[llvm] [MachineCopyPropagation] Recognise and delete no-op moves produced after forwarded uses (PR #129889)

[Alex Bradbury via llvm-commits]

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@@ -971,6 +971,18 @@ void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
 
     forwardUses(MI);
 
+    // It's possible that the previous transformation has resulted in a no-op
+    // register move (i.e. one where source and destination registers are the
+    // same). If so, delete it.
+    CopyOperands = isCopyInstr(MI, *TII, UseCopyInstr);
+    if (CopyOperands &&
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asb wrote:

I've added a check for reserved registers.

Yes, my understanding and intent is that this only applies for precise register/subregister matches.

Thanks for taking a look through other mechanisms in the file. I'd also looked for something else to piggyback on and came to the same conclusion that there's not existing logic that's quite right.

https://github.com/llvm/llvm-project/pull/129889