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

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Wed Jun 14 18:14:05 PDT 2017 

On Wed, Jun 14, 2017 at 1:41 PM Peter Collingbourne <peter at pcc.me.uk> wrote:

> On Wed, Jun 14, 2017 at 12:47 PM, David Blaikie <dblaikie at gmail.com>
> wrote:
>
>>
>>
>> On Tue, Jun 13, 2017, 11:30 PM Peter Collingbourne <peter at pcc.me.uk>
>> wrote:
>>
>>> On Tue, Jun 13, 2017 at 11:06 PM, David Blaikie <dblaikie at gmail.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> On Tue, Jun 13, 2017 at 10:05 PM Peter Collingbourne <peter at pcc.me.uk>
>>>> wrote:
>>>>
>>>>> On Tue, Jun 13, 2017 at 8:48 PM, David Blaikie <dblaikie at gmail.com>
>>>>> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Tue, Jun 13, 2017 at 8:43 PM Peter Collingbourne <peter at pcc.me.uk>
>>>>>> wrote:
>>>>>>
>>>>>>> On Tue, Jun 13, 2017 at 7:54 PM, David Blaikie <dblaikie at gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Tue, Jun 13, 2017 at 6:34 PM Peter Collingbourne via cfe-dev <
>>>>>>>> cfe-dev at lists.llvm.org> wrote:
>>>>>>>>
>>>>>>>>> On Wed, Jun 7, 2017 at 11:28 PM, Peter Collingbourne <
>>>>>>>>> peter at pcc.me.uk> wrote:
>>>>>>>>>
>>>>>>>>>> On Wed, Jun 7, 2017 at 8:06 PM, Sean Silva <chisophugis at gmail.com
>>>>>>>>>> > wrote:
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On Wed, Jun 7, 2017 at 4:31 PM, Peter Collingbourne <
>>>>>>>>>>> peter at pcc.me.uk> wrote:
>>>>>>>>>>>
>>>>>>>>>>>> On Wed, Jun 7, 2017 at 12:17 AM, Sean Silva <
>>>>>>>>>>>> chisophugis at gmail.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> Very nice and simple implementation!
>>>>>>>>>>>>>
>>>>>>>>>>>>> Do you have any statistics on how large these odr tables are
>>>>>>>>>>>>> compared to other object file data? I assume that if these tables contain
>>>>>>>>>>>>> full mangled symbol names, they could end up being very large and may want
>>>>>>>>>>>>> to share the symbol name strings with the overall string table in the .o
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Looking at Chromium's object files it looks like the total size
>>>>>>>>>>>> of the odrtabs is about 50% of the total size of the object files, which
>>>>>>>>>>>> isn't great. The current implementation only looks at records, so I imagine
>>>>>>>>>>>> that it would be hard to share any of the strings that I'm currently
>>>>>>>>>>>> creating. (I guess it's possible that some types will have a mangled vtable
>>>>>>>>>>>> name in the string table, so we may be able to share a little that way.)
>>>>>>>>>>>> Note however that this was without debug info.
>>>>>>>>>>>>
>>>>>>>>>>>> One option for reducing size would be to
>>>>>>>>>>>> 1) store hashes of ODR names in ODR tables, per Rui's
>>>>>>>>>>>> suggestion (alongside a reference to the name itself in the string table)
>>>>>>>>>>>> 2) compress the string table for the ODR names with a standard
>>>>>>>>>>>> compression algorithm like gzip.
>>>>>>>>>>>> This wouldn't seem to affect link time performance much because
>>>>>>>>>>>> I think we should only need to look at the strings if we see a ODR name
>>>>>>>>>>>> hash match together with an ODR hash mismatch, which would mean an ODR
>>>>>>>>>>>> violation with a high probability (i.e. unless there was an ODR name hash
>>>>>>>>>>>> collision, we have found an ODR violation). If we don't expect a lot of
>>>>>>>>>>>> sharing with regular string tables (see below), it seems even more
>>>>>>>>>>>> reasonable.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Neat observation!
>>>>>>>>>>>
>>>>>>>>>>> FWIW, it is a birthday problem type situation though, so for a
>>>>>>>>>>> 32-bit hash, we would expect a collision in about 1 in 2^16 distinct hashes
>>>>>>>>>>> (and 2^16 seems pretty easy to hit in a large project). So 64-bit hashes
>>>>>>>>>>> might be preferable.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Oh right, good point, using a 64-bit hash does seem like a good
>>>>>>>>>> idea here.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>> Also, do you have any numbers on the performance of your
>>>>>>>>>>>>> initial implementation?
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> I measured the link time for chromium's unit_tests (the largest
>>>>>>>>>>>> single binary in chromium) at 5.05s without ODR checks and 6.61s with ODR
>>>>>>>>>>>> checks. So about 30% overhead, but in absolute terms it doesn't seem too
>>>>>>>>>>>> bad. So I think this may be acceptable for an initial implementation, but
>>>>>>>>>>>> it certainly seems worth trying to do better.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> I know that things aren't currently apples-to-apples, but how
>>>>>>>>>>> does that compare to gold?
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> I will measure it.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> For that unit_tests binary I measured the overhead at about 5
>>>>>>>>> seconds (average of 10 runs). That is with debug info, of course.
>>>>>>>>>
>>>>>>>>> W.r.t. LLD and having it always on by default (and hence making it
>>>>>>>>>>>>> as fast as possible), it seems like right now you are implementing the
>>>>>>>>>>>>> checking process with a hash table. That's simple and fine for a first
>>>>>>>>>>>>> implementation, but it's probably worth mentioning in a comment the problem
>>>>>>>>>>>>> of checking the tables, at least from the linker's perspective, does fit
>>>>>>>>>>>>> into a map-reduce pattern and could be easily parallelized if needed. E.g.
>>>>>>>>>>>>> a parallel sort to coalesce all entries for symbols of the same name
>>>>>>>>>>>>> followed by a parallel forEach to check each bucket with the same symbol
>>>>>>>>>>>>> name (roughly speaking).
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Right, that's one approach. I was thinking of a simpler
>>>>>>>>>>>> approach where at compile time we sort ODR names by hash and partition them
>>>>>>>>>>>> using (say) the upper bits of the hash, so that at link time we can have N
>>>>>>>>>>>> threads each building a hash table for a specific partition.
>>>>>>>>>>>>
>>>>>>>>>>>> And of course this work can be started right after symbol
>>>>>>>>>>>> resolution finishes and parallelised with the rest of the work done by the
>>>>>>>>>>>> linker.
>>>>>>>>>>>>
>>>>>>>>>>>> Even better than doing it faster is just doing less work.
>>>>>>>>>>>>> There's a lot of work that the linker is already doing that may be reusable
>>>>>>>>>>>>> for the ODR checking.
>>>>>>>>>>>>> E.g.
>>>>>>>>>>>>> - maybe we could get the coalescing step as a byproduct of our
>>>>>>>>>>>>> existing string deduping, which we are generally doing anyway.
>>>>>>>>>>>>> - we are already coalescing symbol names for the symbol table.
>>>>>>>>>>>>> If the ODR table is keyed off of symbols in the binary that we are
>>>>>>>>>>>>> inserting into the symbol table, then I think we could do the entire ODR
>>>>>>>>>>>>> check with no extra "string" work on LLD's part.
>>>>>>>>>>>>>
>>>>>>>>>>>>> I see Rui already mentioned some of this in
>>>>>>>>>>>>> https://bugs.chromium.org/p/chromium/issues/detail?id=726071#c4
>>>>>>>>>>>>> .
>>>>>>>>>>>>> You mentioned that not everything is necessarily directly
>>>>>>>>>>>>> keyed on a symbol (such as types), but I think that it would really
>>>>>>>>>>>>> simplify things if the check was done as such. Do you have any idea exactly
>>>>>>>>>>>>> how much of the things that we want to check are not keyed on symbols? If
>>>>>>>>>>>>> most things are keyed on symbols, for the things we are not we can just
>>>>>>>>>>>>> emit extra symbols prefixed by __clang_odr_check_ or whatever.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Since the current implementation only works with records there
>>>>>>>>>>>> is basically zero overlap right now between ODR names and symbols. I
>>>>>>>>>>>> suppose that I could estimate the amount of function overlap in a
>>>>>>>>>>>> hypothetical implementation that computes ODR hashes of functions by
>>>>>>>>>>>> comparing the number of *_odr functions after clang has finished IRgen with
>>>>>>>>>>>> the number after optimization finishes. This of course would be strictly
>>>>>>>>>>>> more than functions + types.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Wouldn't any function or symbol using the record type have the
>>>>>>>>>>> type name somewhere in it? If we used an offset+length encoding (instead of
>>>>>>>>>>> offset + NUL termination) we might be able to reuse it then (at some cost
>>>>>>>>>>> in finding the reference).
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> That may be possible with some work in the string table builder.
>>>>>>>>>> But at that point of course we're not dealing with regular symbols any
>>>>>>>>>> more. I guess we could have two ODR tables per object file: an array of
>>>>>>>>>> (ODR hash, location) tuples for ODR names that correspond to symbol table
>>>>>>>>>> symbols (i.e. Rui's proposal on the chromium bug), and an array of (ODR
>>>>>>>>>> name, ODR hash, location) tuples for all other ODR names. I guess if we
>>>>>>>>>> wanted a "low overhead" mode we could just omit the second table or put
>>>>>>>>>> fewer symbols in it.
>>>>>>>>>>
>>>>>>>>>> With debug info surely there is some sort of string representing
>>>>>>>>>>> the record name or something like that, no?
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Not the record name on its own (they do appear but a bit
>>>>>>>>>> awkwardly -- each namespace component is stored in a separate string), but
>>>>>>>>>> if the record has at least one member function the mangled type name will
>>>>>>>>>> appear somewhere in .debug_str, so we could in principle reuse that with
>>>>>>>>>> the offset/length trick.
>>>>>>>>>>
>>>>>>>>>> I guess we may have to have our "low-overhead" user-facing
>>>>>>>>>>> behavior be a bit more nuanced. E.g.:
>>>>>>>>>>> 1. does this feature bloat object files significantly
>>>>>>>>>>> 2. does this feature slow down link times significantly
>>>>>>>>>>>
>>>>>>>>>>> Intuitively, it seems like we should be able to get 1. when
>>>>>>>>>>> debug info happens to be enabled (not sure about split dwarf?) and possibly
>>>>>>>>>>> in all cases at the cost of complexity. We may be able to get 2. in all
>>>>>>>>>>> cases with proper design.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> I think that would be my rough assessment as well. I think we
>>>>>>>>>> have a good shot at 1 for all cases with some of the ideas that have been
>>>>>>>>>> mentioned already. If we can avoid creating dependencies on DWARF I think
>>>>>>>>>> that would be ideal -- I'd ideally like this to work for COFF as well,
>>>>>>>>>> where you'd typically expect to find CodeView in object files. If I were to
>>>>>>>>>> try this I think the first thing that I would try is hash/compression
>>>>>>>>>> combined with the two ODR tables (no reuse for non-symbol ODR names to
>>>>>>>>>> start with, as compression may be enough on its own).
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I developed a second prototype which uses hash/compression with no
>>>>>>>>> attempt to reuse. It is available here:
>>>>>>>>> https://github.com/pcc/llvm-project/tree/odr-checker2
>>>>>>>>>
>>>>>>>>> For Chromium the object file size overhead was 536566007 bytes, or
>>>>>>>>> in relative terms about 25%, or about 4% with debug info. I measured perf
>>>>>>>>> overhead for unit_tests at about 6%, but after I moved the checker onto
>>>>>>>>> another thread, the overhead disappeared into the noise.
>>>>>>>>>
>>>>>>>>
>>>>>>>> Still seems like quite a big increase.
>>>>>>>>
>>>>>>>> Any chance of compression?
>>>>>>>>
>>>>>>>
>>>>>>> That was with compression -- the implementation compresses the parts
>>>>>>> of the ODR table that aren't hashes (aside from the header and the Clang
>>>>>>> version, which is a small fixed cost), as well as the string table. The
>>>>>>> hashes were left uncompressed because they are in the critical path of the
>>>>>>> linker and because I imagine that they wouldn't really be that compressible.
>>>>>>>
>>>>>>
>>>>>> I'd be a bit surprised if they weren't especially compressible -
>>>>>>
>>>>>
>>>>> Maybe I'm wrong, but my intuition about compression is that it works
>>>>> best when the data contains repeated patterns. If we use a hash function
>>>>> with good dispersion then I'd expect each hash to have little in common
>>>>> with other hashes.
>>>>>
>>>>>
>>>>>> and how much of the size increase is the compressed data V the
>>>>>> uncompressed data?
>>>>>>
>>>>>
>>>>> The ratio was roughly 60% compressed data to 40% uncompressed data.
>>>>>
>>>>>
>>>>>> Is it still in the hot path when parallelized?
>>>>>>
>>>>>
>>>>> Not right now according to my benchmarking, but decompression could
>>>>> push it into the critical path if it ends up taking longer than the rest of
>>>>> the work done by the linker after symbol resolution. On the same machine
>>>>> that I used for benchmarking, gunzip'ing 200MB of /dev/urandom (which is
>>>>> roughly what I'd expect the hashes to look like) takes around 1.1s, i.e. a
>>>>> not insignificant fraction of lld's runtime.
>>>>>
>>>>>
>>>>>> So I think the remaining gains would either be through limiting the
>>>>>>> number of ODR table entries, or through reuse of data.
>>>>>>>
>>>>>>> Limiting might be something to explore -- one possibility is that we
>>>>>>> could limit the ODR table entries to the declarations that are "used" by a
>>>>>>> particular translation unit (it appears that Clang tracks something like
>>>>>>> that in Decl::Used/Decl::Referenced, but I'm not sure if that is exactly
>>>>>>> what we need -- I think we would basically need to test for reference
>>>>>>> reachability from the functions/globals that are IRgen'd).
>>>>>>>
>>>>>>
>>>>>> Currently it has every type and function that is in the AST? Yeah,
>>>>>> that's a lot - perhaps it should be more like the things that go in the
>>>>>> DWARF? (though would need to add some cases there - since the DWARF logic
>>>>>> already relies on the ODR to not emit duplicates in some cases)
>>>>>>
>>>>>
>>>>> Just every record declaration -- Clang only supports ODR hashes for
>>>>> record declarations right now. I understand that function declarations
>>>>> (including function bodies) are still works in progress.
>>>>>
>>>>> I think it should indeed just be roughly the things that go in the
>>>>> DWARF. I think that at one point I observed that every record declaration,
>>>>> even unused ones, were going into the DWARF, but I might have been mistaken
>>>>> because I can no longer reproduce that. I'll take a closer look to see if I
>>>>> can reuse what logic presumably already exists for DWARF.
>>>>>
>>>>> In terms of reuse, it seems that of the 536566007 bytes of
>>>>>>> overhead, 319309579 were the compressed part of the ODR tables. So even if
>>>>>>> we achieved 100% sharing,
>>>>>>>
>>>>>>
>>>>>> 100% sharing? You mean if all the data were compressed, and assuming
>>>>>> the hashes were compressible at the same ratio as the other data?
>>>>>>
>>>>>
>>>>> Sorry, I mean if 100% of the data in the compressed part of the ODR
>>>>> table could be eliminated by reusing data stored elsewhere (e.g. in the
>>>>> object file string table or in the DWARF).
>>>>>
>>>>> with the current scheme I think that our minimum achievable overhead
>>>>>>> would be ~15% (no debug info) or ~2% (with debug info).
>>>>>>>
>>>>>>>
>>>>>>>> Could this go into .dwo files with Fission and be checked by dwp
>>>>>>>> instead, perhaps?
>>>>>>>>
>>>>>>>
>>>>>>> I think it could also work that way, yes.
>>>>>>>
>>>>>>> I'm reasonably happy with these figures, at least for a first
>>>>>>>>> implementation. We may be able to do even better for file size with reuse,
>>>>>>>>> but I'd leave that for version 2.
>>>>>>>>>
>>>>>>>>
>>>>>>>> What's the story with compatibility between versions, then? Is
>>>>>>>> there a version header?
>>>>>>>>
>>>>>>>
>>>>>>> Yes, the header contains a version number.
>>>>>>>
>>>>>>>
>>>>>>>> Will old formats be supported by lld indefinitely? Not at all?
>>>>>>>>
>>>>>>>
>>>>>>> I think we should drop support for old formats when we introduce a
>>>>>>> new format. My understanding is that the ODR hash can change whenever Clang
>>>>>>> changes (the implementation only performs ODR checking if all ODR tables
>>>>>>> were produced by the same revision of Clang), so there wouldn't seem to be
>>>>>>> a huge benefit in keeping support for old formats around.
>>>>>>>
>>>>>>
>>>>>> I imagine it's possible people aren't necessarily going to rev lld in
>>>>>> exact lock-step with clang, but I could be wrong. (certainly binutils ld or
>>>>>> gold aren't released/kept in lock-step with GCC, for example)
>>>>>>
>>>>>
>>>>> That's certainly possible, but I'd say that the bar for dropping
>>>>> backwards compatibility is lower because ODR tables are not required for
>>>>> correctness. We could keep compatibility with the last version or so if it
>>>>> isn't too burdensome, or otherwise print a warning.
>>>>>
>>>>
>>>> They aren't required for correctness, but upgrading your compiler or
>>>> linker then silently losing ODR checking would be bad (or even not silently
>>>> losing it, but having no choice but to rev both to keep the functionality &
>>>> hold the ODR-cleanliness bar) - it's the sort of thing where if you lost
>>>> the checking, then gained it back again later, the regression cleanup would
>>>> be annoying/an impediment to using the feature.
>>>>
>>>
>>> Makes sense I guess. I'd be fine with a policy where the Nth open source
>>> release should be able to read ODR tables produced by the N-1th and
>>> possibly the N-2th release.
>>>
>>
>> Still strikes me as a bit awkward - wonder how that compared to other
>> (similar or different) linker features.
>>
>
> I think the most similar existing feature is .gdb_index. They have already
> gone through a few format revisions:
> https://sourceware.org/gdb/onlinedocs/gdb/Index-Section-Format.html
> and have deprecated/removed support for older formats.
>

Not sure it's quite the same - that's an output of the linker & produces a
performance degredation in GDB when not present. Here the discussion is a
format input to the linker that produces warnings, potentially errors
(Werror) for users that they might want to maintain compatibility for.
Seems like a higher bar.

Feels to me like a bit more time could be spent on a design/size impact/etc
to give a better chance of ongoing stability (and/or backcompat at least)
for users. But just a thought.

- Dave


>
> Because the requirements for ODR tables are simpler than those for
> .gdb_index, I'd expect us to converge on a final format sooner, so in
> practice the window of compatibility would end up being longer than a year.
>
> Peter
>
>
>>
>>> Any idea what Daniel Jasper & co have been working on WRT ODR checking &
>>>> how this feature integrates or doesn't with their work? I imagine they
>>>> might be working on something more like a Clang Tooling style approach, but
>>>> I'm not sure.
>>>>
>>>
>>> I'm not aware of any work like that, only of Richard Trieu's efforts for
>>> modules that I'm piggybacking on.
>>>
>>
>> +Djasper - perhaps you could provide some context on other odr detection
>> efforts?
>>
>>
>>> Peter
>>>
>>>>
>>>> - Dave
>>>>
>>>>
>>>>>
>>>>> Peter
>>>>>
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> Peter
>>>>>>>
>>>>>>>>
>>>>>>>> - Dave
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Peter
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Peter
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> -- Sean Silva
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>> The issue of retaining the ODR check for functions even if
>>>>>>>>>>>>> they get inlined may inherently pose an extra cost that can't be folded
>>>>>>>>>>>>> into existing work the linker is doing, so there might be a reason for
>>>>>>>>>>>>> clang to have a default mode that has practically no linking overhead and
>>>>>>>>>>>>> one that does more thorough checking but imposes extra linking overhead.
>>>>>>>>>>>>> Think something like a crazy boost library with thousands of functions that
>>>>>>>>>>>>> get inlined away, but have gigantic mangled names and so precisely are the
>>>>>>>>>>>>> ones that are going to impose extra cost on the linker. Simply due to the
>>>>>>>>>>>>> extra volume of strings that the linker would need to look at, I don't
>>>>>>>>>>>>> think there's a way to include checking of all inlined function "for free"
>>>>>>>>>>>>> at the linker level using the symbol approach.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>> I guess those inlined functions would still have those symbol
>>>>>>>>>>>>> names in debug info (I think?), so piggybacking on the string deduplication
>>>>>>>>>>>>> we're already doing might make it possible to fold away the work in that
>>>>>>>>>>>>> case (but then again, would still impose extra cost with split dwarf...).
>>>>>>>>>>>>>
>>>>>>>>>>>>> Anyway, let's wait to see what the actual performance numbers
>>>>>>>>>>>>> are.
>>>>>>>>>>>>>
>>>>>>>>>>>>> -- Sean Silva
>>>>>>>>>>>>>
>>>>>>>>>>>>> On Tue, Jun 6, 2017 at 10:40 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
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>> --
>>>>>>>>>> Peter
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> --
>>>>>>>>> Peter
>>>>>>>>> _______________________________________________
>>>>>>>>> cfe-dev mailing list
>>>>>>>>> cfe-dev at lists.llvm.org
>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> --
>>>>>>> Peter
>>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> --
>>>>> Peter
>>>>>
>>>>
>>>
>>>
>>> --
>>> --
>>> Peter
>>>
>>
>
>
> --
> --
> Peter
>
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