[llvm] [CodeGen] Keep physreg copies when successors use-before-def (PR #182732)

[via llvm-commits]

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@@ -563,6 +563,26 @@ void MachineCopyPropagation::readSuccessorLiveIns(
   }
 }
 
+// Conservatively detect a physical-register use in a successor block before
+// the register is redefined there. This guards against stale/incomplete
+// live-in information, which can otherwise make a required copy look
+// dead at block exit
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Sightem wrote:

Indeed, you were right. upon a closer look this ended up being a backend issue. 

root cause was in our out of tree eZ80 backend, several `PUSH16/24AF` preservation paths were marking the AF use as `undef`. That made a required A value copy look dead to MCP, so MCP deleted it.

Upon removing those AF `setIsUndef()`s and reverting the MCP workaround entirely the miscompile is gone and rendering is correct again. I will close this PR accordingly.

https://github.com/llvm/llvm-project/pull/182732