[Seq][Arc] Allow seq.initial to take immutable operands. Add cast ope…

…rations

This commit adds support for seq.initial ops to take immutable operands and
introduces new cast operations:

- Update InitialOp to accept input operands of ImmutableType
- Add FromImmutableOp and ToImmutableOp for casting between immutable and regular types
- Add mergeInitialOps helper to handle operands and topological sorting
  of initial ops when lowering (SV flow -> SeqToSV, Arc flow ->
  LowerState).
- Update lowering passes and dialects to work with new initial op

include/circt/Dialect/Seq/SeqOps.h

@@ -70,6 +70,12 @@ createConstantInitialValue(OpBuilder builder, Operation *constantLike);
 // initial op.
 Value unwrapImmutableValue(mlir::TypedValue<seq::ImmutableType> immutableVal);
 
+// Helper function to merge initial ops within the block into a single initial
+// op. Return failure if we cannot topologically sort the initial ops.
+// Return null if there is no initial op in the block. Return the initial op
+// otherwise.
+FailureOr<seq::InitialOp> mergeInitialOps(Block *block);
+
 } // namespace seq
 } // namespace circt
 

include/circt/Dialect/Seq/SeqOps.td

@@ -711,7 +711,7 @@ def InitialOp : SeqOp<"initial", [SingleBlock,
     See the Seq dialect rationale for a longer description.
   }];
 
-  let arguments = (ins);
+  let arguments = (ins Variadic<ImmutableType>: $inputs);
   let results = (outs Variadic<ImmutableType>); // seq.immutable values
   let regions = (region SizedRegion<1>:$body);
   let hasVerifier = 1;
@@ -721,7 +721,7 @@ def InitialOp : SeqOp<"initial", [SingleBlock,
   ];
 
   let assemblyFormat = [{
-    $body attr-dict `:` type(results)
+    `(` $inputs `)` $body attr-dict `:` functional-type($inputs, results)
   }];
 
   let extraClassDeclaration = [{
@@ -740,3 +740,25 @@ def YieldOp : SeqOp<"yield",
 
   let assemblyFormat = "attr-dict ($operands^ `:` type($operands))?";
 }
+
+def FromImmutableOp : SeqOp<"from_immutable", [Pure]> {
+  let summary = "Cast from an immutable type to a wire type";
+
+  let arguments = (ins ImmutableType:$input);
+  let results = (outs AnyType:$output);
+
+  let assemblyFormat = "$input attr-dict `:` functional-type(operands, results)";
+}
+
+def ToImmutableOp : SeqOp<"to_immutable", [Pure]> {
+  let summary = "Cast from a wire type to an immutable type.";
+  let description = [{
+    This is an unsafe cast op that converts a HW type to an immutable type. It assumes
+    the value is valid at initialization phase.
+  }];
+
+  let arguments = (ins AnyType:$input);
+  let results = (outs ImmutableType:$output);
+
+  let assemblyFormat = "$input attr-dict `:` functional-type(operands, results)";
+}

integration_test/Bindings/Python/dialects/seq.py

@@ -27,10 +27,10 @@ def top(module):
       poweron_value = hw.ConstantOp.create(i32, 42).result
       # CHECK: %[[INPUT_VAL:.+]] = hw.constant 45
       reg_input = hw.ConstantOp.create(i32, 45).result
-      # CHECK-NEXT: %[[POWERON_VAL:.+]] = seq.initial {
+      # CHECK-NEXT: %[[POWERON_VAL:.+]] = seq.initial() {
       # CHECK-NEXT:   %[[C42:.+]] = hw.constant 42 : i32
       # CHECK-NEXT:   seq.yield %[[C42]] : i32
-      # CHECK-NEXT: } : !seq.immutable<i32>
+      # CHECK-NEXT: } : () -> !seq.immutable<i32>
       # CHECK: %[[DATA_VAL:.+]] = seq.compreg %[[INPUT_VAL]], %clk reset %rst, %[[RESET_VAL]] initial %[[POWERON_VAL]]
       reg = seq.CompRegOp(i32,
                           reg_input,

integration_test/arcilator/JIT/reg.mlir

@@ -19,12 +19,12 @@ hw.module @counter(in %clk: i1, out o1: i8, out o2: i8) {
 
   %r0 = seq.compreg %added1, %seq_clk initial %0#0 : i8
   %r1 = seq.compreg %added2, %seq_clk initial %0#1 : i8
-  %0:2 = seq.initial {
+  %0:2 = seq.initial () {
     %1 = func.call @random() : () -> i32
     %2 = comb.extract %1 from 0 : (i32) -> i8
     %3 = hw.constant 5 : i8
     seq.yield %2, %3: i8, i8
-  } : !seq.immutable<i8>, !seq.immutable<i8>
+  } : () -> (!seq.immutable<i8>, !seq.immutable<i8>)
 
   %one = hw.constant 1 : i8
   %added1 = comb.add %r0, %one : i8

lib/Bindings/Python/dialects/seq.py

@@ -99,7 +99,7 @@ def __init__(self,
         if power_on_value.owner is None:
           assert False, "Initial value must not be port"
         elif isinstance(power_on_value.owner.opview, hw.ConstantOp):
-          init = InitialOp([seq.ImmutableType.get(power_on_value.type)])
+          init = InitialOp([seq.ImmutableType.get(power_on_value.type)], [])
           init.body.blocks.append()
           with InsertionPoint(init.body.blocks[0]):
             cloned_constant = power_on_value.owner.clone()

lib/Conversion/ConvertToArcs/ConvertToArcs.cpp

@@ -35,12 +35,10 @@ static LogicalResult convertInitialValue(seq::CompRegOp reg,
   if (!reg.getInitialValue())
     return values.push_back({}), success();
 
-  // unrealized_conversion_cast to normal type
+  // Use from_immutable cast to convert the seq.immutable type to the reg's type.
   OpBuilder builder(reg);
-  auto init = builder
-                  .create<mlir::UnrealizedConversionCastOp>(
-                      reg.getLoc(), reg.getType(), reg.getInitialValue())
-                  .getResult(0);
+  auto init = builder.create<seq::FromImmutableOp>(reg.getLoc(), reg.getType(),
+                                                   reg.getInitialValue());
 
   values.push_back(init);
   return success();

lib/Conversion/SeqToSV/SeqToSV.cpp

@@ -23,6 +23,7 @@
 #include "circt/Dialect/SV/SVOps.h"
 #include "circt/Dialect/Seq/SeqOps.h"
 #include "circt/Support/Naming.h"
+#include "mlir/Analysis/TopologicalSortUtils.h"
 #include "mlir/IR/Builders.h"
 #include "mlir/IR/DialectImplementation.h"
 #include "mlir/IR/ImplicitLocOpBuilder.h"
@@ -59,11 +60,10 @@ struct SeqToSVPass : public impl::LowerSeqToSVBase<SeqToSVPass> {
 namespace {
 struct ModuleLoweringState {
   ModuleLoweringState(HWModuleOp module)
-      : initalOpLowering(module), module(module) {}
+      : immutableValueLowering(module), module(module) {}
 
-  struct InitialOpLowering {
-    InitialOpLowering(hw::HWModuleOp module)
-        : builder(module.getModuleBody()), module(module) {}
+  struct ImmutableValueLowering {
+    ImmutableValueLowering(hw::HWModuleOp module) : module(module) {}
 
     // Lower initial ops.
     LogicalResult lower();
@@ -82,9 +82,8 @@ struct ModuleLoweringState {
     // defined in SV initial op.
     MapVector<mlir::TypedValue<seq::ImmutableType>, Value> mapping;
 
-    OpBuilder builder;
     hw::HWModuleOp module;
-  } initalOpLowering;
+  } immutableValueLowering;
 
   struct FragmentInfo {
     bool needsRegFragment = false;
@@ -94,21 +93,39 @@ struct ModuleLoweringState {
   HWModuleOp module;
 };
 
-LogicalResult ModuleLoweringState::InitialOpLowering::lower() {
-  auto loweringFailed = module
-                            .walk([&](seq::InitialOp initialOp) {
-                              if (failed(lower(initialOp)))
-                                return mlir::WalkResult::interrupt();
-                              return mlir::WalkResult::advance();
-                            })
-                            .wasInterrupted();
-  return LogicalResult::failure(loweringFailed);
+LogicalResult ModuleLoweringState::ImmutableValueLowering::lower() {
+  auto result = mergeInitialOps(module.getBodyBlock());
+  if (failed(result))
+    return failure();
+
+  auto initialOp = *result;
+  if (!initialOp)
+    return success();
+
+  return lower(initialOp);
 }
 
 LogicalResult
-ModuleLoweringState::InitialOpLowering::lower(seq::InitialOp initialOp) {
+ModuleLoweringState::ImmutableValueLowering::lower(seq::InitialOp initialOp) {
+  OpBuilder builder = OpBuilder::atBlockBegin(module.getBodyBlock());
   if (!svInitialOp)
     svInitialOp = builder.create<sv::InitialOp>(initialOp->getLoc());
+  // Replace immutable operands passed to initial op with already lowered
+  // values.
+  for (auto [blockArgument, operand] :
+       llvm::zip(initialOp.getBodyBlock()->getArguments(),
+                 initialOp->getOpOperands())) {
+
+    auto immut = operand.get().getDefiningOp<seq::ToImmutableOp>();
+    if (!immut)
+      return initialOp.emitError()
+             << "invalid operand to initial op: " << operand.get();
+    blockArgument.replaceAllUsesWith(immut.getInput());
+    operand.drop();
+    if (immut.use_empty())
+      immut.erase();
+  }
+
   auto loc = initialOp.getLoc();
   llvm::SmallVector<Value> results;
 
@@ -127,10 +144,10 @@ ModuleLoweringState::InitialOpLowering::lower(seq::InitialOp initialOp) {
   }
 
   svInitialOp.getBodyBlock()->getOperations().splice(
-      svInitialOp.begin(), initialOp.getBodyBlock()->getOperations());
+      svInitialOp.end(), initialOp.getBodyBlock()->getOperations());
 
   assert(initialOp->use_empty());
-  initialOp->erase();
+  initialOp.erase();
   yieldOp->erase();
   return success();
 }
@@ -200,7 +217,7 @@ class CompRegLower : public OpConversionPattern<OpTy> {
       auto module = reg->template getParentOfType<hw::HWModuleOp>();
       const auto &initial =
           moduleLoweringStates.find(module.getModuleNameAttr())
-              ->second.initalOpLowering;
+              ->second.immutableValueLowering;
 
       Value initialValue = initial.lookupImmutableValue(init);
 
@@ -247,6 +264,49 @@ void CompRegLower<CompRegClockEnabledOp>::createAssign(
   });
 }
 
+/// Lower FromImmutable to `sv.reg` and `sv.initial`.
+class FromImmutableLowering : public OpConversionPattern<FromImmutableOp> {
+public:
+  FromImmutableLowering(
+      TypeConverter &typeConverter, MLIRContext *context,
+      const MapVector<StringAttr, ModuleLoweringState> &moduleLoweringStates)
+      : OpConversionPattern<FromImmutableOp>(typeConverter, context),
+        moduleLoweringStates(moduleLoweringStates) {}
+
+  using OpAdaptor = typename OpConversionPattern<FromImmutableOp>::OpAdaptor;
+
+  LogicalResult
+  matchAndRewrite(FromImmutableOp fromImmutableOp, OpAdaptor adaptor,
+                  ConversionPatternRewriter &rewriter) const final {
+    Location loc = fromImmutableOp.getLoc();
+
+    auto regTy = ConversionPattern::getTypeConverter()->convertType(
+        fromImmutableOp.getType());
+    auto svReg = rewriter.create<sv::RegOp>(loc, regTy);
+
+    auto regVal = rewriter.create<sv::ReadInOutOp>(loc, svReg);
+
+    // Lower initial values.
+    auto module = fromImmutableOp->template getParentOfType<hw::HWModuleOp>();
+    const auto &initial = moduleLoweringStates.find(module.getModuleNameAttr())
+                              ->second.immutableValueLowering;
+
+    Value initialValue =
+        initial.lookupImmutableValue(fromImmutableOp.getInput());
+
+    OpBuilder::InsertionGuard guard(rewriter);
+    auto in = initial.getSVInitial();
+    rewriter.setInsertionPointToEnd(in.getBodyBlock());
+    rewriter.create<sv::BPAssignOp>(fromImmutableOp->getLoc(), svReg,
+                                    initialValue);
+
+    rewriter.replaceOp(fromImmutableOp, regVal);
+    return success();
+  }
+
+private:
+  const MapVector<StringAttr, ModuleLoweringState> &moduleLoweringStates;
+};
 // Lower seq.clock_gate to a fairly standard clock gate implementation.
 //
 class ClockGateLowering : public OpConversionPattern<ClockGateOp> {
@@ -537,7 +597,7 @@ void SeqToSVPass::runOnOperation() {
     moduleLoweringStates.try_emplace(module.getModuleNameAttr(),
                                      ModuleLoweringState(module));
 
-  mlir::parallelForEach(
+  auto result = mlir::failableParallelForEach(
       &getContext(), moduleLoweringStates, [&](auto &moduleAndState) {
         auto &state = moduleAndState.second;
         auto module = state.module;
@@ -561,9 +621,12 @@ void SeqToSVPass::runOnOperation() {
           }
           needsMemRandomization = true;
         }
-        (void)state.initalOpLowering.lower();
+        return state.immutableValueLowering.lower();
       });
 
+  if (failed(result))
+    return signalPassFailure();
+
   auto randomInitFragmentName =
       FlatSymbolRefAttr::get(context, "RANDOM_INIT_FRAGMENT");
   auto randomInitRegFragmentName =
@@ -605,6 +668,8 @@ void SeqToSVPass::runOnOperation() {
                                         moduleLoweringStates);
   patterns.add<CompRegLower<CompRegClockEnabledOp>>(
       typeConverter, context, lowerToAlwaysFF, moduleLoweringStates);
+  patterns.add<FromImmutableLowering>(typeConverter, context,
+                                      moduleLoweringStates);
   patterns.add<ClockCastLowering<seq::FromClockOp>>(typeConverter, context);
   patterns.add<ClockCastLowering<seq::ToClockOp>>(typeConverter, context);
   patterns.add<ClockGateLowering>(typeConverter, context);
@@ -683,8 +748,7 @@ void SeqToSVPass::runOnOperation() {
 
   // Helper function to emit #ifndef guard.
   auto emitGuard = [&](const char *guard, llvm::function_ref<void(void)> body) {
-    b.create<sv::IfDefOp>(
-        guard, []() {}, body);
+    b.create<sv::IfDefOp>(guard, []() {}, body);
   };
 
   b.create<emit::FragmentOp>(randomInitFragmentName.getAttr(), [&] {

lib/Dialect/Arc/Transforms/LowerState.cpp

@@ -190,6 +190,10 @@ Value ClockLowering::materializeValue(Value value) {
     return {};
   if (auto mapped = materializedValues.lookupOrNull(value))
     return mapped;
+  if (auto fromImmutable = value.getDefiningOp<seq::FromImmutableOp>())
+    // Immutable value is pre-materialized so directly lookup the input.
+    return materializedValues.lookup(fromImmutable.getInput());
+
   if (!shouldMaterialize(value))
     return value;
 
@@ -427,19 +431,27 @@ LogicalResult ModuleLowering::lowerPrimaryOutputs() {
 }
 
 LogicalResult ModuleLowering::lowerInitials() {
-  // Move all operations except for seq.yield to arc.initial op.
-  for (auto op : moduleOp.getOps<seq::InitialOp>()) {
-    auto terminator = cast<seq::YieldOp>(op.getBodyBlock()->getTerminator());
-    getInitial().builder.getBlock()->getOperations().splice(
-        getInitial().builder.getBlock()->begin(),
-        op.getBodyBlock()->getOperations());
-
-    // Map seq.initial results to operands of the seq.yield op.
-    for (auto [result, operand] :
-         llvm::zip(op.getResults(), terminator.getOperands()))
-      getInitial().materializedValues.map(result, operand);
-    terminator.erase();
-  }
+  // Merge all seq.initial ops into a single seq.initial op.
+  auto result = circt::seq::mergeInitialOps(moduleOp.getBodyBlock());
+  if (failed(result))
+    return moduleOp.emitError() << "initial ops cannot be topologically sorted";
+
+  auto initialOp = *result;
+  if (!initialOp) // There is no seq.initial op.
+    return success();
+
+  // Move the operations of the merged initial op into the builder's block.
+  auto terminator =
+      cast<seq::YieldOp>(initialOp.getBodyBlock()->getTerminator());
+  getInitial().builder.getBlock()->getOperations().splice(
+      getInitial().builder.getBlock()->begin(),
+      initialOp.getBodyBlock()->getOperations());
+
+  // Map seq.initial results to their corresponding operands.
+  for (auto [result, operand] :
+       llvm::zip(initialOp.getResults(), terminator.getOperands()))
+    getInitial().materializedValues.map(result, operand);
+  terminator.erase();
 
   return success();
 }