[llvm] [AArch64] Add a subvector extract cost. (PR #121472)

[Alexey Bataev via llvm-commits]

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@@ -4706,10 +4706,21 @@ InstructionCost AArch64TTIImpl::getShuffleCost(
   }
 
   Kind = improveShuffleKindFromMask(Kind, Mask, Tp, Index, SubTp);
-  // Treat extractsubvector as single op permutation.
   bool IsExtractSubvector = Kind == TTI::SK_ExtractSubvector;
-  if (IsExtractSubvector && LT.second.isFixedLengthVector())
+  // A sebvector extract can be implemented with a ext (or trivial extract, if
+  // from lane 0). This currently only handles low or high extracts to prevent
+  // SLP vectorizer regressions.
+  if (IsExtractSubvector && LT.second.isFixedLengthVector()) {
+    if (LT.second.is128BitVector() &&
+        cast<FixedVectorType>(SubTp)->getNumElements() ==
+            LT.second.getVectorNumElements() / 2) {
+      if (Index == 0)
+        return 0;
+      else if (Index == LT.second.getVectorNumElements() / 2)
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alexey-bataev wrote:

Drop `else`

https://github.com/llvm/llvm-project/pull/121472