[llvm] [LoopVectorizer] Add support for chaining partial reductions (PR #120272)

[Sam Tebbs via llvm-commits]

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@@ -2453,13 +2453,16 @@ class VPPartialReductionRecipe : public VPSingleDefRecipe {
       : VPSingleDefRecipe(VPDef::VPPartialReductionSC,
                           ArrayRef<VPValue *>({Op0, Op1}), ReductionInst),
         Opcode(Opcode) {
-    assert(isa<VPReductionPHIRecipe>(getOperand(1)->getDefiningRecipe()) &&
+    auto *DefiningRecipe = getOperand(1)->getDefiningRecipe();
+    assert((isa<VPReductionPHIRecipe>(DefiningRecipe) ||
+            isa<VPPartialReductionRecipe>(DefiningRecipe)) &&
            "Unexpected operand order for partial reduction recipe");
   }
   ~VPPartialReductionRecipe() override = default;
 
   VPPartialReductionRecipe *clone() override {
-    return new VPPartialReductionRecipe(Opcode, getOperand(0), getOperand(1));
+    return new VPPartialReductionRecipe(Opcode, getOperand(0), getOperand(1),
+                                        getUnderlyingInstr());
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SamTebbs33 wrote:

Why do we need to set the underlying instruction here? Partial reductions aren't equivalent to their underlying add/sub instruction.

https://github.com/llvm/llvm-project/pull/120272