[llvm] [BasicTTI] Prefer SK_InsertSubvector over SK_Select when both are legal (PR #85302)

[Alexey Bataev via llvm-commits]

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@@ -996,7 +996,17 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
         return TTI::SK_Splice;
       break;
     }
-    case TTI::SK_Select:
+    case TTI::SK_Select: {
+      int NumSubElts;
+      if (Mask.size() > 2 && ShuffleVectorInst::isInsertSubvectorMask(
+                                 Mask, NumSrcElts, NumSubElts, Index)) {
+        if (Index + NumSubElts > NumSrcElts)
+          return Kind;
+        SubTy = FixedVectorType::get(Ty->getElementType(), NumSubElts);
+        return TTI::SK_InsertSubvector;
+      }
+      break;
+    }
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alexey-bataev wrote:

I think better to fix the order in TargetTransformInfoImpl.h::getInstructionCost

https://github.com/llvm/llvm-project/pull/85302