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Do not let MaskedValueIsZero consider undef to be zero, for reasons

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explained in the comment.

This fixes UnitTests/2003-09-18-BitFieldTest on darwin


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22483 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2005-07-20 18:49:28 +00:00
parent e884dc2c58
commit f9944f109c
1 changed files with 8 additions and 2 deletions
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10
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -1317,7 +1317,13 @@ struct FoldSetCCLogical {
/// this predicate to simplify operations downstream. V and Mask are known to
/// be the same type.
static bool MaskedValueIsZero(Value *V, ConstantIntegral *Mask) {
if (isa<UndefValue>(V) || Mask->isNullValue())
// Note, we cannot consider 'undef' to be "IsZero" here. The problem is that
// we cannot optimize based on the assumption that it is zero without changing
// to to an explicit zero. If we don't change it to zero, other code could
// optimized based on the contradictory assumption that it is non-zero.
// Because instcombine aggressively folds operations with undef args anyway,
// this won't lose us code quality.
if (Mask->isNullValue())
return true;
if (ConstantIntegral *CI = dyn_cast<ConstantIntegral>(V))
return ConstantExpr::getAnd(CI, Mask)->isNullValue();
@ -3134,7 +3140,7 @@ Instruction *InstCombiner::visitShiftInst(ShiftInst &I) {
return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
}
if (isa<UndefValue>(Op1)) {
if (isLeftShift || I.getType()->isUnsigned())
if (isLeftShift || I.getType()->isUnsigned())// X << undef, X >>u undef -> 0
return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
else
return ReplaceInstUsesWith(I, Op0); // X >>s undef -> X