From 2bb11d189f6f6aa1137e60396f4170c5435a54a0 Mon Sep 17 00:00:00 2001
From: Luke Lau <luke@igalia.com>
Date: Mon, 30 Jun 2025 14:33:36 +0100
Subject: [PATCH] [InstCombine] Pull vector reverse through fneg

This follows on from https://github.com/llvm/llvm-project/pull/144933#issuecomment-2992372627, and allows us to remove the reverse (fneg (reverse x)) combine.

A separate patch will handle the case for fabs. I haven't checked if we perform this canonicalization for either unops or binops for vp.reverse
---
 .../InstCombine/InstCombineAddSub.cpp          |  6 ++++++
 .../InstCombine/InstCombineCalls.cpp           | 18 +-----------------
 .../Transforms/InstCombine/vector-reverse.ll   |  6 +++---
 3 files changed, 10 insertions(+), 20 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index e721f0cd5f9e3..f727eb0a63e05 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -3049,6 +3049,12 @@ Instruction *InstCombinerImpl::visitFNeg(UnaryOperator &I) {
   if (match(OneUse, m_Shuffle(m_Value(X), m_Poison(), m_Mask(Mask))))
     return new ShuffleVectorInst(Builder.CreateFNegFMF(X, &I), Mask);
 
+  // fneg (reverse x) --> reverse (fneg x)
+  if (match(OneUse, m_VecReverse(m_Value(X)))) {
+    Value *Reverse = Builder.CreateVectorReverse(Builder.CreateFNegFMF(X, &I));
+    return replaceInstUsesWith(I, Reverse);
+  }
+
   return nullptr;
 }
 
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index e33d111167c04..47cc424f4ac07 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -3549,23 +3549,6 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     }
     break;
   }
-  case Intrinsic::vector_reverse: {
-    Value *Vec = II->getArgOperand(0);
-    // Note: We canonicalize reverse after binops, so we don't need a
-    // corresponding binop case here. TODO: Consider canonicalizing
-    // reverse after fneg?
-
-    // rev(unop rev(X)) --> unop X
-    Value *X;
-    if (match(Vec, m_OneUse(m_UnOp(m_VecReverse(m_Value(X)))))) {
-      auto *OldUnOp = cast<UnaryOperator>(Vec);
-      auto *NewUnOp = UnaryOperator::CreateWithCopiedFlags(
-          OldUnOp->getOpcode(), X, OldUnOp, OldUnOp->getName(),
-          II->getIterator());
-      return replaceInstUsesWith(CI, NewUnOp);
-    }
-    break;
-  }
   case Intrinsic::experimental_vp_reverse: {
     Value *X;
     Value *Vec = II->getArgOperand(0);
@@ -3573,6 +3556,7 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     if (!match(Mask, m_AllOnes()))
       break;
     Value *EVL = II->getArgOperand(2);
+    // TODO: Canonicalize experimental.vp.reverse after unop/binops?
     // rev(unop rev(X)) --> unop X
     if (match(Vec,
               m_OneUse(m_UnOp(m_Intrinsic<Intrinsic::experimental_vp_reverse>(
diff --git a/llvm/test/Transforms/InstCombine/vector-reverse.ll b/llvm/test/Transforms/InstCombine/vector-reverse.ll
index c9c68d2241b34..8421d6082dc44 100644
--- a/llvm/test/Transforms/InstCombine/vector-reverse.ll
+++ b/llvm/test/Transforms/InstCombine/vector-reverse.ll
@@ -161,9 +161,9 @@ define <vscale x 4 x i32> @binop_reverse_splat_LHS_1(<vscale x 4 x i32> %a, i32
 
 define <vscale x 4 x float> @unop_reverse(<vscale x 4 x float> %a) {
 ; CHECK-LABEL: @unop_reverse(
-; CHECK-NEXT:    [[A_REV:%.*]] = tail call <vscale x 4 x float> @llvm.vector.reverse.nxv4f32(<vscale x 4 x float> [[A:%.*]])
-; CHECK-NEXT:    [[NEG:%.*]] = fneg fast <vscale x 4 x float> [[A_REV]]
-; CHECK-NEXT:    ret <vscale x 4 x float> [[NEG]]
+; CHECK-NEXT:    [[NEG:%.*]] = fneg fast <vscale x 4 x float> [[A_REV:%.*]]
+; CHECK-NEXT:    [[NEG1:%.*]] = call <vscale x 4 x float> @llvm.vector.reverse.nxv4f32(<vscale x 4 x float> [[NEG]])
+; CHECK-NEXT:    ret <vscale x 4 x float> [[NEG1]]
 ;
   %a.rev = tail call <vscale x 4 x float> @llvm.vector.reverse.nxv4f32(<vscale x 4 x float> %a)
   %neg = fneg fast <vscale x 4 x float> %a.rev