protected override void GenerateCodeForConditionalBranch(CodeGenContext context, BranchTargetLabel target, bool reverse)
{
// NB: reverse=false is the normal case as would be used in:
// if ( a || b ) { c=1; }
// where we branch around the then-part ( {c=1; } ) when the sub-expression a || b is false
//
// Reverse=true would apply in situation such as:
// if ( ! (a || b) ) { c=1; }
// where we would apply reverse=true toward the inner evaluation of (a || b)
// or many other cases involving reversal of evaluation purpose due to the || expression's larger context
if (reverse)
{
context.EvalBoth(Left, Right, target, true);
}
else
{
context.EvalEither(Left, Right, target, false);
}
}
// 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);
}
}
