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

[Manuel Klimek via cfe-dev]

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).


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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