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

[Gaëtan Bossu via llvm-commits]

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@@ -2370,6 +2370,23 @@ struct VPFirstOrderRecurrencePHIRecipe : public VPHeaderPHIRecipe {
 #endif
 };
 
+// Possible variants of a reduction.
+
+// This reduction is ordered and in-loop.
+struct RdxOrderedInLoop {};
+// This reduction is in-loop.
+struct RdxInLoop {};
+// This reduction isn't partial, ordered or in-loop.
+struct RdxNormal {};
+// This reduction is partial and its output is a vector whose length is scaled
+// by the VF.
+struct RdxPartial {
+  // The factor by which the output is scaled down from the VF.
+  unsigned VFScaleFactor;
+};
+typedef std::variant<RdxOrderedInLoop, RdxInLoop, RdxNormal, RdxPartial>
+    RdxStyle;
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gbossu wrote:

Maybe also replace `typedef` with `using`?

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