[llvm] [CostModel][AArch64] Make extractelement, with fmul user, free whenev… (PR #111479)

[David Sherwood via llvm-commits]

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@@ -11633,6 +11633,17 @@ InstructionCost BoUpSLP::getTreeCost(ArrayRef<Value *> VectorizedVals) {
   std::optional<DenseMap<Value *, unsigned>> ValueToExtUses;
   DenseMap<const TreeEntry *, DenseSet<Value *>> ExtractsCount;
   SmallPtrSet<Value *, 4> ScalarOpsFromCasts;
+  // Keep track of {Scalar, Index} -> User and User -> {Scalar, Index}.
+  // On AArch64, this helps in fusing a mov instruction, associated with
+  // extractelement, with fmul in the backend so that extractelement is free.
+  DenseMap<std::pair<Value *, unsigned>, SmallVector<Value *, 4>>
+      ScalarAndIdxToUser;
+  DenseMap<Value *, SmallVector<std::pair<Value *, unsigned>, 4>>
+      UserToScalarAndIdx;
+  for (ExternalUser &EU : ExternalUses) {
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david-arm wrote:

I'm not sure if this is important or not, but as far as I understand `Use` and `User` are different things. I think you can have a single user with multiple uses, i.e. `fadd double %0, %0` is a single user of `%0` but there are two uses, or something like that. Perhaps `ExternalUses` is just a bit misleading and actually refers to `Users`?

https://github.com/llvm/llvm-project/pull/111479