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

[Graham Hunter via llvm-commits]

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@@ -6871,6 +6974,18 @@ void LoopVectorizationCostModel::collectValuesToIgnore() {
     const SmallVectorImpl<Instruction *> &Casts = IndDes.getCastInsts();
     VecValuesToIgnore.insert(Casts.begin(), Casts.end());
   }
+
+  // Ignore any values that we know will be flattened
+  for (auto It : getPartialReductionChains()) {
+    PartialReductionChain Chain = It.second;
+    SmallVector<Value *> PartialReductionValues{Chain.Reduction, Chain.BinOp,
+                                                Chain.ExtendA, Chain.ExtendB,
+                                                Chain.Accumulator};
+    ValuesToIgnore.insert(PartialReductionValues.begin(),
+                          PartialReductionValues.end());
+    VecValuesToIgnore.insert(PartialReductionValues.begin(),
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huntergr-arm wrote:

I'd like that too, but we need to make sure the higher VFs are available to consider. I guess another way to do that would be to make `AArch64TTIImpl::shouldMaximizeVectorBandwidth()` return true for SVE registers as well as NEON registers.

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