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

[Gábor Horváth via cfe-dev]

On 4 May 2016 at 15:28, Manuel Klimek <klimek at google.com> wrote:

> On Wed, May 4, 2016 at 3:09 PM 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.
>>
>
> I agree that we want cross translation unit analysis to be simpler to
> implement, but I think that parallelization of single steps will still be
> key for usability. Thus, I'm not convinced the "one large glob" approach is
> going to play out (but I might well be wrong).
>
>
Sorry for omitting this information, the serialized ASTs are actually not
one large glob, but one for each translation unit. Are you comfortable with
this level of granularity?


>
> 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
>>
>>
>>
>>
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