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

[Sam Tebbs via llvm-commits]

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@@ -8663,6 +8663,113 @@ VPReplicateRecipe *VPRecipeBuilder::handleReplication(Instruction *I,
   return Recipe;
 }
 
+void VPRecipeBuilder::collectScaledReductions(VFRange &Range) {
+  // Find all possible partial reductions
+  SmallVector<PartialReductionChain, 1> PartialReductionChains;
+  for (const auto &[Phi, RdxDesc] : Legal->getReductionVars())
+    if (std::optional<PartialReductionChain> Chain =
+            getScaledReduction(Phi, RdxDesc, Range))
+      PartialReductionChains.push_back(*Chain);
+
+  // A partial reduction is invalid if any of its extends are used by
+  // something that isn't another partial reduction. This is because the
+  // extends are intended to be lowered along with the reduction itself.
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SamTebbs33 wrote:

I've looked into what the assembly looks like if we remove the restriction, and it looks pretty bad so I think removing the restriction should come in a future patch with additional work on the cost model.

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