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

[Philip Reames 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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preames wrote:

I think you need to check against reserved registers here.

I'm running through the checks in forwardUses, and it looks like none of the others apply.  Just to confirm, the fact the registers are *equal* means that if there are sub-registers involved they must be the same sub-register right?  

I glanced at the MaybeDeadCopies mechanism which seems related, but I don't think we want to use it here as we don't need to worry about block out liveness, or updating debug info here.  Nor does the deadness of this copy depend on following regmasks.  

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