with ObjC interop, there is additionally a platform-specific bit in the pointer representation of a BridgeObject that is reserved to indicate whether the referenced object has native Swift refcounting. It is undefined behavior to set this bit when the first operand references an Objective-C object.

bridge_object_to_ref

sil-instruction ::= 'bridge_object_to_ref' sil-operand 'to' sil-type

%1 = bridge_object_to_ref %0 : $Builtin.BridgeObject to $C
// $C must be a reference type
// %1 will be of type $C

Extracts the object reference from a Builtin.BridgeObject, masking out any spare bits.

bridge_object_to_word

sil-instruction ::= 'bridge_object_to_word' sil-operand 'to' sil-type

%1 = bridge_object_to_word %0 : $Builtin.BridgeObject to $Builtin.Word
// %1 will be of type $Builtin.Word

Provides the bit pattern of a Builtin.BridgeObject as an integer.

thin_to_thick_function

sil-instruction ::= 'thin_to_thick_function' sil-operand 'to' sil-type

%1 = thin_to_thick_function %0 : $@convention(thin) T -> U to $T -> U
// %0 must be of a thin function type $@convention(thin) T -> U
// The destination type must be the corresponding thick function type
// %1 will be of type $T -> U

Converts a thin function value, that is, a bare function pointer with no context information, into a thick function value with ignored context. Applying the resulting thick function value is equivalent to applying the original thin value. The thin_to_thick_function conversion may be eliminated if the context is proven not to be needed.

thick_to_objc_metatype

sil-instruction ::= 'thick_to_objc_metatype' sil-operand 'to' sil-type

%1 = thick_to_objc_metatype %0 : $@thick T.Type to $@objc_metatype T.Type
// %0 must be of a thick metatype type $@thick T.Type
// The destination type must be the corresponding Objective-C metatype type
// %1 will be of type $@objc_metatype T.Type

Converts a thick metatype to an Objective-C class metatype. T must be of class, class protocol, or class protocol composition type.

objc_to_thick_metatype

sil-instruction ::= 'objc_to_thick_metatype' sil-operand 'to' sil-type

%1 = objc_to_thick_metatype %0 : $@objc_metatype T.Type to $@thick T.Type
// %0 must be of an Objective-C metatype type $@objc_metatype T.Type
// The destination type must be the corresponding thick metatype type
// %1 will be of type $@thick T.Type

Converts an Objective-C class metatype to a thick metatype. T must be of class, class protocol, or class protocol composition type.

objc_metatype_to_object

TODO

objc_existential_metatype_to_object

TODO

Checked Conversions

Some user-level cast operations can fail and thus require runtime checking.

The unconditional_checked_cast_addr and unconditional_checked_cast instructions performs an unconditional checked cast; it is a runtime failure if the cast fails. The checked_cast_addr_br and checked_cast_br terminator instruction performs a conditional checked cast; it branches to one of two destinations based on whether the cast succeeds or not.

unconditional_checked_cast

sil-instruction ::= 'unconditional_checked_cast' sil-operand 'to' sil-type

%1 = unconditional_checked_cast %0 : $A to $B
%1 = unconditional_checked_cast %0 : $*A to $*B
// $A and $B must be both objects or both addresses
// %1 will be of type $B or $*B

Performs a checked scalar conversion, causing a runtime failure if the conversion fails. Casts that require changing representation or ownership are unsupported.

unconditional_checked_cast_addr

sil-instruction ::= 'unconditional_checked_cast_addr'
                     sil-type 'in' sil-operand 'to'
                     sil-type 'in' sil-operand

unconditional_checked_cast_addr $A in %0 : $*@thick A to $B in %1 : $*@thick B
// $A and $B must be both addresses
// %1 will be of type $*B
// $A is destroyed during the conversion. There is no implicit copy.

Performs a checked indirect conversion, causing a runtime failure if the conversion fails.

Runtime Failures

cond_fail

sil-instruction ::= 'cond_fail' sil-operand, string-literal

cond_fail %0 : $Builtin.Int1, "failure reason"
// %0 must be of type $Builtin.Int1

This instruction produces a runtime failure if the operand is one. Execution proceeds normally if the operand is zero. The second operand is a static failure message, which is displayed by the debugger in case the failure is triggered.

Terminators

These instructions terminate a basic block. Every basic block must end with a terminator. Terminators may only appear as the final instruction of a basic block.

unreachable

sil-terminator ::= 'unreachable'

unreachable

Indicates that control flow must not reach the end of the current basic block. It is a dataflow error if an unreachable terminator is reachable from the entry point of a function and is not immediately preceded by an apply of a no-return function.

return

sil-terminator ::= 'return' sil-operand

return %0 : $T
// $T must be the return type of the current function

Exits the current function and returns control to the calling function. If the current function was invoked with an apply instruction, the result of that function will be the operand of this return instruction. If the current function was invoked with a try_apply instruction, control resumes at the normal destination, and the value of the basic block argument will be the operand of this return instruction.

If the current function is a yield_once coroutine, there must not be a path from the entry block to a return which does not pass through a yield instruction. This rule does not apply in the raw SIL stage.

return does not retain or release its operand or any other values.

A function must not contain more than one return instruction.

throw

sil-terminator ::= 'throw' sil-operand

throw %0 : $T
// $T must be the error result type of the current function

Exits the current function and returns control to the calling function. The current function must have an error result, and so the function must have been invoked with a try_apply instruction. Control will resume in the error destination of that instruction, and the basic block argument will be the operand of the throw.

throw does not retain or release its operand or any other values.

A function must not contain more than one throw instruction.

yield

sil-terminator ::= 'yield' sil-yield-values
                     ',' 'resume' sil-identifier
                     ',' 'unwind' sil-identifier
sil-yield-values ::= sil-operand
sil-yield-values ::= '(' (sil-operand (',' sil-operand)*)? ')'

Temporarily suspends the current function and provides the given values to the calling function. The current function must be a coroutine, and the yield values must match the yield types of the coroutine. If the calling function resumes the coroutine normally, control passes to the resume destination. If the calling function aborts the coroutine, control passes to the unwind destination.

The resume and unwind destination blocks must be uniquely referenced by the yield instruction. This prevents them from becoming critical edges.

In a yield_once coroutine, there must not be a control flow path leading from the resume edge to another yield instruction in this function. This rule does not apply in the raw SIL stage.

There must not be a control flow path leading from the unwind edge to a return instruction, to a throw instruction, or to any block reachable from the entry block via a path that does not pass through an unwind edge. That is, the blocks reachable from unwind edges must jointly form a disjoint subfunction of the coroutine.

unwind

sil-terminator ::= 'unwind'

Exits the current function and returns control to the calling function, completing an unwind from a yield. The current function must be a coroutine.

unwind is only permitted in blocks reachable from the unwind edges of yield instructions.

br

sil-terminator ::= 'br' sil-identifier
                     '(' (sil-operand (',' sil-operand)*)? ')'

br label (%0 : $A, %1 : $B, ...)
// `label` must refer to a basic block label within the current function
// %0, %1, etc. must be of the types of `label`'s arguments

Unconditionally transfers control from the current basic block to the block labeled label, binding the given values to the arguments of the destination basic block.

cond_br

sil-terminator ::= 'cond_br' sil-operand ','
                     sil-identifier '(' (sil-operand (',' sil-operand)*)? ')' ','
                     sil-identifier '(' (sil-operand (',' sil-operand)*)? ')'

cond_br %0 : $Builtin.Int1, true_label (%a : $A, %b : $B, ...), \
                               false_label (%x : $X, %y : $Y, ...)
// %0 must be of $Builtin.Int1 type
// `true_label` and `false_label` must refer to block labels within the
//   current function and must not be identical
// %a, %b, etc. must be of the types of `true_label`'s arguments
// %x, %y, etc. must be of the types of `false_label`'s arguments

Conditionally branches to true_label if %0 is equal to 1 or to false_label if %0 is equal to 0, binding the corresponding set of values to the arguments of the chosen destination block.

switch_value

sil-terminator ::= 'switch_value' sil-operand
                     (',' sil-switch-value-case)*
                     (',' sil-switch-default)?
sil-switch-value-case ::= 'case' sil-value ':' sil-identifier
sil-switch-default ::= 'default' sil-identifier

switch_value %0 : $Builtin.Int<n>, case %1: label1, \
                                   case %2: label2, \
                                   ...,            \
                                   default labelN

// %0 must be a value of builtin integer type $Builtin.Int<n>
// `label1` through `labelN` must refer to block labels within the current
//   function
// FIXME: All destination labels currently must take no arguments

Conditionally branches to one of several destination basic blocks based on a value of builtin integer or function type. If the operand value matches one of the case values of the instruction, control is transferred to the corresponding basic block. If there is a default basic block, control is transferred to it if the value does not match any of the case values. It is undefined behavior if the value does not match any cases and no default branch is provided.

select_value

sil-instruction ::= 'select_value' sil-operand sil-select-value-case*
                    (',' 'default' sil-value)?
                    ':' sil-type
sil-select-value-case ::= 'case' sil-value ':' sil-value


%n = select_value %0 : $U, \
  case %c1: %r1,           \
  case %c2: %r2, /* ... */ \
  default %r3 : $T

// $U must be a builtin type. Only integers types are supported currently.
// c1, c2, etc must be of type $U
// %r1, %r2, %r3, etc. must have type $T
// %n has type $T

Selects one of the "case" or "default" operands based on the case of a value. This is equivalent to a trivial switch_value branch sequence:

entry:
  switch_value %0 : $U,            \
    case %c1: bb1,           \
    case %c2: bb2, /* ... */ \
    default bb_default
bb1:
  br cont(%r1 : $T) // value for %c1
bb2:
  br cont(%r2 : $T) // value for %c2
bb_default:
  br cont(%r3 : $T) // value for default
cont(%n : $T):
  // use argument %n

but turns the control flow dependency into a data flow dependency.

switch_enum

sil-terminator ::= 'switch_enum' sil-operand
                     (',' sil-switch-enum-case)*
                     (',' sil-switch-default)?
sil-switch-enum-case ::= 'case' sil-decl-ref ':' sil-identifier

switch_enum %0 : $U, case #U.Foo!enumelt: label1, \
                      case #U.Bar!enumelt: label2, \
                      ...,                 \
                      default labelN

// %0 must be a value of enum type $U
// #U.Foo, #U.Bar, etc. must be 'case' declarations inside $U
// `label1` through `labelN` must refer to block labels within the current
//   function
// label1 must take either no basic block arguments, or a single argument
//   of the type of #U.Foo's data
// label2 must take either no basic block arguments, or a single argument
//   of the type of #U.Bar's data, etc.
// labelN must take no basic block arguments

Conditionally branches to one of several destination basic blocks based on the discriminator in a loadable enum value. Unlike switch_int, switch_enum requires coverage of the operand type: If the enum type cannot be switched exhaustively in the current function, the default branch is required; otherwise, the default branch is required unless a destination is assigned to every case of the enum. The destination basic block for a case may take an argument of the corresponding enum case's data type (or of the address type, if the operand is an address). If the branch is taken, the destination's argument will be bound to the associated data inside the original enum value. For example:

enum Foo {
  case Nothing
  case OneInt(Int)
  case TwoInts(Int, Int)
}

sil @sum_of_foo : $Foo -> Int {
entry(%x : $Foo):
  switch_enum %x : $Foo,       \
    case #Foo.Nothing!enumelt: nothing, \
    case #Foo.OneInt!enumelt:  one_int, \
    case #Foo.TwoInts!enumelt: two_ints

nothing:
  %zero = integer_literal $Int, 0
  return %zero : $Int

one_int(%y : $Int):
  return %y : $Int

two_ints(%ab : $(Int, Int)):
  %a = tuple_extract %ab : $(Int, Int), 0
  %b = tuple_extract %ab : $(Int, Int), 1
  %add = function_ref @add : $(Int, Int) -> Int
  %result = apply %add(%a, %b) : $(Int, Int) -> Int
  return %result : $Int
}

On a path dominated by a destination block of switch_enum, copying or destroying the basic block argument has equivalent reference counting semantics to copying or destroying the switch_enum operand:

  // This retain_value...
  retain_value %e1 : $Enum
  switch_enum %e1, case #Enum.A: a, case #Enum.B: b
a(%a : $A):
  // ...is balanced by this release_value
  release_value %a
b(%b : $B):
  // ...and this one
  release_value %b

switch_enum_addr

sil-terminator ::= 'switch_enum_addr' sil-operand
                     (',' sil-switch-enum-case)*
                     (',' sil-switch-default)?

switch_enum_addr %0 : $*U, case #U.Foo!enumelt: label1, \
                                        case #U.Bar!enumelt: label2, \
                                        ...,                 \
                                        default labelN

// %0 must be the address of an enum type $*U
// #U.Foo, #U.Bar, etc. must be cases of $U
// `label1` through `labelN` must refer to block labels within the current
//   function
// The destinations must take no basic block arguments

Conditionally branches to one of several destination basic blocks based on the discriminator in the enum value referenced by the address operand.

Unlike switch_int, switch_enum requires coverage of the operand type: If the enum type cannot be switched exhaustively in the current function, the default branch is required; otherwise, the default branch is required unless a destination is assigned to every case of the enum. Unlike switch_enum, the payload value is not passed to the destination basic blocks; it must be projected out separately with unchecked_take_enum_data_addr.

dynamic_method_br

sil-terminator ::= 'dynamic_method_br' sil-operand ',' sil-decl-ref
                     ',' sil-identifier ',' sil-identifier

dynamic_method_br %0 : $P, #X.method, bb1, bb2
// %0 must be of protocol type
// #X.method must be a reference to an @objc method of any class
// or protocol type

Looks up the implementation of an Objective-C method with the same selector as the named method for the dynamic type of the value inside an existential container. The "self" operand of the result function value is represented using an opaque type, the value for which must be projected out as a value of type Builtin.ObjCPointer.

If the operand is determined to have the named method, this instruction branches to bb1, passing it the uncurried function corresponding to the method found. If the operand does not have the named method, this instruction branches to bb2.

checked_cast_br

sil-terminator ::= 'checked_cast_br' sil-checked-cast-exact?
                    sil-operand 'to' sil-type ','
                    sil-identifier ',' sil-identifier
sil-checked-cast-exact ::= '[' 'exact' ']'

checked_cast_br %0 : $A to $B, bb1, bb2
checked_cast_br %0 : $*A to $*B, bb1, bb2
checked_cast_br [exact] %0 : $A to $A, bb1, bb2
// $A and $B must be both object types or both address types
// bb1 must take a single argument of type $B or $*B
// bb2 must take no arguments

Performs a checked scalar conversion from $A to $B. If the conversion succeeds, control is transferred to bb1, and the result of the cast is passed into bb1 as an argument. If the conversion fails, control is transferred to bb2.

An exact cast checks whether the dynamic type is exactly the target type, not any possible subtype of it. The source and target types must be class types.

checked_cast_addr_br

sil-terminator ::= 'checked_cast_addr_br'
                    sil-cast-consumption-kind
                    sil-type 'in' sil-operand 'to'
                    sil-stype 'in' sil-operand ','
                    sil-identifier ',' sil-identifier
sil-cast-consumption-kind ::= 'take_always'
sil-cast-consumption-kind ::= 'take_on_success'
sil-cast-consumption-kind ::= 'copy_on_success'

checked_cast_addr_br take_always $A in %0 : $*@thick A to $B in %2 : $*@thick B, bb1, bb2
// $A and $B must be both address types
// bb1 must take a single argument of type $*B
// bb2 must take no arguments

Performs a checked indirect conversion from $A to $B. If the conversion succeeds, control is transferred to bb1, and the result of the cast is left in the destination. If the conversion fails, control is transferred to bb2.

try_apply

sil-terminator ::= 'try_apply' sil-value
                      sil-apply-substitution-list?
                      '(' (sil-value (',' sil-value)*)? ')'
                      ':' sil-type
  'normal' sil-identifier, 'error' sil-identifier

try_apply %0(%1, %2, ...) : $(A, B, ...) -> (R, @error E),
  normal bb1, error bb2
bb1(%3 : R):
bb2(%4 : E):

// Note that the type of the callee '%0' is specified *after* the arguments
// %0 must be of a concrete function type $(A, B, ...) -> (R, @error E)
// %1, %2, etc. must be of the argument types $A, $B, etc.

Transfers control to the function specified by %0, passing it the given arguments. When %0 returns, control resumes in either the normal destination (if it returns with return) or the error destination (if it returns with throw).

%0 must have a function type with an error result.

The rules on generic substitutions are identical to those of apply.

await_async_continuation

sil-terminator ::= 'await_async_continuation' sil-value
                      ',' 'resume' sil-identifier
                      (',' 'error' sil-identifier)?

await_async_continuation %0 : $UnsafeContinuation<T>, resume bb1
await_async_continuation %0 : $UnsafeThrowingContinuation<T>, resume bb1, error bb2

bb1(%1 : @owned $T):
bb2(%2 : @owned $Error):

Suspends execution of an @async function until the continuation is resumed. The continuation must be the result of a get_async_continuation or get_async_continuation_addr instruction within the same function; see the documentation for get_async_continuation for discussion of further constraints on the IR between get_async_continuation[_addr] and await_async_continuation. This terminator can only appear inside an @async function. The instruction must always have a resume successor, but must have an error successor if and only if the operand is an UnsafeThrowingContinuation<T>.

If the operand is the result of a get_async_continuation instruction, then the resume successor block must take an argument whose type is the maximally-abstracted lowered type of T, matching the type argument of the Unsafe[Throwing]Continuation<T> operand. The value of the resume argument is owned by the current function. If the operand is the result of a get_async_continuation_addr instruction, then the resume successor block must not take an argument; the resume value will be written to the memory referenced by the operand to the get_async_continuation_addr instruction, after which point the value in that memory becomes owned by the current function. With either variant, if the await_async_continuation instruction has an error successor block, the error block must take a single Error argument, and that argument is owned by the enclosing function. The memory referenced by a get_async_continuation_addr instruction remains uninitialized when await_async_continuation resumes on the error successor.

It is possible for a continuation to be resumed before await_async_continuation. In this case, the resume operation returns immediately to its caller. When the await_async_continuation instruction later executes, it then immediately transfers control to its resume or error successor block, using the resume or error value that the continuation was already resumed with.

Differentiable Programming

differentiable_function

sil-instruction ::= 'differentiable_function'
                    sil-differentiable-function-parameter-indices
                    sil-value ':' sil-type
                    sil-differentiable-function-derivative-functions-clause?

sil-differentiable-function-parameter-indices ::=
    '[' 'parameters' [0-9]+ (' ' [0-9]+)* ']'
sil-differentiable-derivative-functions-clause ::=
    'with_derivative'
    '{' sil-value ':' sil-type ',' sil-value ':' sil-type '}'

differentiable_function [parameters 0] %0 : $(T) -> T \
  with_derivative {%1 : $(T) -> (T, (T) -> T), %2 : $(T) -> (T, (T) -> T)}

Creates a @differentiable function from an original function operand and derivative function operands (optional). There are two derivative function kinds: a Jacobian-vector products (JVP) function and a vector-Jacobian products (VJP) function.

[parameters ...] specifies parameter indices that the original function is differentiable with respect to.

The with_derivative clause specifies the derivative function operands associated with the original function.

The differentiation transformation canonicalizes all differentiable_function instructions, generating derivative functions if necessary to fill in derivative function operands.

In raw SIL, the with_derivative clause is optional. In canonical SIL, the with_derivative clause is mandatory.

linear_function

sil-instruction ::= 'linear_function'
                    sil-linear-function-parameter-indices
                    sil-value ':' sil-type
                    sil-linear-function-transpose-function-clause?

sil-linear-function-parameter-indices ::=
    '[' 'parameters' [0-9]+ (' ' [0-9]+)* ']'
sil-linear-transpose-function-clause ::=
    with_transpose sil-value ':' sil-type

linear_function [parameters 0] %0 : $(T) -> T with_transpose %1 : $(T) -> T

Bundles a function with its transpose function into a @differentiable(_linear) function.

[parameters ...] specifies parameter indices that the original function is linear with respect to.

A with_transpose clause specifies the transpose function associated with the original function. When a with_transpose clause is not specified, the mandatory differentiation transform will add a with_transpose clause to the instruction.

In raw SIL, the with_transpose clause is optional. In canonical SIL, the with_transpose clause is mandatory.

differentiable_function_extract

sil-instruction ::= 'differentiable_function_extract'
                    '[' sil-differentiable-function-extractee ']'
                    sil-value ':' sil-type
                    ('as' sil-type)?

sil-differentiable-function-extractee ::= 'original' | 'jvp' | 'vjp'

differentiable_function_extract [original] %0 : $@differentiable (T) -> T
differentiable_function_extract [jvp] %0 : $@differentiable (T) -> T
differentiable_function_extract [vjp] %0 : $@differentiable (T) -> T
differentiable_function_extract [jvp] %0 : $@differentiable (T) -> T \
  as $(@in_constant T) -> (T, (T.TangentVector) -> T.TangentVector)

Extracts the original function or a derivative function from the given @differentiable function. The extractee is one of the following: [original], [jvp], or [vjp].

In lowered SIL, an explicit extractee type may be provided. This is currently used by the LoadableByAddress transformation, which rewrites function types.

linear_function_extract

sil-instruction ::= 'linear_function_extract'
                    '[' sil-linear-function-extractee ']'
                    sil-value ':' sil-type

sil-linear-function-extractee ::= 'original' | 'transpose'

linear_function_extract [original] %0 : $@differentiable(_linear) (T) -> T
linear_function_extract [transpose] %0 : $@differentiable(_linear) (T) -> T

Extracts the original function or a transpose function from the given @differentiable(_linear) function. The extractee is one of the following: [original] or [transpose].

differentiability_witness_function

sil-instruction ::=
    'differentiability_witness_function'
    '[' sil-differentiability-witness-function-kind ']'
    '[' differentiability-kind ']'
    '[' 'parameters' sil-differentiability-witness-function-index-list ']'
    '[' 'results' sil-differentiability-witness-function-index-list ']'
    generic-parameter-clause?
    sil-function-name ':' sil-type

sil-differentiability-witness-function-kind ::= 'jvp' | 'vjp' | 'transpose'
sil-differentiability-witness-function-index-list ::= [0-9]+ (' ' [0-9]+)*

differentiability_witness_function [vjp] [reverse] [parameters 0] [results 0] \
  <T where T: Differentiable> @foo : $(T) -> T

Looks up a differentiability witness function (JVP, VJP, or transpose) for a referenced function via SIL differentiability witnesses.

The differentiability witness function kind identifies the witness function to look up: [jvp], [vjp], or [transpose].

The remaining components identify the SIL differentiability witness:

  • Original function name.
  • Differentiability kind.
  • Parameter indices.
  • Result indices.
  • Witness generic parameter clause (optional). When parsing SIL, the parsed witness generic parameter clause is combined with the original function's generic signature to form the full witness generic signature.

Optimizer Dataflow Marker Instructions

mark_must_check

sil-instruction ::= 'mark_must_check'
                    '[' sil-optimizer-analysis-marker ']'

sil-optimizer-analysis-marker ::= 'consumable_and_assignable'
                              ::= 'no_consume_or_assign'

A canary value inserted by a SIL generating frontend to signal to the move checker to check a specific value. Valid only in Raw SIL. The relevant checkers should remove the mark_must_check instruction after successfully running the relevant diagnostic. The idea here is that instead of needing to introduce multiple "flagging" instructions for the optimizer, we can just reuse this one instruction by varying the kind.

If the sil optimizer analysis marker is consumable_and_assignable then the move checker is told to check that the result of this instruction is consumed at most once. If the marker is no_consume_or_assign, then the move checker will validate that the result of this instruction is never consumed or assigned over.

No Implicit Copy and No Escape Value Instructions

copyable_to_moveonlywrapper

sil-instruction ::= 'copyable_to_moveonlywrapper'

copyable_to_moveonlywrapper takes in a 'T' and maps it to a move only wrapped '@moveOnly T'. This is semantically used by a code generator initializing a new moveOnly binding from a copyable value. It semantically destroys its input @owned value and returns a brand new independent @owned @moveOnly value. It also is used to convert a trivial copyable value with type 'Trivial' into an owned non-trivial value of type '@moveOnly Trivial'. If one thinks of '@moveOnly' as a monad, this is how one injects a copyable value into the move only space.

moveonlywrapper_to_copyable

sil-instruction ::= 'moveonlywrapper_to_copyable [owned]'
sil-instruction ::= 'moveonlywrapper_to_copyable [guaranteed]'

moveonlywrapper_to_copyable takes in a '@moveOnly T' and produces a new 'T' value. This is a 'forwarding' instruction where at parse time, we only allow for one to choose it to be [owned] or [guaranteed]. With time, we may eliminate the need for the guaranteed form in the future.

  • moveonlywrapper_to_copyable [owned] is used to signal the end of lifetime of the '@moveOnly' wrapper. SILGen inserts these when ever a move only value has its ownership passed to a situation where a copyable value is needed. Since it is consuming, we know that the no implicit copy or no-escape checker will ensure that if we need a copy for it, the program will emit a diagnostic.
  • moveonlywrapper_to_copyable [guaranteed] is used to pass a @moveOnly T value as a copyable guaranteed parameter with type 'T' to a function. In the case of using no-implicit-copy checking this is always fine since no-implicit-copy is a local pattern. This would be an error when performing no escape checking. Importantly, this instruction also is where in the case of an @moveOnly trivial type, we convert from the non-trivial representation to the trivial representation.

copyable_to_moveonlywrapper_addr

sil-instruction ::= 'copyable_to_moveonlywrapper_addr'

copyable_to_moveonlywrapper_addr takes in a '*T' and maps it to a move only wrapped '*@moveOnly T'. This is semantically used by a code generator initializing a new moveOnly binding from a copyable value. It semantically acts as an address cast. If one thinks of '@moveOnly' as a monad, this is how one injects a copyable value into the move only space.

moveonlywrapper_to_copyable_addr

sil-instruction ::= 'moveonlywrapper_to_copyable_addr'

moveonlywrapper_to_copyable_addr takes in a '*@moveOnly T' and produces a new '*T' value. This instruction acts like an address cast that projects out the underlying T from an @moveOnly T.

NOTE: From the perspective of the address checker, a trivial load with a moveonlywrapper_to_copyable_addr operand is considered to be a use of a noncopyable type.

Assertion configuration

To be able to support disabling assertions at compile time there is a builtin assertion_configuration function. A call to this function can be replaced at compile time by a constant or can stay opaque.

All calls to the assert_configuration function are replaced by the constant propagation pass to the appropriate constant depending on compile time settings. Subsequent passes remove dependent unwanted control flow. Using this mechanism we support conditionally enabling/disabling of code in SIL libraries depending on the assertion configuration selected when the library is linked into user code.

There are three assertion configurations: Debug (0), Release (1) and DisableReplacement (-1).

The optimization flag or a special assert configuration flag determines the value. Depending on the configuration value assertions in the standard library will be executed or not.

The standard library uses this builtin to define an assert that can be disabled at compile time.

func assert(...) {
  if (Int32(Builtin.assert_configuration()) == 0) {
    _fatal_error_message(message, ...)
  }
}

The assert_configuration function application is serialized when we build the standard library (we recognize the -parse-stdlib option and don't do the constant replacement but leave the function application to be serialized to sil).

The compiler flag that influences the value of the assert_configuration function application is the optimization flag: at -Onone the application will be replaced by Debug at higher optimization levels the instruction will be replaced by Release. Optionally, the value to use for replacement can be specified with the -assert-config flag which overwrites the value selected by the optimization flag (possible values are Debug, Release, DisableReplacement).

If the call to the assert_configuration function stays opaque until IRGen, IRGen will replace the application by the constant representing Debug mode (0). This happens we can build the standard library .dylib. The generate sil will retain the function call but the generated .dylib will contain code with assertions enabled.

Weak linking support

has_symbol

sil-instruction ::= 'has_symbol' sil-decl-ref

Returns true if each of the underlying symbol addresses associated with the given declaration are non-null. This can be used to determine whether a weakly-imported declaration is available at runtime.