protected override AbstractSyntaxTree GenerateCodeForValue(CodeGenContext context, EvaluationIntention purpose)
{
bool so = false;
switch (purpose)
{
case EvaluationIntention.SideEffectsOnly:
so = true;
break;
case EvaluationIntention.Value:
case EvaluationIntention.ValueOrNode:
break;
default:
throw new AssertionFailedException("unexpected evaluation intention" + purpose);
}
var secondChoice = context.CreateLabel();
Pre.GenerateCodeForConditionalBranchWithPrettyPrint(context, secondChoice, false);
Mid.GenerateCodeForValueWithPrettyPrint(context, so ? EvaluationIntention.SideEffectsOnly : EvaluationIntention.Value);
var joinPoint = context.CreateLabel();
context.GenerateUnconditionalBranch(joinPoint);
context.PlaceLabelHere(secondChoice);
Post.GenerateCodeForValueWithPrettyPrint(context, so ? EvaluationIntention.SideEffectsOnly : EvaluationIntention.Value);
context.PlaceLabelHere(joinPoint);
return(null);
}
// NB: Short Circut Evaluation (Left || Right): if "Left" evaluates to true we must not evaluate "Right"
protected override AbstractSyntaxTree GenerateCodeForValue(CodeGenContext context, EvaluationIntention purpose)
{
switch (purpose)
{
case EvaluationIntention.SideEffectsOnly:
// We have something like an expression statement:
// a || b;
// or perhaps a parameter as in
// f(a||b);
// This requires differentiated evaluation for left/first and right/second as follows:
// a's truth value determines whether b is executed or skipped (short-circuted).
// if a is false, then b is executed; otherwise if a is true, b is skipped.
// thus, we need to evaluate "a" for conditional branch, and to branch around "b"!
// however, b does not affect anything further (and we don't need a final value for (a||b) either).
// Thus, b is evaluated for side effects only.
var joinPoint1 = context.CreateLabel();
Left.GenerateCodeForConditionalBranchWithPrettyPrint(context, joinPoint1, true);
Right.GenerateCodeForValueWithPrettyPrint(context, EvaluationIntention.SideEffectsOnly);
context.PlaceLabelHere(joinPoint1);
return(null);
case EvaluationIntention.Value:
case EvaluationIntention.ValueOrNode:
// we have an expression like (a || b) + 3 so we treat that like: ((a || b) ? 1 : 0) + 3;
var zero = context.CreateLabel();
context.EvalEither(Left, Right, zero, false);
// get temp, so easy on stack machine
context.GenerateInstruction("PUSH", "#1");
var joinPoint2 = context.CreateLabel();
context.GenerateUnconditionalBranch(joinPoint2);
context.PlaceLabelHere(zero);
context.GenerateInstruction("PUSH", "#0");
context.PlaceLabelHere(joinPoint2);
return(null);
default:
throw new AssertionFailedException("unexpected evaluation intention" + purpose);
}
}
