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

[Rui Ueyama via cfe-dev]

I think you are right -- I always confuse false positives with false
negatives. What I wanted to say is collisions increase a chance that an ODR
violation is not detected.

On Wed, Jun 14, 2017 at 6:15 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> 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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