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InstCombine: Correctly propagate NSW/NUW for x-(-A) -> x+A

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We can only propagate the nsw bits if both subtraction instructions are
marked with the appropriate bit.

N.B.  We only propagate the nsw bit in InstCombine because the nuw case
is already handled in InstSimplify.

This fixes PR20189.

llvm-svn: 214385
This commit is contained in:
David Majnemer 2014-07-31 04:49:29 +00:00
parent ad214c8e9e
commit 066fbe5798
2 changed files with 18 additions and 3 deletions
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12
lib/Transforms/InstCombine/InstCombineAddSub.cpp
View File

@ -1462,11 +1462,17 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
if (Value *V = SimplifyUsingDistributiveLaws(I))
return ReplaceInstUsesWith(I, V);
// If this is a 'B = x-(-A)', change to B = x+A. This preserves NSW/NUW.
// If this is a 'B = x-(-A)', change to B = x+A.
if (Value *V = dyn_castNegVal(Op1)) {
BinaryOperator *Res = BinaryOperator::CreateAdd(Op0, V);
Res->setHasNoSignedWrap(I.hasNoSignedWrap());
Res->setHasNoUnsignedWrap(I.hasNoUnsignedWrap());
if (const auto *BO = dyn_cast<BinaryOperator>(Op1)) {
assert(BO->getOpcode() == Instruction::Sub &&
"Expected a subtraction operator!");
if (BO->hasNoSignedWrap() && I.hasNoSignedWrap())
Res->setHasNoSignedWrap(true);
}
return Res;
}

9
test/Transforms/InstCombine/sub.ll
View File

@ -464,3 +464,12 @@ define i32 @test38(i32 %A) {
; CHECK-NEXT: [[SEXT:%.*]] = sext i1 [[ICMP]] to i32
; CHECK-NEXT: ret i32 [[SEXT]]
}
define i32 @test39(i32 %A, i32 %x) {
%B = sub i32 0, %A
%C = sub nsw i32 %x, %B
ret i32 %C
; CHECK-LABEL: @test39(
; CHECK: %C = add i32 %x, %A
; CHECK: ret i32 %C
}