[llvm-dev] [cfe-dev] RFC: ODR checker for Clang and LLD

Sat Jun 10 12:07:11 PDT 2017

On Sat, Jun 10, 2017 at 11:51 AM, Mehdi AMINI <joker.eph at gmail.com> wrote:

>
>
> 2017-06-10 11:39 GMT-07:00 Peter Collingbourne <peter at pcc.me.uk>:
>
>> On Sat, Jun 10, 2017 at 11:00 AM, Mehdi AMINI <joker.eph at gmail.com>
>> wrote:
>>
>>>
>>>
>>> 2017-06-07 16:32 GMT-07:00 Peter Collingbourne via llvm-dev <
>>> llvm-dev at lists.llvm.org>:
>>>
>>>> On Wed, Jun 7, 2017 at 8:18 AM, David Blaikie <dblaikie at gmail.com>
>>>> wrote:
>>>>
>>>>> Does this need LLVM support - or is there some generic representation
>>>>> that could be used instead? (I guess LLVM would want to be aware of it when
>>>>> merging modules though, so maybe it's worth having a first-class
>>>>> representation - though LLVM module linking could special case a section
>>>>> the same way the linker could/would - not sure what's the better choice
>>>>> there)
>>>>>
>>>>
>>>> The only thing that LLVM needs to do is to have some way to store a
>>>> blob that will be emitted to the object file. In my prototype I just create
>>>> a GlobalVariable with private linkage, I think in the final version I will
>>>> use an MDString referenced by a named MD node. I don't think we would want
>>>> a higher level representation -- I'd imagine that the blob would be
>>>> entirely a property of the source code, so I can't see anything that an
>>>> IR-level pass would want to do with it. It's similar to some parts of debug
>>>> info in that there's no real benefit to representing it as anything other
>>>> than a blob.
>>>>
>>>
>>> If you IR Link two modules, you'd want to check that the hash matches
>>> and diagnose immediately before dropping link once_odr functions, right?
>>>
>>
>> The ODR table (including its string table) is created by the compiler and
>> is unaffected by dropped functions.
>>
>
> Does it mean that when we merge two bitcodes files, the two ODR tables for
> these files are merged as well? In which case if both files contain
> function `foo` we will have two hashes for `foo` in the table of the
> resulting file?
>

The resulting file would have two ODR tables, both of which would have a
entry for the function 'foo'.

Peter

>
> --
> Mehdi
>
>
>
>
>> Functions can be dropped by the optimizer in the non-LTO case as well, of
>> course, and we'd still want to be able to diagnose ODR mismatches in those
>> functions.
>>
>> In any case, I think we'd want the ODR checker to always be driven by the
>> linker, not the IRMover. This is so that we can properly diagnose ODR
>> mismatches between bitcode files and regular object files, and avoid
>> needing to deal with duplicate diagnostics (say if we import the same pair
>> of ODR-mismatching modules into two different ThinLTO backends), and avoid
>> bad interactions with the ThinLTO cache (we don't want to miss diagnostics
>> because of a ThinLTO cache hit). The way I imagine that this would work is
>> that we'd add something to lto::InputFile that gives the client access to
>> the ODR table.
>>
>> That said, we may want to reconsider the blob representation if we want
>> to try and share strings between the object file's string table and the ODR
>> table, as discussed elsewhere.
>>
>> Peter
>>
>>
>>> --
>>> Mehdi
>>>
>>>
>>>
>>>>
>>>> I was thinking (hand-wavingly vague since I don't know that much about
>>>>> object files, etc) one of those auto-appending sections and an array of
>>>>> constchar*+hash attributed to that section. (then even without an
>>>>> odr-checking aware linker (which would compare and discard these sections)
>>>>> the data could be merged & a post-processing pass on the binary could still
>>>>> point out ODR violations without anything in the toolchain (except clang)
>>>>> needing to support this extra info)
>>>>>
>>>>
>>>> Linkers merge section contents by section name, so you wouldn't need
>>>> anything other than for the object files to agree on a section name. The
>>>> odrtab header in my prototype has a size field, so we could use that to
>>>> split an .odrtab section into multiple odrtabs.
>>>>
>>>> Peter
>>>>
>>>> On Tue, Jun 6, 2017 at 10:41 PM Peter Collingbourne via cfe-dev <
>>>>> cfe-dev at lists.llvm.org> wrote:
>>>>>
>>>>>> Hi all,
>>>>>>
>>>>>> I'd like to propose an ODR checker feature for Clang and LLD. The
>>>>>> feature would be similar to gold's --detect-odr-violations feature, but
>>>>>> better: we can rely on integration with clang to avoid relying on debug
>>>>>> info and to perform more precise matching.
>>>>>>
>>>>>> The basic idea is that we use clang's ability to create ODR hashes
>>>>>> for declarations. ODR hashes are computed using all information about a
>>>>>> declaration that is ODR-relevant. If the flag -fdetect-odr-violations is
>>>>>> passed, Clang will store the ODR hashes in a so-called ODR table in each
>>>>>> object file. Each ODR table will contain a mapping from mangled declaration
>>>>>> names to ODR hashes. At link time, the linker will read the ODR table and
>>>>>> report any mismatches.
>>>>>>
>>>>>> To make this work:
>>>>>> - LLVM will be extended with the ability to represent ODR tables in
>>>>>> the IR and emit them to object files
>>>>>> - Clang will be extended with the ability to emit ODR tables using
>>>>>> ODR hashes
>>>>>> - LLD will be extended to read ODR tables from object files
>>>>>>
>>>>>> I have implemented a prototype of this feature. It is available here:
>>>>>> https://github.com/pcc/llvm-project/tree/odr-checker and some
>>>>>> results from applying it to chromium are here: crbug.com/726071
>>>>>> As you can see it did indeed find a number of real ODR violations in
>>>>>> Chromium, including some that wouldn't be detectable using debug info.
>>>>>>
>>>>>> If you're interested in what the format of the ODR table would look
>>>>>> like, that prototype shows pretty much what I had in mind, but I expect
>>>>>> many other aspects of the implementation to change as it is upstreamed.
>>>>>>
>>>>>> Thanks,
>>>>>> --
>>>>>> --
>>>>>> Peter
>>>>>> _______________________________________________
>>>>>> cfe-dev mailing list
>>>>>> cfe-dev at lists.llvm.org
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> --
>>>> Peter
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>
>>
>>
>> --
>> --
>> Peter
>>
>
>

-- 
-- 
Peter
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