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

[David Green via llvm-commits]

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@@ -512,6 +512,16 @@ class VectorType : public Type {
                            EltCnt.divideCoefficientBy(2));
   }
 
+  /// This static method returns a VectorType with quarter as many elements as the
+  /// input type and the same element type.
+  static VectorType *getQuarterElementsVectorType(VectorType *VTy) {
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davemgreen wrote:

I think this should be more generic than just 4x wider. I believe an ADDP would be a 2 x wider partial reduction for example. The input type needs to be a multiple of the output type, and it might be easier to keep it to a power-2 factor.

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