[llvm] [ExpandReductions] Add support for non-pow2 vector counts (PR #208738)

[Matt Arsenault via llvm-commits]

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@@ -127,6 +149,10 @@ bool expandReductions(Function &F, const TargetTransformInfo *TTI,
         break;
       }
       unsigned RdxOpcode = getArithmeticReductionInstruction(ID);
+      if (!isPowerOf2_32(NumElts)) {
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arsenm wrote:

Why not just fix getShuffleReduction to naturally handle the odd case instead of requiring pre-guarding every use 

https://github.com/llvm/llvm-project/pull/208738