[llvm] [LV] Use VPReductionRecipe for partial reductions (PR #147513)

[Florian Hahn via llvm-commits]

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@@ -2392,6 +2392,29 @@ struct VPFirstOrderRecurrencePHIRecipe : public VPHeaderPHIRecipe {
 #endif
 };
 
+// Possible variants of a reduction.
+
+// This reduction is ordered and in-loop.
+struct RdxOrdered {};
+// This reduction is in-loop.
+struct RdxInLoop {};
+// This reduction is unordered with the partial result scaled down by some
+// factor.
+struct RdxUnordered {
+  unsigned VFScaleFactor;
+};
+using ReductionStyle = std::variant<RdxOrdered, RdxInLoop, RdxUnordered>;
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fhahn wrote:

Would be good to use `///` here so this gets included in the docs properly.

https://github.com/llvm/llvm-project/pull/147513