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

[Peter Collingbourne via cfe-dev]

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.

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).

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, 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.

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
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