[llvm] [AArch64] Add costs for LD3/LD4 shuffles. (PR #89268)

[Simon Pilgrim via llvm-commits]

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@@ -2978,6 +2978,29 @@ bool ShuffleVectorInst::isInterleaveMask(
   return true;
 }
 
+/// Check if the mask is a DE-interleave mask of the given factor
+/// \p Factor like:
+///     <Index, Index+Factor, ..., Index+(NumElts-1)*Factor>
+bool ShuffleVectorInst::isDeInterleaveMaskOfFactor(ArrayRef<int> Mask,
+                                                   unsigned Factor,
+                                                   unsigned &Index) {
+  // Check all potential start indices from 0 to (Factor - 1).
+  for (Index = 0; Index < Factor; Index++) {
+    unsigned I = 0;
+
+    // Check that elements are in ascending order by Factor. Ignore undef
+    // elements.
+    for (; I < Mask.size(); I++)
+      if (Mask[I] >= 0 && static_cast<unsigned>(Mask[I]) != Index + I * Factor)
+        break;
+
+    if (I == Mask.size())
+      return true;
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RKSimon wrote:

Maybe only set the output Index on success?

https://github.com/llvm/llvm-project/pull/89268