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

[Florian Hahn via llvm-commits]

================
@@ -8663,6 +8663,110 @@ VPReplicateRecipe *VPRecipeBuilder::handleReplication(Instruction *I,
   return Recipe;
 }
 
+/// Find all possible partial reductions in the loop and track all of those that
+/// are valid so recipes can be formed later.
+void VPRecipeBuilder::collectScaledReductions(VFRange &Range) {
+  // Find all possible partial reductions.
+  SmallVector<std::pair<PartialReductionChain, unsigned>, 1>
+      PartialReductionChains;
+  for (const auto &[Phi, RdxDesc] : Legal->getReductionVars())
+    if (std::optional<std::pair<PartialReductionChain, unsigned>> Pair =
+            getScaledReduction(Phi, RdxDesc, Range))
+      PartialReductionChains.push_back(*Pair);
+
+  // 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.
+
+  // Build up a set of partial reduction bin ops for efficient use checking.
+  SmallSet<User *, 4> PartialReductionBinOps;
+  for (auto It : PartialReductionChains) {
----------------
fhahn wrote:

```suggestion
  for (const auto &[_, PartialRdx] : PartialReductionChains) {
```

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