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InstCombine: Don't unconditionally preserve 'nuw' when shrinking constants

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Consider:
  %add = add nuw i32 %a, -16777216
  %and = and i32 %add, 255

Regardless of whether or not we demand the sign bit of %add, we cannot
replace -16777216 with 2130706432 without also removing 'nuw' from the
instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216273 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2014-08-22 17:11:04 +00:00
parent 0e4fc41b0d
commit 5939f08f5e
2 changed files with 24 additions and 6 deletions
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18
lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
View File

@ -44,12 +44,18 @@ static bool ShrinkDemandedConstant(Instruction *I, unsigned OpNo,
Demanded &= OpC->getValue();
I->setOperand(OpNo, ConstantInt::get(OpC->getType(), Demanded));
// If 'nsw' is set and the constant is negative, removing *any* bits from the
// constant could make overflow occur. Remove 'nsw' from the instruction in
// this case.
if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(I))
if (OBO->hasNoSignedWrap() && OpC->getValue().isNegative())
cast<BinaryOperator>(OBO)->setHasNoSignedWrap(false);
// If either 'nsw' or 'nuw' is set and the constant is negative,
// removing *any* bits from the constant could make overflow occur.
// Remove 'nsw' and 'nuw' from the instruction in this case.
if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(I)) {
assert(OBO->getOpcode() == Instruction::Add);
if (OBO->hasNoSignedWrap() || OBO->hasNoUnsignedWrap()) {
if (OpC->getValue().isNegative()) {
cast<BinaryOperator>(OBO)->setHasNoSignedWrap(false);
cast<BinaryOperator>(OBO)->setHasNoUnsignedWrap(false);
}
}
}
return true;
}

12
test/Transforms/InstCombine/cast.ll
View File

@ -1038,3 +1038,15 @@ define i8 @test84(i32 %a) {
; CHECK: [[SHR:%.*]] = lshr exact i32 [[ADD]], 23
; CHECK: [[CST:%.*]] = trunc i32 [[SHR]] to i8
}
define i8 @test85(i32 %a) {
%add = add nuw i32 %a, -16777216
%shr = lshr exact i32 %add, 23
%trunc = trunc i32 %shr to i8
ret i8 %trunc
; CHECK-LABEL: @test85(
; CHECK: [[ADD:%.*]] = add i32 %a, 2130706432
; CHECK: [[SHR:%.*]] = lshr exact i32 [[ADD]], 23
; CHECK: [[CST:%.*]] = trunc i32 [[SHR]] to i8
}