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[InstCombine] Clear out nsw/nuw if we modify computation in the chain 

An nsw/nuw operation relies on the values feeding into it to not
overflow if 'poison' is not to be produced.  This means that
optimizations which make modifications to the bottom of a chain (like
SimplifyDemandedBits) must strip out nsw/nuw if they cannot ensure that
they will be preserved.

This fixes PR23309.

llvm-svn: 235544
This commit is contained in:
David Majnemer 2015-04-22 20:59:28 +00:00
parent a327644534
commit fe58d13a17
2 changed files with 22 additions and 3 deletions
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13
llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
View File

@ -83,11 +83,18 @@ bool InstCombiner::SimplifyDemandedInstructionBits(Instruction &Inst) {
bool InstCombiner::SimplifyDemandedBits(Use &U, APInt DemandedMask,
APInt &KnownZero, APInt &KnownOne,
unsigned Depth) {
Value *NewVal =
SimplifyDemandedUseBits(U.get(), DemandedMask, KnownZero, KnownOne, Depth,
dyn_cast<Instruction>(U.getUser()));
auto *UserI = dyn_cast<Instruction>(U.getUser());
Value *NewVal = SimplifyDemandedUseBits(U.get(), DemandedMask, KnownZero,
KnownOne, Depth, UserI);
if (!NewVal) return false;
U = NewVal;
// Shrinking a constant might cause a nsw/nuw violation to occur in
// instructions which are themselves demanded.
if (auto *UserOBO = dyn_cast<OverflowingBinaryOperator>(UserI)) {
cast<BinaryOperator>(UserOBO)->setHasNoSignedWrap(false);
cast<BinaryOperator>(UserOBO)->setHasNoUnsignedWrap(false);
}
return true;
}

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

@ -1113,3 +1113,15 @@ define float @sitofp_zext(i16 %a) {
%sitofp = sitofp i32 %zext to float
ret float %sitofp
}
define i1 @PR23309(i32 %A, i32 %B) {
; CHECK-LABEL: @PR23309(
; CHECK-NEXT: %[[sub:.*]] = sub i32 %A, %B
; CHECK-NEXT: %[[and:.*]] = and i32 %[[sub]], 1
; CHECK-NEXT: %[[cmp:.*]] = icmp ne i32 %[[and]], 0
; CHECK-NEXT: ret i1 %[[cmp]]
%add = add i32 %A, -4
%sub = sub nsw i32 %add, %B
%trunc = trunc i32 %sub to i1
ret i1 %trunc
}