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

[Graham Hunter via llvm-commits]

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@@ -8962,8 +9108,16 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
           Legal->isInvariantAddressOfReduction(SI->getPointerOperand()))
         continue;
 
-      VPRecipeBase *Recipe =
-          RecipeBuilder.tryToCreateWidenRecipe(Instr, Operands, Range, VPBB);
+      VPRecipeBase *Recipe = nullptr;
+
+      PartialReductionChain Chain;
+      if (getInstructionsPartialReduction(Instr, Chain))
+        Recipe =
+            RecipeBuilder.tryToCreatePartialReduction(Range, Chain, Operands);
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huntergr-arm wrote:

I think this creation attempt should be moved to `VPRecipeBuilder::tryToWiden` in the Instruction::Add case, with a fallthrough to the normal add case if we don't find a matching partial reduction.

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