[llvm] [MachineCP] Correctly handle register masks and sub-registers (PR #122734)

[Oliver Stannard via llvm-commits]

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@@ -117,7 +117,32 @@ class CopyTracker {
 
   DenseMap<MCRegUnit, CopyInfo> Copies;
 
+  // Memoised sets of register units which are preserved by each register mask,
+  // needed to efficiently remove copies which are invalidated by call
+  // instructions.
+  DenseMap<const uint32_t *, BitVector> RegMaskToPreservedRegUnits;
+
 public:
+  /// Get the set of register units which are preserved by RegMaskOp.
+  BitVector &getPreservedRegUnits(const MachineOperand &RegMaskOp,
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ostannard wrote:

I agree with the general principle, but I'm not sure that would help here, because `LiveRegUnits` only tracks liveness, but this pass also needs to track other information about the `COPY` instruction which defined that unit (if any). If that is possible, I don't think I'm qualified to do that re-write, since this is the first time I've looked at this pass. Are you OK with this fix as-is, or do you think that re-write needs to happen first?

https://github.com/llvm/llvm-project/pull/122734