caio/clang-p2996
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[InstSimplify] Don't fold vectors of partial undef in SimplifySelectInst if the non-undef element value might produce poison

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We can't fold to the non-undef value unless we know it isn't poison. So check each element with isGuaranteedNotToBeUndefOrPoison. This currently rules out all constant expressions.

Differential Revision: https://reviews.llvm.org/D83442
This commit is contained in:
Craig Topper
2020-07-09 11:01:11 -07:00
parent f5f58f1f73
commit 122b0640fc
2 changed files with 18 additions and 2 deletions
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6
llvm/lib/Analysis/InstructionSimplify.cpp
View File

@@ -4135,9 +4135,11 @@ static Value *SimplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
// one element is undef, choose the defined element as the safe result.
if (TEltC == FEltC)
NewC.push_back(TEltC);
else if (isa<UndefValue>(TEltC))
else if (isa<UndefValue>(TEltC) &&
isGuaranteedNotToBeUndefOrPoison(FEltC))
NewC.push_back(FEltC);
else if (isa<UndefValue>(FEltC))
else if (isa<UndefValue>(FEltC) &&
isGuaranteedNotToBeUndefOrPoison(TEltC))
NewC.push_back(TEltC);
else
break;

14
llvm/test/Transforms/InstSimplify/select.ll
View File

@@ -848,3 +848,17 @@ define i32 @false_undef_false_freeze(i1 %cond, i32 %x) {
%s = select i1 %cond, i32 undef, i32 %xf
ret i32 %s
}
@g = external global i32, align 1
; Make sure we don't fold partial undef vectors when constexprs are involved.
; We would need to prove the constexpr doesn't result in poison which we aren't
; equiped to do yet.
define <2 x i32> @false_undef_true_constextpr_vec(i1 %cond) {
; CHECK-LABEL: @false_undef_true_constextpr_vec(
; CHECK-NEXT: [[S:%.*]] = select i1 [[COND:%.*]], <2 x i32> <i32 undef, i32 ptrtoint (i32* @g to i32)>, <2 x i32> <i32 ptrtoint (i32* @g to i32), i32 undef>
; CHECK-NEXT: ret <2 x i32> [[S]]
;
%s = select i1 %cond, <2 x i32> <i32 undef, i32 ptrtoint (i32* @g to i32)>, <2 x i32> <i32 ptrtoint (i32* @g to i32), i32 undef>
ret <2 x i32> %s
}