[llvm] [LV] Use VPReductionRecipe for partial reductions (PR #144908)

[Sander de Smalen via llvm-commits]

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@@ -2408,23 +2414,32 @@ class VPInterleaveRecipe : public VPRecipeBase {
   Instruction *getInsertPos() const { return IG->getInsertPos(); }
 };
 
-/// A recipe to represent inloop reduction operations, performing a reduction on
-/// a vector operand into a scalar value, and adding the result to a chain.
-/// The Operands are {ChainOp, VecOp, [Condition]}.
+/// A recipe to represent inloop, ordered or partial reduction operations. It
+/// performs a reduction on a vector operand into a scalar (vector in the case
+/// of a partial reduction) value, and adds the result to a chain. The Operands
+/// are {ChainOp, VecOp, [Condition]}.
 class VPReductionRecipe : public VPRecipeWithIRFlags {
   /// The recurrence kind for the reduction in question.
   RecurKind RdxKind;
   bool IsOrdered;
   /// Whether the reduction is conditional.
   bool IsConditional = false;
+  /// The scaling factor, relative to the VF, that this recipe's output is
+  /// divided by.
+  /// For outer-loop reductions this is equal to 1.
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sdesmalen-arm wrote:

Out-of-order in-loop reductions are a special kind of a partial reduction, in that they reduce to a VF of 1. Conversely a partial reduction is an out-of-order in-loop reduction reducing to a smaller VF. Adding partial reductions is therefore a natural extension and both can be described with a single property (VFScaleFactor).

With hindsight, I believe that it was a mistake to keep partial reductions in a separate subclass from all the other ones. If this all would have used the same class from the start, the work done for VPMulAccumulateReductionRecipe, VPExtendedReductionRecipe and VPBundle would all have benefited partial reductions.

I'm not saying that we should always keep this as a single class, but I would first like to see the behaviours of partial reductions merged with the functionality for in-loop reductions, so that we don't have to keep adding new code specifically for partial reductions when it can just piggy-back on existing code.

https://github.com/llvm/llvm-project/pull/144908