[llvm] [GlobalISel] Turn shuffle a, b, mask -> shuffle undef, b, mask iff mask does not reference a (PR #115377)

[Matt Arsenault via llvm-commits]

================
@@ -7726,3 +7726,65 @@ bool CombinerHelper::matchShuffleUndefRHS(MachineInstr &MI,
 
   return true;
 }
+
+static void commuteMask(MutableArrayRef<int> Mask, const unsigned NumElems) {
+  const unsigned MaskSize = Mask.size();
+  for (unsigned I = 0; I < MaskSize; ++I) {
+    int Idx = Mask[I];
+    if (Idx < 0)
+      continue;
+
+    if (Idx < (int)NumElems)
+      Mask[I] = Idx + NumElems;
+    else
+      Mask[I] = Idx - NumElems;
+  }
+}
+
+bool CombinerHelper::matchShuffleDisjointMask(MachineInstr &MI,
+                                              BuildFnTy &MatchInfo) {
+
+  auto &Shuffle = cast<GShuffleVector>(MI);
+  // If any of the two inputs is already undef, don't check the mask again to
+  // prevent infinite loop
+  if (getOpcodeDef(TargetOpcode::G_IMPLICIT_DEF, Shuffle.getSrc1Reg(), MRI))
+    return false;
+
+  if (getOpcodeDef(TargetOpcode::G_IMPLICIT_DEF, Shuffle.getSrc2Reg(), MRI))
+    return false;
+
+  ArrayRef<int> Mask = Shuffle.getMask();
+  const LLT Src1Ty = MRI.getType(Shuffle.getSrc1Reg());
+
+  const unsigned NumSrcElems = Src1Ty.isVector() ? Src1Ty.getNumElements() : 1;
+
+  bool TouchesSrc1 = false;
+  bool TouchesSrc2 = false;
+  const unsigned NumElems = Mask.size();
+  for (unsigned Idx = 0; Idx < NumElems; ++Idx) {
+    if (Mask[Idx] < 0)
+      continue;
+
+    if (Mask[Idx] < (int)NumSrcElems)
+      TouchesSrc1 = true;
+    else
+      TouchesSrc2 = true;
+  }
+
+  if (!(TouchesSrc1 ^ TouchesSrc2))
+    return false;
+
+  Register NewSrc1 = Shuffle.getSrc1Reg();
+  SmallVector<int, 16> NewMask(Mask);
+  if (TouchesSrc2) {
+    NewSrc1 = Shuffle.getSrc2Reg();
+    commuteMask(NewMask, NumSrcElems);
+  }
+
+  MatchInfo = [=, &Shuffle](MachineIRBuilder &B) {
+    Register Undef = B.buildUndef(Src1Ty).getReg(0);
----------------
arsenm wrote:

> COPY might be different. But for G_IMPLCIT_DEF state for which types it is legal (maybe all) in your legalizer. Never ever assume that something is legal.

And there are restrictions on what the legality rules must be. We should enforce in the verifier that G_IMPLICIT_DEF is legal for all types used in any operand, such that it should never be necessary to check if it is legal 

https://github.com/llvm/llvm-project/pull/115377