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

[Roman Lebedev via llvm-dev]

On Mon, Mar 15, 2021 at 11:06 AM Jeroen Dobbelaere via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
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> Thanks for all the feedback !
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> The issue I encountered is likely not happening on the main llvm, but I believe the issue is interesting enough to
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> document:
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> - the problematic code construct consists of a cycle of 'dead' basic blocks that conditionally jumps into live blocks.
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>   That jump introduce a connection of a dead self-referring 'getelementptr' to live code through a PHI node.
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> - during SROA, the scoped noalias analysis (full restrict version) does an unbounded (MaxLookup == 0) 'getUnderlyingObject' call on a pointer.
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> - 'getUnderlyingObject' also follows the path to the dead blocks and ends up in the self-referring  'getelementpr'.
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> Does this mean that 'getUnderlyingObject' should also be prepared to be handling 'invalid instructions' in 'dead code' ?
I've just adjusted that function in 36f1c3db66f7268ea3183bcf0bbf05b3e1c570b4
to assert that no cycles are encountered instead of endlessly looping.

But i *think* the problem is in your SROA changes.
Can you link me to the particular patch in question?

> Greetings,
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>
>
> Jeroen Dobbelaere
Roman

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> From: Mehdi AMINI <joker.eph at gmail.com>
> Sent: Sunday, March 14, 2021 05:05
> To: Jeroen Dobbelaere <dobbel at synopsys.com>
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] Invalid llvm-ir and unreachable code
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> On Fri, Mar 12, 2021 at 12:47 AM Jeroen Dobbelaere via llvm-dev <llvm-dev at lists.llvm.org> wrote:
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> Hi all,
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> 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)
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> The invalid code and the unreachable basic blocks were introduced by the function inliner.
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> I am wondering what is valid for these cases ?
> (I did not find anything in LangRef, but I might as well have missed it)
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> - clearly 'dead code' (aka unreachable basic blocks) is valid in an IR ?
> - I assume that the self reference in dead code is not valid ?
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>
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> 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).
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> --
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> Mehdi
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> - should inlining do an extra effort of cleaning up such dead code blocks ?
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> Thanks,
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> Jeroen Dobbelaere
>
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