/// 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();
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