[llvm] [AArch64][GlobalISel] Add combine for build_vector(unmerge, unmerge, undef, undef) (PR #165539)

[David Green via llvm-commits]

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@@ -133,6 +134,99 @@ bool isZeroExtended(Register R, MachineRegisterInfo &MRI) {
   return MRI.getVRegDef(R)->getOpcode() == TargetOpcode::G_ZEXT;
 }
 
+// This pattern aims to match the following shape to avoid extra mov
+// instructions
+// G_BUILD_VECTOR(
+//   G_UNMERGE_VALUES(src, 0)
+//   G_UNMERGE_VALUES(src, 1)
+//   G_IMPLICIT_DEF
+//   G_IMPLICIT_DEF
+// )
+// ->
+// G_CONCAT_VECTORS(
+//   undef
+//   src
+// )
----------------
davemgreen wrote:

// G_CONCAT_VECTORS(
//   src,
//   undef
// )

https://github.com/llvm/llvm-project/pull/165539