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

[Nicholas Guy via llvm-commits]

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@@ -2374,6 +2380,44 @@ class VPReductionPHIRecipe : public VPHeaderPHIRecipe,
   bool isInLoop() const { return IsInLoop; }
 };
 
+/// A recipe for forming partial reductions. In the loop, an accumulator and
+/// vector operand are added together and passed to the next iteration as the
+/// next accumulator. After the loop body, the accumulator is reduced to a
+/// scalar value.
+class VPPartialReductionRecipe : public VPRecipeWithIRFlags {
+  Instruction &ReductionInst;
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NickGuy-Arm wrote:

Can `getUnderlyingInst()` be used for cloning instead? There's little benefit to storing the instruction twice

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