[llvm] [LoopVectorizer][AArch64] Add support for partial reduce subtraction (PR #123636)

[Nicholas Guy via llvm-commits]

https://github.com/NickGuy-Arm created https://github.com/llvm/llvm-project/pull/123636

None

>From 8b8443afead9c0034e79579032210e8921dc7c54 Mon Sep 17 00:00:00 2001
From: Nick Guy <nicholas.guy at arm.com>
Date: Mon, 20 Jan 2025 15:51:22 +0000
Subject: [PATCH] [LoopVectorizer][AArch64] Add support for partial reduce
 subtraction

---
 .../lib/Target/AArch64/AArch64TargetTransformInfo.h |  2 +-
 llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp      | 13 ++++++++++---
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h
index 8e7e590c173ff2..c6cebcca679353 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h
@@ -368,7 +368,7 @@ class AArch64TTIImpl : public BasicTTIImplBase<AArch64TTIImpl> {
     InstructionCost Invalid = InstructionCost::getInvalid();
     InstructionCost Cost(TTI::TCC_Basic);
 
-    if (Opcode != Instruction::Add)
+    if (Opcode != Instruction::Add && Opcode != Instruction::Sub)
       return Invalid;
 
     if (InputTypeA != InputTypeB)
diff --git a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
index 979a8e0768a991..e1fffafa594849 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
@@ -317,13 +317,20 @@ void VPPartialReductionRecipe::execute(VPTransformState &State) {
   State.setDebugLocFrom(getDebugLoc());
   auto &Builder = State.Builder;
 
-  assert(getOpcode() == Instruction::Add &&
-         "Unhandled partial reduction opcode");
-
   Value *BinOpVal = State.get(getOperand(0));
   Value *PhiVal = State.get(getOperand(1));
   assert(PhiVal && BinOpVal && "Phi and Mul must be set");
 
+  unsigned Opcode = getOpcode();
+
+  if (Opcode == Instruction::Sub) {
+    bool HasNSW = cast<Instruction>(BinOpVal)->hasNoSignedWrap();
+    BinOpVal = Builder.CreateNeg(BinOpVal, "", HasNSW);
+    Opcode = Instruction::Add;
+  }
+
+  assert(Opcode == Instruction::Add && "Unhandled partial reduction opcode");
+
   Type *RetTy = PhiVal->getType();
 
   CallInst *V = Builder.CreateIntrinsic(