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[InstCombine] Let shrinkSplatShuffle act on vectors of different lengths #148593

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Merged
nikic merged 2 commits into from
Jul 28, 2025
Merged

[InstCombine] Let shrinkSplatShuffle act on vectors of different lengths #148593

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6fa9358
Expand move trunc through shuffle splat
Jul 14, 2025
fb5b9c0
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Jul 15, 2025
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12 changes: 9 additions & 3 deletions llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -708,11 +708,17 @@ static Instruction *shrinkSplatShuffle(TruncInst &Trunc,
auto *Shuf = dyn_cast<ShuffleVectorInst>(Trunc.getOperand(0));
if (Shuf && Shuf->hasOneUse() && match(Shuf->getOperand(1), m_Undef()) &&
all_equal(Shuf->getShuffleMask()) &&
Shuf->getType() == Shuf->getOperand(0)->getType()) {
Shuf->getType()->getScalarType() ==
Shuf->getOperand(0)->getType()->getScalarType()) {
// trunc (shuf X, Undef, SplatMask) --> shuf (trunc X), Poison, SplatMask
// trunc (shuf X, Poison, SplatMask) --> shuf (trunc X), Poison, SplatMask
Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(0), Trunc.getType());
return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask());
auto *const NewTruncTy = VectorType::get(
Trunc.getType()->getScalarType(),
cast<VectorType>(Shuf->getOperand(0)->getType())->getElementCount());
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@nikic nikic Jul 14, 2025

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I think this is Shuf->getOperand(0)->getType()->getWithNewType(Trunc.getType()->getScalarType())?

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@Adar-Dagan Adar-Dagan Jul 15, 2025

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Yeah, done

Value *NarrowOp =
Builder.CreateTrunc(Shuf->getOperand(0), NewTruncTy, Trunc.getName());
return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask(),
Shuf->getName());
}

return nullptr;
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4 changes: 2 additions & 2 deletions llvm/test/Transforms/InstCombine/trunc-inseltpoison.ll
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Expand Up @@ -959,8 +959,8 @@ define <3 x i31> @wide_splat3(<3 x i33> %x) {

define <8 x i8> @wide_lengthening_splat(<4 x i16> %v) {
; CHECK-LABEL: @wide_lengthening_splat(
; CHECK-NEXT: [[SHUF:%.*]] = shufflevector <4 x i16> [[V:%.*]], <4 x i16> poison, <8 x i32> zeroinitializer
; CHECK-NEXT: [[TR:%.*]] = trunc <8 x i16> [[SHUF]] to <8 x i8>
; CHECK-NEXT: [[TMP1:%.*]] = trunc <4 x i16> [[V:%.*]] to <4 x i8>
; CHECK-NEXT: [[TR:%.*]] = shufflevector <4 x i8> [[TMP1]], <4 x i8> poison, <8 x i32> zeroinitializer
; CHECK-NEXT: ret <8 x i8> [[TR]]
;
%shuf = shufflevector <4 x i16> %v, <4 x i16> %v, <8 x i32> zeroinitializer
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4 changes: 2 additions & 2 deletions llvm/test/Transforms/InstCombine/trunc.ll
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Expand Up @@ -960,8 +960,8 @@ define <3 x i31> @wide_splat3(<3 x i33> %x) {

define <8 x i8> @wide_lengthening_splat(<4 x i16> %v) {
; CHECK-LABEL: @wide_lengthening_splat(
; CHECK-NEXT: [[SHUF:%.*]] = shufflevector <4 x i16> [[V:%.*]], <4 x i16> poison, <8 x i32> zeroinitializer
; CHECK-NEXT: [[TR:%.*]] = trunc <8 x i16> [[SHUF]] to <8 x i8>
; CHECK-NEXT: [[TMP1:%.*]] = trunc <4 x i16> [[V:%.*]] to <4 x i8>
; CHECK-NEXT: [[TR:%.*]] = shufflevector <4 x i8> [[TMP1]], <4 x i8> poison, <8 x i32> zeroinitializer
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@nikic nikic Jul 14, 2025

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Also test shortening splat? I think in that case the profitability is less clear.

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@lukel97 lukel97 Jul 15, 2025

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Yeah I think we want to restrict it to only when Shuf->getOperand(0) is shorter than Shuf?

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@Adar-Dagan Adar-Dagan Jul 15, 2025

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I don't understand why it wouldn't be profitable in that case, could you please elaborate?

Also added test

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@lukel97 lukel97 Jul 15, 2025

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We're now performing trunc on a wider type than before, and that can be slower on some targets. E.g. a <8 x i16> trunc may take twice as many uops as a <4 x i16> trunc. There is definitely at least hardware on RISC-V where this is the case.

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@Adar-Dagan Adar-Dagan Jul 16, 2025

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Thanks! changed

; CHECK-NEXT: ret <8 x i8> [[TR]]
;
%shuf = shufflevector <4 x i16> %v, <4 x i16> %v, <8 x i32> zeroinitializer
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