[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:

Looking into the implementation of `LiveRegUnits`, it works in a similar way to this, but to re-use it we'd need to create a dummy LRU object for each call, which would probably have worse performance than this. The alternative would be to switch this whole pass over to `LiveRegUnits`, but that would be a lot of work, and I don't think it would be enough anyway because this pass tracks more than just liveness.

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