[llvm] [VectorCombine] Fold vector.interleave2 with two constant splats (PR #125144)

[Min-Yih Hsu via llvm-commits]

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@@ -3145,6 +3146,45 @@ bool VectorCombine::foldInsExtVectorToShuffle(Instruction &I) {
   return true;
 }
 
+bool VectorCombine::foldInterleaveIntrinsics(Instruction &I) {
+  // If we're interleaving 2 constant splats, for instance `<vscale x 8 x i32>
+  // <splat of 666>` and `<vscale x 8 x i32> <splat of 777>`, we can create a
+  // larger splat
+  // `<vscale x 8 x i64> <splat of ((777 << 32) | 666)>` first before casting it
+  // back into `<vscale x 16 x i32>`.
+  using namespace PatternMatch;
+  const APInt *SplatVal0, *SplatVal1;
+  if (!match(&I, m_Intrinsic<Intrinsic::vector_interleave2>(
+                     m_APInt(SplatVal0), m_APInt(SplatVal1))))
+    return false;
+
+  LLVM_DEBUG(dbgs() << "VC: Folding interleave2 with two splats: " << I
+                    << "\n");
+
+  auto *VTy =
+      cast<VectorType>(cast<IntrinsicInst>(I).getArgOperand(0)->getType());
+  auto *ExtVTy = VectorType::getExtendedElementVectorType(VTy);
+  unsigned Width = VTy->getElementType()->getIntegerBitWidth();
+
+  if (TTI.getInstructionCost(&I, CostKind) <
+      TTI.getCastInstrCost(Instruction::BitCast, I.getType(), ExtVTy,
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mshockwave wrote:

We're really just worrying about the legalization cost here, should `ExtVTy` be an illegal type.

https://github.com/llvm/llvm-project/pull/125144