[llvm-dev] Invalid llvm-ir and unreachable code

[Mehdi AMINI via llvm-dev]

On Fri, Mar 12, 2021 at 12:47 AM Jeroen Dobbelaere via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
>
> I have found a case where an optimization pass is barfing on invalid code
> in an unreachable
> basic block. A self referencing GEP '%x = getelementptr %x, 0, 1' is found
> inside a cycle of
> two unreachable basic blocks)
>
> The invalid code and the unreachable basic blocks were introduced by the
> function inliner.
>
> I am wondering what is valid for these cases ?
> (I did not find anything in LangRef, but I might as well have missed it)
>
> - clearly 'dead code' (aka unreachable basic blocks) is valid in an IR ?
> - I assume that the self reference in dead code is not valid ?
>

Yes, self-references are valid in unreachable code, we have test cases for
this, it'll pass the verifier. Passes should be resilient against this
(even though a pass could always do a quick reachability check and delete
all dead blocks as a pre-step).

-- 
Mehdi



> - should inlining do an extra effort of cleaning up such dead code blocks ?
>
> Thanks,
>
> Jeroen Dobbelaere
>
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