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

[Roman Lebedev via llvm-dev]

On Fri, Mar 12, 2021 at 11:47 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 ?
Yep.

> - I assume that the self reference in dead code is not valid ?
Unreachable code is permitted to take various weird shapes and forms.
So no, that is valid.

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

Which pass?
Generally, it (or the users of that code) should either use DominatoTree
to avoid handling unreachable blocks, or it should be hardened to deal
with such situations gracefully.

> Thanks,
>
> Jeroen Dobbelaere
Roman

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