[llvm] [VectorCombine] Fold insertelement chains of scalar parts to a bitcast and shuffle (PR #226224)

[Tim Besard via llvm-commits]

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@@ -6036,6 +6037,114 @@ bool VectorCombine::foldInsExtVectorToShuffle(Instruction &I) {
   return true;
 }
 
+/// Try to replace a chain of insertelements of parts of the same scalar with a
+/// bitcast and a shuffle (little endian):
+///   insert (insert poison, (trunc (lshr X, 32)), 0), (trunc X), 1 -->
+///   shuffle (bitcast X to <2 x i32>), poison, <1, 0>
+bool VectorCombine::foldInsertScalarPartsToShuffle(Instruction &I) {
+  auto *VecTy = dyn_cast<FixedVectorType>(I.getType());
+  if (!VecTy)
+    return false;
+
+  // Start from the last insertelement of the chain.
+  if (I.hasOneUse() && isa<InsertElementInst>(I.user_back()) &&
+      I.user_back()->getOperand(0) == &I)
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maleadt wrote:

Done.

https://github.com/llvm/llvm-project/pull/226224