[cfe-dev] Two pass analysis framework: AST merging approach

Tue Jul 12 05:36:32 PDT 2016

On 12 July 2016 at 14:09, Manuel Klimek <klimek at google.com> wrote:

> On Tue, Jul 12, 2016 at 1:59 PM Gábor Horváth <xazax.hun at gmail.com> wrote:
>
>> Hi!
>>
>> (As a ping), I would like to summarize the measurements I done since the
>> original e-mail:
>>
>> The approach is to first serialize all the translation units to the
>> storage, create an index of the functions, and then load them lazily on
>> demand to achieve cross translation unit support. This does not modify the
>> inter-procedural analysis of the Static Analyzer and could be used for
>> Clang Tidy as well. Once a new inter-procedural analysis is introduced for
>> the Static Analyzer, the cross tu support would profit from it immediately.
>>
>> Benchmarks:
>> rAthena, a 150k LOC C project:
>> The size of the serialized ASTs was: 140MB
>> The size of the indexes: 4.4MB
>> The time of the analysis was bellow 4X
>> The amount of memory consumed was bellow 2X
>> The number of reports is 3 times more
>>
>> Xerces, 150k LOC C++ project:
>> The size of the serialized ASTs was:  800MB
>> The size of the indexes: 90MB
>> The analysis time using CTU was the half of the one without CTU
>>
>> LLVM + Clang + Clang tools extra:
>> The size of the serialized ASTs was: 45.4 GB
>> The size of the indexes:  1,6GB
>>
>> Some optimization effort to reduce te size of the CFG:
>> TU ASTs after omitting function bodies from headers: 42.7 GB
>> TU ASTs after omitting STL: 34.0 GB
>> TU ASTs after skipping implicit instantiations: 21.5 GB
>> TU ASTs after omitting STL and implicit instantiations: 16.0 GB
>>
>> Considering that the build directory of a debug build is also about 40 GB
>> on my platform, I do not consider the size of the serialized ASTs a
>> blocking issue. However, in case it is a concern, according to the attached
>> statistics about the serialized AST dumps, there are some optimization
>> possibilities in the binary format.
>>
>> This solution also works in a multi process build/analysis environment.
>> Some of the necessary framework, for example ASTImporter code is being
>> accepted into mainline clang right now.
>>
>> All in all, this approach:
>> - Can discover new bug reports as is.
>> - Feasible to implement, does not need sever modifications to the Static
>> Analyzer or Clang Tidy.
>> - Has acceptable performance for lots of the real world projects.
>>
>> I think, this would be a useful addition to the clang source tree. Do you
>> agree?
>>
>
> I definitely think this is interesting - I'd be curious if we could design
> the interfaces in a way that we can
> a) fully split out indexing from analysis
>

In the current implementation the analysis and dumping and indexing ASTs
are two separate passes, so they can be used completely independently.
Which is a very good design choice in my opinion.

> b) allow storing the indexing data in a distributed storage system
>

I do not have much experience with distributed storage systems, but I think
that should work. The only problem might be the size of the individual AST
dumps.

>
>
>> Regards,
>> Gábor
>>
>>
>> On 4 May 2016 at 15:09, Gábor Horváth <xazax.hun at gmail.com> wrote:
>>
>>> Hi!
>>>
>>> This e-mail is a proposal based on the work done by Yury Gibrov et al.:
>>> http://lists.llvm.org/pipermail/cfe-dev/2015-December/046299.html
>>>
>>> They accomplished a two pass analysis, the first pass is serializing the
>>> AST of every translation unit and creates an index of functions, the second
>>> pass does the real analysis, which can load the AST of function bodies on
>>> demand.
>>>
>>> This approach can be used to achieve cross translation unit analysis for
>>> the clang Static Analyzer to some extent, but similar approach could be
>>> applicable to Clang Tidy and other clang based tools.
>>>
>>> While this method is not likely to be a silver bullet for the Static
>>> Analyzer, I did some benchmarks to see how feasible this approach is. The
>>> baseline was running the Static Analyzer without the two pass analyis, the
>>> second one was running using the framework linked above.
>>>
>>> For a 150k LOC C projects I got the following results:
>>> The size of the serialized ASTs was: 140MB
>>> The size of the indexes (textual representation): 4.4MB
>>> The time of the analysis was bellow 4X
>>> The amount of memory consumed was bellow 2X
>>>
>>> All in all it looks like a feasible approach for some use cases.
>>>
>>> I also tried to do a benchmark on the LLVM+Clang codebase. Unfortunately
>>> I was not able to run the analysis due to some missing features in the AST
>>> Importer. But I was able to serialize the ASTs and generate the indices:
>>> The siye of the serialized ASTs: 45.4 GB
>>> The siye of the function index: 1,6GB
>>>
>>> While these numbers are less promising, I think there are some
>>> opportunities to reduce them significantly.
>>>
>>> I propose the introduction of an analysis mode for exporting ASTs. In
>>> analysis mode the AST exporter would not emit the function body of a
>>> function for several cases:
>>> - In case a function is defined in a header, do not emit the body.
>>> - In case the function was defined in an implicit template
>>> specialisation, do not emit the body.
>>>
>>> I think after similar optimizations it might be feasible to use this
>>> approach on LLVM scale projects as well, and it would be much easier to
>>> implement Clang based tools that can utilize cross translation unit
>>> capabilities.
>>>
>>> In case the analyzer gets a new interprocedural analysis method that
>>> would increase the performance the users of this framework would profit
>>> from that approach immediately.
>>>
>>> Does a framework like this worth mainlining and working on? What do you
>>> think?
>>>
>>> (Note that, AST Importer related improvements are already being
>>> mainlined by Yury et al. My question is about the "analysis mode" for
>>> exporting ASTs, and a general framework to consume those exported ASTs.)
>>>
>>> Regards,
>>> Gábor
>>>
>>>
>>>
>>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20160712/c9371d1a/attachment.html>