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

[Florian Hahn via llvm-commits]

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@@ -318,13 +332,20 @@ 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;
+  }
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fhahn wrote:

I think in general it should be possible to create multiple recipes for a single source instruction (we do it in multiple places already IIRC), as long as the additional instructions do not need to be mapped to IR instructions for lookup later.

A recent example is https://github.com/llvm/llvm-project/pull/124268/files#diff-da321d454a7246f8ae276bf1db2782bf26b5210b8133cb59e4d7fd45d0905decR8900

https://github.com/llvm/llvm-project/pull/123636