[llvm] [VPlan] Remove legacy costing inside VPBlendRecipe::computeCost (PR #171846)

[Luke Lau via llvm-commits]

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@@ -7081,6 +7081,11 @@ static bool planContainsAdditionalSimplifications(VPlan &Plan,
           return true;
       }
 
+      // The legacy cost model costs non-header phis with a scalar VF as a phi,
+      // but scalar unrolled VPlans will have VPBlendRecipes which emit selects.
+      if (VF.isScalar() && isa<VPBlendRecipe>(&R))
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lukel97 wrote:

I think it's likelier to lead to mismatches if we use vputils::onlyFirstLaneUsed. The loop vectorizer cost model checks VF.isScalar() and AFAIK we don't convert a VPBlendRecipe to a phi if only the first lane is used

https://github.com/llvm/llvm-project/blob/8a4cc440f200154d6514df6db55789991aae5a4f/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp#L6177-L6181

https://github.com/llvm/llvm-project/pull/171846