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

[David Green via llvm-commits]

================
@@ -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
+// )
+bool matchCombineBuildUnmerge(MachineInstr &MI, MachineRegisterInfo &MRI,
+                              Register &UnmergeSrc) {
+  assert(MI.getOpcode() == TargetOpcode::G_BUILD_VECTOR);
+
+  unsigned UnmergeInstrCount = 0;
+  unsigned UndefInstrCount = 0;
+
+  unsigned UnmergeEltCount = 0;
+  unsigned UnmergeEltSize = 0;
+
+  Register UnmergeSrcTemp;
+
+  std::set<int> KnownRegs;
+
+  for (auto Use : MI.all_uses()) {
+    auto *Def = getDefIgnoringCopies(Use.getReg(), MRI);
+
+    if (!Def) {
+      return false;
+    }
+
+    unsigned Opcode = Def->getOpcode();
+
+    switch (Opcode) {
+    default:
+      return false;
+    case TargetOpcode::G_IMPLICIT_DEF:
+      ++UndefInstrCount;
+      break;
+    case TargetOpcode::G_UNMERGE_VALUES:
----------------
davemgreen wrote:

What would happen if we had this?
```
// G_BUILD_VECTOR(
//   G_UNMERGE_VALUES(src, 0)
//   G_IMPLICIT_DEF
//   G_UNMERGE_VALUES(src, 1)
//   G_IMPLICIT_DEF
// )
```
or 
```
// G_BUILD_VECTOR(
//   G_UNMERGE_VALUES(src, 1)
//   G_UNMERGE_VALUES(src, 0)
//   G_IMPLICIT_DEF
//   G_IMPLICIT_DEF
// )
```

I believe it needs to check that the first operand is an unmerge from def 0, the remaining n/2-1 operands are the same unmerge from the correct def number, and the remaining n2/defs are undef.

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