[InstCombine] Pull shuffles out of binops with splatted ops #137948

lukel97 · 2025-04-30T10:09:47Z

Given a binary op on splatted vector and a splatted constant, InstCombine will normally pull the shuffle out in InstCombinerImpl::foldVectorBinop:

define <4 x i32> @f(i32 %x) {
  %x.insert = insertelement <4 x i32> poison, i32 %x, i64 0
  %x.splat = shufflevector <4 x i32> %x.insert, <4 x i32> poison, <4 x i32> zeroinitializer
  %res = add <4 x i32> %x.splat, splat (i32 42)
  ret <4 x i32> %res
}

define <4 x i32> @f(i32 %x) {
  %x.insert = insertelement <4 x i32> poison, i32 %x, i64 0
  %1 = add <4 x i32> %x.insert, <i32 42, i32 poison, i32 poison, i32 poison>
  %res = shufflevector <4 x i32> %1, <4 x i32> poison, <4 x i32> zeroinitializer
  ret <4 x i32> %res
}

However, this currently only operates on fixed length vectors. Splats of scalable vectors don't currently have their shuffle pulled out, e.g:

define <vscale x 4 x i32> @f(i32 %x) {
  %x.insert = insertelement <vscale x 4 x i32> poison, i32 %x, i64 0
  %x.splat = shufflevector <vscale x 4 x i32> %x.insert, <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
  %res = add <vscale x 4 x i32> %x.splat, splat (i32 42)
  ret <vscale x 4 x i32> %res
}

Having this canonical form with the shuffle pulled out is important as VectorCombine relies on it in order to scalarize binary ops in scalarizeBinopOrCmp, which would prevent the need for #137786. This also brings it in line for scalable binary ops with two non-constant operands: https://godbolt.org/z/M9f7ebzca

This adds a combine just after the fixed-length version, but restricted to splats at index 0 so that it also handles the scalable case:

So the whilst the existing combine looks like: Op(shuffle(V1, Mask), C) -> shuffle(Op(V1, NewC), Mask)

This patch adds: Op(shuffle(V1, 0), (splat C)) -> shuffle(Op(V1, (splat C)), 0)

I think this could be generalized to other splat indexes that aren't zero, but I think it would be dead code since only fixed-length vectors can have non-zero shuffle indices, which would be covered by the existing combine.

llvmbot · 2025-04-30T10:10:18Z

@llvm/pr-subscribers-llvm-transforms

Author: Luke Lau (lukel97)

Changes

Given a binary op on splatted vector and a splatted constant, InstCombine will normally pull the shuffle out in InstCombinerImpl::foldVectorBinop:

define &lt;4 x i32&gt; @<!-- -->f(i32 %x) {
  %x.insert = insertelement &lt;4 x i32&gt; poison, i32 %x, i64 0
  %x.splat = shufflevector &lt;4 x i32&gt; %x.insert, &lt;4 x i32&gt; poison, &lt;4 x i32&gt; zeroinitializer
  %res = add &lt;4 x i32&gt; %x.splat, splat (i32 42)
  ret &lt;4 x i32&gt; %res
}

define &lt;4 x i32&gt; @<!-- -->f(i32 %x) {
  %x.insert = insertelement &lt;4 x i32&gt; poison, i32 %x, i64 0
  %1 = add &lt;4 x i32&gt; %x.insert, &lt;i32 42, i32 poison, i32 poison, i32 poison&gt;
  %res = shufflevector &lt;4 x i32&gt; %1, &lt;4 x i32&gt; poison, &lt;4 x i32&gt; zeroinitializer
  ret &lt;4 x i32&gt; %res
}

However, this currently only operates on fixed length vectors. Splats of scalable vectors don't currently have their shuffle pulled out, e.g:

define &lt;vscale x 4 x i32&gt; @<!-- -->f(i32 %x) {
  %x.insert = insertelement &lt;vscale x 4 x i32&gt; poison, i32 %x, i64 0
  %x.splat = shufflevector &lt;vscale x 4 x i32&gt; %x.insert, &lt;vscale x 4 x i32&gt; poison, &lt;vscale x 4 x i32&gt; zeroinitializer
  %res = add &lt;vscale x 4 x i32&gt; %x.splat, splat (i32 42)
  ret &lt;vscale x 4 x i32&gt; %res
}

Having this canonical form with the shuffle pulled out is important as VectorCombine relies on it in order to scalarize binary ops in scalarizeBinopOrCmp, which would prevent the need for #137786. This also brings it in line for scalable binary ops with two non-constant operands: https://godbolt.org/z/M9f7ebzca

This adds a combine just after the fixed-length version, but restricted to splats at index 0 so that it also handles the scalable case:

So the whilst the existing combine looks like: Op(shuffle(V1, Mask), C) -> shuffle(Op(V1, NewC), Mask)

This patch adds: Op(shuffle(V1, 0), (splat C)) -> shuffle(Op(V1, (splat C)), 0)

I think this could be generalized to other splat indexes that aren't zero, but I think it would be dead code since only fixed-length vectors can have non-zero shuffle indices, which would be covered by the existing combine.

Full diff: https://github.com/llvm/llvm-project/pull/137948.diff

3 Files Affected:

(modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+25)
(modified) llvm/test/Transforms/InstCombine/getelementptr.ll (+2-2)
(modified) llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll (+23)

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index f807f5f4519fc..ecb3899eafe67 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2271,6 +2271,31 @@ Instruction *InstCombinerImpl::foldVectorBinop(BinaryOperator &Inst) {
     }
   }
 
+  // Similar to the combine above, but handles the case for scalable vectors
+  // where both V1 and C are splats.
+  //
+  // Op(shuffle(V1, 0), (splat C)) -> shuffle(Op(V1, (splat C)), 0)
+  if (isa<ScalableVectorType>(Inst.getType()) &&
+      match(&Inst, m_c_BinOp(m_OneUse(m_Shuffle(m_Value(V1), m_Poison(),
+                                                m_ZeroMask())),
+                             m_ImmConstant(C)))) {
+    if (Constant *Splat = C->getSplatValue()) {
+      bool ConstOp1 = isa<Constant>(RHS);
+      VectorType *V1Ty = cast<VectorType>(V1->getType());
+      Constant *NewC = ConstantVector::getSplat(V1Ty->getElementCount(), Splat);
+
+      Value *NewLHS = ConstOp1 ? V1 : NewC;
+      Value *NewRHS = ConstOp1 ? NewC : V1;
+      Value *XY = Builder.CreateBinOp(Opcode, NewLHS, NewRHS);
+      if (auto *BO = dyn_cast<BinaryOperator>(XY))
+        BO->copyIRFlags(&Inst);
+
+      VectorType *VTy = cast<VectorType>(Inst.getType());
+      SmallVector<int> NewM(VTy->getElementCount().getKnownMinValue(), 0);
+      return new ShuffleVectorInst(XY, NewM);
+    }
+  }
+
   // Try to reassociate to sink a splat shuffle after a binary operation.
   if (Inst.isAssociative() && Inst.isCommutative()) {
     // Canonicalize shuffle operand as LHS.
diff --git a/llvm/test/Transforms/InstCombine/getelementptr.ll b/llvm/test/Transforms/InstCombine/getelementptr.ll
index feba952919b9a..61236df80bfa6 100644
--- a/llvm/test/Transforms/InstCombine/getelementptr.ll
+++ b/llvm/test/Transforms/InstCombine/getelementptr.ll
@@ -282,8 +282,8 @@ define <2 x i1> @test13_fixed_scalable(i64 %X, ptr %P, <2 x i64> %y) nounwind {
 define <vscale x 2 x i1> @test13_scalable_scalable(i64 %X, ptr %P, <vscale x 2 x i64> %y) nounwind {
 ; CHECK-LABEL: @test13_scalable_scalable(
 ; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <vscale x 2 x i64> poison, i64 [[X:%.*]], i64 0
-; CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <vscale x 2 x i64> [[DOTSPLATINSERT]], <vscale x 2 x i64> poison, <vscale x 2 x i32> zeroinitializer
-; CHECK-NEXT:    [[A_IDX:%.*]] = shl nsw <vscale x 2 x i64> [[DOTSPLAT]], splat (i64 3)
+; CHECK-NEXT:    [[TMP3:%.*]] = shl nsw <vscale x 2 x i64> [[DOTSPLATINSERT]], splat (i64 3)
+; CHECK-NEXT:    [[A_IDX:%.*]] = shufflevector <vscale x 2 x i64> [[TMP3]], <vscale x 2 x i64> poison, <vscale x 2 x i32> zeroinitializer
 ; CHECK-NEXT:    [[TMP1:%.*]] = call i64 @llvm.vscale.i64()
 ; CHECK-NEXT:    [[TMP2:%.*]] = shl i64 [[TMP1]], 4
 ; CHECK-NEXT:    [[DOTSPLATINSERT1:%.*]] = insertelement <vscale x 2 x i64> poison, i64 [[TMP2]], i64 0
diff --git a/llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll b/llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll
index c6329af164623..926f272d36a4b 100644
--- a/llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll
+++ b/llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll
@@ -1789,3 +1789,26 @@ define <4 x i32> @PR46872(<4 x i32> %x) {
   ret <4 x i32> %a
 }
 
+define <vscale x 4 x i32> @scalable_splat_binop_constant_rhs(<vscale x 4 x i32> %x) {
+; CHECK-LABEL: @scalable_splat_binop_constant_rhs(
+; CHECK-NEXT:    [[R1:%.*]] = add <vscale x 4 x i32> [[R:%.*]], splat (i32 42)
+; CHECK-NEXT:    [[R2:%.*]] = shufflevector <vscale x 4 x i32> [[R1]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <vscale x 4 x i32> [[R2]]
+;
+
+  %splatx = shufflevector <vscale x 4 x i32> %x, <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
+  %r = add <vscale x 4 x i32> %splatx, splat (i32 42)
+  ret <vscale x 4 x i32> %r
+}
+
+define <vscale x 4 x float> @scalable_splat_binop_constant_lhs(<vscale x 4 x float> %x) {
+; CHECK-LABEL: @scalable_splat_binop_constant_lhs(
+; CHECK-NEXT:    [[R1:%.*]] = fadd <vscale x 4 x float> [[R:%.*]], splat (float 4.200000e+01)
+; CHECK-NEXT:    [[R2:%.*]] = shufflevector <vscale x 4 x float> [[R1]], <vscale x 4 x float> poison, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <vscale x 4 x float> [[R2]]
+;
+
+  %splatx = shufflevector <vscale x 4 x float> %x, <vscale x 4 x float> poison, <vscale x 4 x i32> zeroinitializer
+  %r = fadd <vscale x 4 x float> splat (float 42.0), %splatx
+  ret <vscale x 4 x float> %r
+}

dtcxzyw · 2025-04-30T12:07:09Z

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

  }

+  // Similar to the combine above, but handles the case for scalable vectors
+  // where both V1 and C are splats.


Suggested change

// where both V1 and C are splats.

// where both shuffle(V1, 0) and C are splats.

V1 is not required to be a splat vector.

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

dtcxzyw

LGTM. Thank you!

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

preames

LGTM.

lukel97 added 2 commits April 30, 2025 17:32

Precommit tests

5ebf099

[InstCombine] Pull shuffles out of binops with splatted ops

8522d21

lukel97 requested review from dtcxzyw and paulwalker-arm April 30, 2025 10:09

lukel97 requested a review from nikic as a code owner April 30, 2025 10:09

llvmbot added llvm:instcombine Covers the InstCombine, InstSimplify and AggressiveInstCombine passes llvm:transforms labels Apr 30, 2025

lukel97 mentioned this pull request Apr 30, 2025

[VectorCombine] Scalarize bin ops and cmps with two splatted operands #137786

Closed

Reuse createBinOpShuffle

401ffa6

dtcxzyw reviewed Apr 30, 2025

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Update comment, add test for ub

e9d51e7

dtcxzyw approved these changes Apr 30, 2025

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preames reviewed Apr 30, 2025

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preames approved these changes May 9, 2025

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Merge branch 'main' into instcombine/foldVectorBinop-shuffle-scalable

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lukel97 merged commit cf3242f into llvm:main May 12, 2025
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[InstCombine] Pull shuffles out of binops with splatted ops #137948

[InstCombine] Pull shuffles out of binops with splatted ops #137948

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lukel97 commented Apr 30, 2025 •

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	// where both V1 and C are splats.
	// where both shuffle(V1, 0) and C are splats.

[InstCombine] Pull shuffles out of binops with splatted ops #137948

[InstCombine] Pull shuffles out of binops with splatted ops #137948

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lukel97 commented Apr 30, 2025 •

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