diff options
Diffstat (limited to 'lib/Transforms/Scalar/InstructionCombining.cpp')
| -rw-r--r-- | lib/Transforms/Scalar/InstructionCombining.cpp | 10 |
1 files changed, 8 insertions, 2 deletions
diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp index b838289e59..41b8f3bc29 100644 --- a/lib/Transforms/Scalar/InstructionCombining.cpp +++ b/lib/Transforms/Scalar/InstructionCombining.cpp @@ -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 |