diff options
| -rw-r--r-- | llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp | 8 |
1 files changed, 4 insertions, 4 deletions
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp index 2dea4c45cbd..ba77696f1f1 100644 --- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp +++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp @@ -171,8 +171,8 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift, // But for a mask we need to get rid of old masking instruction. if (!Masked->hasOneUse()) return nullptr; // Else we can't perform the fold. - // We should produce compute the mask in wider type, and truncate later! - // Get type twice as wide element-wise (same number of elements!). + // The mask must be computed in a type twice as wide to ensure + // that no bits are lost if the sum-of-shifts is wider than the base type. Type *ExtendedScalarTy = Type::getIntNTy(Ty->getContext(), 2 * BitWidth); Type *ExtendedTy = Ty->isVectorTy() @@ -211,8 +211,8 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift, return nullptr; // Else we can't perform the fold. Type *Ty = X->getType(); unsigned BitWidth = Ty->getScalarSizeInBits(); - // We should produce compute the mask in wider type, and truncate later! - // Get type twice as wide element-wise (same number of elements!). + // The mask must be computed in a type twice as wide to ensure + // that no bits are lost if the sum-of-shifts is wider than the base type. Type *ExtendedScalarTy = Type::getIntNTy(Ty->getContext(), 2 * BitWidth); Type *ExtendedTy = Ty->isVectorTy() |
