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

[Sam Tebbs via llvm-commits]

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@@ -2266,6 +2275,39 @@ 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 {
+  unsigned Opcode;
+
+public:
+  template <typename IterT>
+  VPPartialReductionRecipe(unsigned ReductionOpcode,
+                           iterator_range<IterT> Operands)
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SamTebbs33 wrote:

I wanted to do that but couldn't figure out how to create the operands list in the call to the `VPRecipeWithIRFlags` constructor if we just have the two operand values. Do let me know if you know how to do that.

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