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

[Sam Tebbs via llvm-commits]

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@@ -8898,8 +8898,24 @@ VPRecipeBuilder::tryToCreatePartialReduction(Instruction *Reduction,
       isa<VPPartialReductionRecipe>(BinOpRecipe))
     std::swap(BinOp, Accumulator);
 
-  return new VPPartialReductionRecipe(Reduction->getOpcode(), BinOp,
-                                      Accumulator, Reduction);
+  unsigned Opcode = Reduction->getOpcode();
+  if (Opcode == Instruction::Sub) {
+    VPBasicBlock *ParentBlock = Builder.getInsertBlock();
+    // TODO Is this ever possible in normal operation, would this check be
+    // better off as an assert (if checked at all)?
+    if (!ParentBlock)
+      return nullptr;
+
+    auto *const Zero = ConstantInt::get(Reduction->getType(), 0);
+    SmallVector<VPValue *, 2> Ops;
+    Ops.push_back(Plan.getOrAddLiveIn(Zero));
+    Ops.push_back(cast<VPWidenRecipe>(BinOp->getDefiningRecipe()));
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SamTebbs33 wrote:

Why does this need to cast? Can it just add `BinOp` to the list?

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