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

[Simon Pilgrim via llvm-commits]

================
@@ -3815,6 +3815,27 @@ InstructionCost AArch64TTIImpl::getSpliceCost(VectorType *Tp, int Index) {
   return LegalizationCost * LT.first;
 }
 
+/// Check if the mask is a DE-interleave mask of the given factor
+/// \p Factor like:
+///     <Index, Index+Factor, ..., Index+(NumElts-1)*Factor>
+static bool isDeInterleaveMaskOfFactor(ArrayRef<int> Mask, unsigned Factor) {
+  // Check all potential start indices from 0 to (Factor - 1).
+  for (unsigned 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;
+  }
+
+  return false;
+}
----------------
RKSimon wrote:

Move this to ShuffleVectorInst? Its likely we can use that in some general interleaved detection code.

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