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

[David Blaikie via cfe-dev]

On Wed, Jun 14, 2017 at 6:01 PM Rui Ueyama <ruiu at google.com> wrote:

> Is the entry in your ODR table 64-bit? Sean mentioned that this is a
> birthday paradox situation, but I don't think we need that large hash
> values, as our aim is not to avoid any collisions. Small number of
> collisions is okay as it just slightly increases false negatives. I think
> it can even be 16-bit if space saving is important. If we choose 16-bit
> hash, the probability that an ODR violation is not detected is 1/65536,
> which is still quite low.
>

If I'm understanding this correctly, it's the opposite though - colliding
hashes will produce false positives, right? (ODR violations will be
detected where none exist in the code) Perhaps I'm misunderstanding.


>
> On Wed, Jun 14, 2017 at 1:41 PM, Peter Collingbourne via cfe-dev <
> cfe-dev at lists.llvm.org> 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.
>>
>> 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
>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>>
>
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