Extract the negating subtraction to its own recipe

Author: NickGuy-Arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8936,6 +8936,20 @@ VPRecipeBuilder::tryToCreatePartialReduction(Instruction *Reduction,
     std::swap(BinOp, Accumulator);
 
   unsigned ReductionOpcode = Reduction->getOpcode();
+  if (ReductionOpcode == Instruction::Sub) {
+    VPBasicBlock *ParentBlock = Builder.getInsertBlock();
+    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()));
+    BinOp = new VPWidenRecipe(*Reduction, make_range(Ops.begin(), Ops.end()));
+    ParentBlock->appendRecipe(BinOp->getDefiningRecipe());
+    ReductionOpcode = Instruction::Add;
+  }
+
   if (CM.blockNeedsPredicationForAnyReason(Reduction->getParent())) {
     assert((ReductionOpcode == Instruction::Add ||
             ReductionOpcode == Instruction::Sub) &&

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -339,15 +339,8 @@ void VPPartialReductionRecipe::execute(VPTransformState &State) {
   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");
+  assert(getOpcode() == Instruction::Add &&
+         "Unhandled partial reduction opcode");
 
   Type *RetTy = PhiVal->getType();