[llvm] [Analysis][SVE] Improve cost model for some extending masked loads (PR #65957)

[David Sherwood via llvm-commits]

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@@ -2461,6 +2461,25 @@ InstructionCost AArch64TTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
             FP16Tbl, ISD, DstTy.getSimpleVT(), SrcTy.getSimpleVT()))
       return AdjustCost(Entry->Cost);
 
+  if ((ISD == ISD::ZERO_EXTEND || ISD == ISD::SIGN_EXTEND) &&
+      CCH == TTI::CastContextHint::Masked && ST->hasSVEorSME() &&
+      TLI->getTypeAction(Src->getContext(), SrcTy) ==
+          TargetLowering::TypePromoteInteger &&
+      TLI->getTypeAction(Dst->getContext(), DstTy) ==
+          TargetLowering::TypeSplitVector) {
+    // The standard behaviour in the backend for these cases is to split the
+    // extend up into two parts:
+    //  1. Perform an extending load or masked load up to the legal type.
+    //  2. Extend the loaded data to the final type.
+    std::pair<InstructionCost, MVT> SrcLT = getTypeLegalizationCost(Src);
+    Type *LegalTy = EVT(SrcLT.second).getTypeForEVT(Src->getContext());
+    InstructionCost Part1 = AArch64TTIImpl::getCastInstrCost(
+        Opcode, LegalTy, Src, CCH, CostKind, I);
+    InstructionCost Part2 = AArch64TTIImpl::getCastInstrCost(
+        Opcode, Dst, LegalTy, CCH, CostKind, I);
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david-arm wrote:

Good spot! I've changed this to pass in None now, although it didn't make any difference to the cost.

https://github.com/llvm/llvm-project/pull/65957