[llvm] [AArch64] Increase fixed-length SVE scalarization costs (PR #226120)

[Utpal Bora via llvm-commits]

================
@@ -4891,12 +4891,26 @@ 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);
-  unsigned VecInstCost =
-      CostKind == TTI::TCK_CodeSize ? 1 : ST->getVectorInsertExtractBaseCost();
-  return DemandedElts.popcount() * (Insert + Extract) * VecInstCost;
+
+  std::pair<InstructionCost, MVT> LT = getTypeLegalizationCost(Ty);
+  if (!ST->useSVEForFixedLengthVectors(LT.second)) {
+    if (Ty->getElementType()->isFloatingPointTy())
+      return BaseT::getScalarizationOverhead(Ty, DemandedElts, Insert, Extract,
+                                             CostKind);
+
+    unsigned VecInstCost = CostKind == TTI::TCK_CodeSize
+                               ? 1
+                               : ST->getVectorInsertExtractBaseCost();
+    return DemandedElts.popcount() * (Insert + Extract) * VecInstCost;
+  }
+
+  // Scalarizing fixed-length SVE vectors is expensive. Add an extra cost to
+  // prevent the SLP vectorizer from selecting unprofitable trees.
+  // TODO: Model the scalarization overhead of wide fixed-length SVE vectors
+  // accurately.
+  return BaseT::getScalarizationOverhead(Ty, DemandedElts, Insert, Extract,
+                                         CostKind) +
+         5;
----------------
utpalbora wrote:

Our experiments with SPEC CPU 2017 and 2026 using a scalarization overhead of 500 did not show any additional benefit over overhead of 5. So 5 appears to be sufficient for this heuristic without unnecessarily inflating the cost.

Longer term, this should be superseded by a more accurate cost model for constant vector materialization.

https://github.com/llvm/llvm-project/pull/226120