Redundancy in compiled form of UNTIL

[NCGThompson]

There is no problem, but by visual inspection of the compiler code, I can see that it outputs a not operation in front of a branch-if-false. This is a double negative.

KOS/src/kOS.Safe/Compilation/KS/Compiler.cs

Lines 2389 to 2390 in c6425ac

	AddOpcode(new OpcodeLogicNot());
	Opcode branch = AddOpcode(new OpcodeBranchIfFalse());

It would be logically equivalent to omit the not and use a branch-if-true. This section of code was written before branch-if-true was added.

There is a difference. not has a try catch that the others do not. For reference, branch-if-false:

KOS/src/kOS.Safe/Compilation/Opcode.cs

Lines 1059 to 1064 in c6425ac

	public override void Execute(ICpu cpu)
	{
	bool condition = Convert.ToBoolean(cpu.PopValueArgument());
	DeltaInstructionPointer = !condition ? Distance : 1;
	}

branch-if-true:

KOS/src/kOS.Safe/Compilation/Opcode.cs

Lines 1080 to 1084 in c6425ac

	public override void Execute(ICpu cpu)
	{
	bool condition = Convert.ToBoolean(cpu.PopValueArgument());
	DeltaInstructionPointer = condition ? Distance : 1;
	}

and not:

KOS/src/kOS.Safe/Compilation/Opcode.cs

Lines 1464 to 1482 in c6425ac

	public override void Execute(ICpu cpu)
	{
	object value = cpu.PopValueArgument();
	object result;
	// Convert to bool instead of cast in case the identifier is stored
	// as an encapsulated BooleanValue, preventing an unboxing collision.
	// Wrapped in a try/catch since the Convert framework doesn't have a
	// way to determine if a type can be converted.
	try
	{
	result = !Convert.ToBoolean(value);
	}
	catch
	{
	throw new KOSCastException(value.GetType(), typeof(BooleanValue));
	}
	cpu.PushArgumentStack(Structure.FromPrimitive(result));
	}