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[InstCombine] dropRedundantMaskingOfLeftShiftInput(): improve comment 

llvm-svn: 372637
This commit is contained in:
Roman Lebedev 2019-09-23 18:21:14 +00:00
parent 8d52016155
commit 1972327d63
1 changed files with 4 additions and 4 deletions
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8
llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
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@ -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()