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[InstCombine] Do not fold shufflevector(select) if the select condition is a vector #113993

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Merged
merged 1 commit into from
Oct 29, 2024
Merged

[InstCombine] Do not fold shufflevector(select) if the select condition is a vector #113993

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8 changes: 6 additions & 2 deletions llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2902,8 +2902,12 @@ Instruction *InstCombinerImpl::visitShuffleVectorInst(ShuffleVectorInst &SVI) {

if (match(RHS, m_Constant())) {
if (auto *SI = dyn_cast<SelectInst>(LHS)) {
if (Instruction *I = FoldOpIntoSelect(SVI, SI))
return I;
// We cannot do this fold for elementwise select since ShuffleVector is
// not elementwise.
if (SI->getCondition()->getType()->isIntegerTy()) {
if (Instruction *I = FoldOpIntoSelect(SVI, SI))
return I;
}
Comment on lines +2907 to +2910
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@MatzeB MatzeB Oct 29, 2024 •
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I think we could still perform this transformation when the shuffle operand types are the same as the shuffle output type? Regardless it is probably best to just land this change as quick as possible to unbreak things. I can look into re-enabling it for same-type myself then.

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There is a counterexample: https://alive2.llvm.org/ce/z/s8saCx.

}
if (auto *PN = dyn_cast<PHINode>(LHS)) {
if (Instruction *I = foldOpIntoPhi(SVI, PN))
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24 changes: 24 additions & 0 deletions llvm/test/Transforms/InstCombine/vec_shuffle.ll
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Expand Up @@ -2387,6 +2387,30 @@ define <2 x i32> @foldselect0(i1 %c) {
ret <2 x i32> %shuf
}

; Make sure we do not crash in this case.
define <4 x float> @shuf_larger_length_vec_select(<2 x i1> %cond) {
; CHECK-LABEL: @shuf_larger_length_vec_select(
; CHECK-NEXT: [[SEL:%.*]] = select <2 x i1> [[COND:%.*]], <2 x float> zeroinitializer, <2 x float> <float 1.000000e+00, float 1.000000e+00>
; CHECK-NEXT: [[SHUF:%.*]] = shufflevector <2 x float> [[SEL]], <2 x float> zeroinitializer, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
; CHECK-NEXT: ret <4 x float> [[SHUF]]
;
%sel = select <2 x i1> %cond, <2 x float> zeroinitializer, <2 x float> splat(float 1.000000e+00)
%shuf = shufflevector <2 x float> %sel, <2 x float> zeroinitializer, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
ret <4 x float> %shuf
}

; Make sure we do not fold in this case.
define <4 x i32> @shuf_same_length_vec_select(<4 x i1> %cond) {
; CHECK-LABEL: @shuf_same_length_vec_select(
; CHECK-NEXT: [[SEL:%.*]] = select <4 x i1> [[COND:%.*]], <4 x i32> <i32 poison, i32 1, i32 2, i32 3>, <4 x i32> <i32 poison, i32 5, i32 6, i32 7>
; CHECK-NEXT: [[SHUF:%.*]] = shufflevector <4 x i32> [[SEL]], <4 x i32> <i32 poison, i32 9, i32 poison, i32 poison>, <4 x i32> <i32 2, i32 1, i32 3, i32 5>
; CHECK-NEXT: ret <4 x i32> [[SHUF]]
;
%sel = select <4 x i1> %cond, <4 x i32> <i32 0, i32 1, i32 2, i32 3>, <4 x i32> <i32 4, i32 5, i32 6, i32 7>
%shuf = shufflevector <4 x i32> %sel, <4 x i32> <i32 8, i32 9, i32 10, i32 11>, <4 x i32> <i32 2, i32 1, i32 3, i32 5>
ret <4 x i32> %shuf
}

declare i1 @cond()
declare <4 x i32> @value()

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