[llvm] [LoopVectorizer] Add support for partial reductions (PR #92418)

[Nicholas Guy via llvm-commits]

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@@ -111,20 +139,46 @@ class VPRecipeBuilder {
   VPHistogramRecipe *tryToWidenHistogram(const HistogramInfo *HI,
                                          ArrayRef<VPValue *> Operands);
 
+  /// Examines reduction operations to see if the target can use a cheaper
+  /// operation with a wider per-iteration input VF and narrower PHI VF.
+  /// Returns a struct containing the ratio between the two VFs and other cached
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NickGuy-Arm wrote:

Is this still accurate with the change to return a `std::pair`?

https://github.com/llvm/llvm-project/pull/92418