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

[Ryan Cowan via llvm-commits]

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@@ -3463,6 +3463,91 @@ static bool isConstValidTrue(const TargetLowering &TLI, unsigned ScalarSizeBits,
          isConstTrueVal(TLI, Cst, IsVector, IsFP);
 }
 
+// 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(
+//   src,
+//   undef
+// )
+bool CombinerHelper::matchCombineBuildUnmerge(MachineInstr &MI,
+                                              MachineRegisterInfo &MRI,
+                                              Register &UnmergeSrc) const {
+  assert(MI.getOpcode() == TargetOpcode::G_BUILD_VECTOR);
+
+  unsigned BuildUseCount = MI.getNumOperands() - 1;
+
+  if (BuildUseCount % 2 != 0)
+    return false;
+
+  unsigned NumUnmerge = BuildUseCount / 2;
+
+  // Check the first operand is an unmerge
+  auto *MaybeUnmerge = getDefIgnoringCopies(MI.getOperand(1).getReg(), MRI);
+  if (MaybeUnmerge->getOpcode() != TargetOpcode::G_UNMERGE_VALUES)
+    return false;
+
+  // Check that the resultant concat will be legal
+  auto UnmergeEltSize =
+      MRI.getType(MaybeUnmerge->getOperand(1).getReg()).getScalarSizeInBits();
+  auto UnmergeEltCount = MaybeUnmerge->getNumDefs();
+
+  if (UnmergeEltCount < 2 || (UnmergeEltSize * UnmergeEltCount != 64 &&
+                              UnmergeEltSize * UnmergeEltCount != 128))
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HolyMolyCowMan wrote:

This optimisation runs both before and after legalisation so I've added checks when run in the post-legalizer pass.

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