[llvm] [LV] Bundle partial reductions inside VPExpressionRecipe (PR #147302)

[Gaëtan Bossu via llvm-commits]

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@@ -2856,6 +2867,19 @@ InstructionCost VPExpressionRecipe::computeCost(ElementCount VF,
     Opcode = Instruction::Sub;
     LLVM_FALLTHROUGH;
   case ExpressionTypes::ExtMulAccReduction: {
+    if (isa<VPPartialReductionRecipe>(ExpressionRecipes.back())) {
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gbossu wrote:

Curious: Were we already implicitly supporting partial reductions because `VPPartialReductionRecipe` is a subclass of `VPReductionRecipe`? Is this patch "just" about computing better costs or does it also add extra capabilities?

https://github.com/llvm/llvm-project/pull/147302