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

[Matt Arsenault 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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arsenm wrote:

This sounds like it's working around something that should have failed the verifier 

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