[llvm] [AArch64] [CostModel] Improve costs for scalar inserts into fixed-length SVE constant vector (PR #223638)

[Utpal Bora via llvm-commits]

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@@ -4866,9 +4866,57 @@ InstructionCost AArch64TTIImpl::getScalarizationOverhead(
     TTI::VectorInstrContext VIC) const {
   if (isa<ScalableVectorType>(Ty))
     return InstructionCost::getInvalid();
-  if (Ty->getElementType()->isFloatingPointTy())
-    return BaseT::getScalarizationOverhead(Ty, DemandedElts, Insert, Extract,
-                                           CostKind);
+  if (Ty->getElementType()->isFloatingPointTy()) {
+    InstructionCost Cost = BaseT::getScalarizationOverhead(
+        Ty, DemandedElts, Insert, Extract, CostKind);
+
+    if (!Insert || VL.empty())
+      return Cost;
+
+    auto LT = getTypeLegalizationCost(Ty);
+    if (!ST->isNeonAvailable() || !LT.second.isFixedLengthVector() ||
+        LT.second.getFixedSizeInBits() <= 128)
+      return Cost;
+
+    auto HasNonUniformConstants = [&VL]() -> bool {
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utpalbora wrote:

Thank you for the review. I agree that splitting this PR will be cleaner.
I am going to split the cost-model work as below:
- Fix the more accurate cost for `BUILD_VECTOR`s for wider vectors using SVE by costing inserts into each 128b segment
- Handle the specific case of splat(constant) + inserts
- Generalize per-lane scalarization costs for wide fixed-length vectors for other types

https://github.com/llvm/llvm-project/pull/223638