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

[Graham Hunter via llvm-commits]

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@@ -437,6 +469,15 @@ Value *VPInstruction::generate(VPTransformState &State) {
     Value *Cond = State.get(getOperand(0));
     Value *Op1 = State.get(getOperand(1));
     Value *Op2 = State.get(getOperand(2));
+    auto *CondVec = dyn_cast<VectorType>(Cond->getType());
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huntergr-arm wrote:

This feels fragile. Perhaps better to change the place where the select is introduced (`LoopVectorizationPlanner::adjustRecipesForReductions`) if it's a partial reduction? I guess that may mean we need to store details of partial reductions in `LoopVectorizationPlanner` and pass those to `VPRecipeBuilder` in the constructor.

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