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

[Sam Tebbs via llvm-commits]

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@@ -8663,6 +8663,81 @@ VPReplicateRecipe *VPRecipeBuilder::handleReplication(Instruction *I,
   return Recipe;
 }
 
+/// 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
+/// information, or null if no scalable reduction was found.
+static std::optional<PartialReductionChain>
+getScaledReduction(PHINode *PHI, const RecurrenceDescriptor &Rdx,
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SamTebbs33 wrote:

Nice idea, done.

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