[llvm] [BasicTTI] Use getTypeLegalizationCost to generalize vector cast cost. (PR #107303)

[Philip Reames via llvm-commits]

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@@ -1171,9 +1171,50 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
                                           CostKind, I));
       }
 
+      // Check if the wider type of Src and Dst needs to be legalized. If it
+      // does, compute the cost of the cast based on vectors with the same
+      // number of elements as the legalized widest type. Don't directly return
+      // the cost, as scalarization may be more profitable, which has its cost
+      // computed below.
+      // TODO: Use the more general logic below to replace the specific logic
+      // above to also handle cost for legalization via splitting above.
+      InstructionCost LegalizedCost = InstructionCost::getInvalid();
+      if (SrcVTy->getElementCount().isVector() &&
+          DstVTy->getElementCount().isVector()) {
+        Type *WiderTy =
+            Src->getScalarSizeInBits() < Dst->getScalarSizeInBits() ? Dst : Src;
+        auto [WiderLegalCost, WiderLegalTy] = getTypeLegalizationCost(WiderTy);
+        if (WiderLegalTy.isVector() &&
+            TLI->getValueType(DL, WiderTy) != WiderLegalTy) {
+          Type *SplitDstTy = VectorType::get(
+              DstVTy->getElementType(), WiderLegalTy.getVectorElementCount());
+          Type *SplitSrcTy = VectorType::get(
+              SrcVTy->getElementType(), WiderLegalTy.getVectorElementCount());
+          if (SplitDstTy != Dst && SplitSrcTy != Src) {
+            bool SplitSrc = TLI->getTypeAction(Src->getContext(),
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preames wrote:

Since you haven't change Src or Dst, this logic should be dead.  The splitting part of this would be handled above.

I think what you're actual implementing here is a mixture of vector widening, and splitting *after* widening.  I think you need to separate those two to avoid costing problems.

https://github.com/llvm/llvm-project/pull/107303