[llvm] [IR] Avoid self-referencing values caused by PHI node removal (PR #129501)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

I am assuming this is what cases the failure: https://github.com/llvm/llvm-project/blob/eef5ea0c42fc07ef2c948be59b57d0df8ec801ca/llvm/lib/IR/Verifier.cpp#L5150-L5151

Having looked closer into it, I think the fault is in the Verifier. Since dominance is not defined in unreachable code, checking dominance is explictly neutred: https://github.com/llvm/llvm-project/blob/b00ad366323cee324b0294f6395c33ae4b047e2c/llvm/include/llvm/Support/GenericDomTree.h#L474-L475
An instruction referencing itself is a special case of violating dominance. Hence, it should be skipped in unreachable code.

Alternatively, InstSimplify needs to be changed in that before replacing an PHIInst (which is allowed to reference itself **IF** the incoming edge is dominated by the PHI -- the check that is skipped in unreachable code) with its only value does not cause any other instruction to reference itself (which is not allowed without exception for unreachable code).

In any case, this is not a problem with JumpThreading and there might also be cases in SimplifyCFG that trigger this. For instance, JumpThreading could just replace the instruction in the BB with an UnreachableInst instead of deleting it. That would just toss the same problem to SimplifyCFG.

https://github.com/llvm/llvm-project/pull/129501