[cfe-dev] Adding indexing support to Clangd

[Ilya Biryukov via cfe-dev]

Thanks for the insights, I think I get the gist of the idea with the
"module" PCH.
One question is: what if the system headers are included after the user
includes? Then we abandon the PCH cache and run the parsing from scratch,
right?

FileSystemStatCache that is reused between compilation units? Sounds like a
low-hanging fruit for indexing, thanks.

On Thu, Jun 1, 2017 at 11:52 AM, Vladimir Voskresensky <
vladimir.voskresensky at oracle.com> wrote:

> Hi Ilia,
>
> Sorry for the late reply.
> Unfortunately mentioned hacks were done long time ago and I couldn't find
> the changes at the first glance :-(
>
> But you can think about reusable chaned PCHs in the "module" way.
> Each system header is a module.
> There are special index_headers.c and index_headers.cpp files which
> includes all standard headers.
> These files are indexed first and create "module" per #include.
> Module is created once or several times if preprocessor contexts are very
> different like C vs. C++98 vs. C++14.
> Then reused.
> Of course it could compromise the accuracy, but for proof of concept was
> enough to see that expected indexing speed can be achieved theoretically.
>
> Btw, another hint: implementing FileSystemStatCache gave the next visible
> speedup. Of course need to carefully invalidate/update it when file was
> modified in IDE or externally.
> So, finally we got just 2x slowdown, but the accuracy of "real" compiler.
> And then as you know we have started Clank :-)
>
> Hope it helps,
> Vladimir.
>
>
> On 29.05.2017 11:58, Ilya Biryukov wrote:
>
> Hi Vladimir,
>
> Thanks for sharing your experience.
>
> We did such measurements when evaluated clang as a technology to be used
>> in NetBeans C/C++, I don't remember the exact absolute numbers now, but the
>> conclusion was:
>>
> to be on par with the existing NetBeans speed we have to use different
>> caching, otherwise it was like 10 times slower.
>>
> It's a good reason to focus on that issue from the very start than. Would
> be nice to have some exact measurements, though. (i.e. on LLVM).
> Just to know how slow exactly was it.
>
> +1. Btw, may be It is worth to set some expectations what is available
>> during and after initial index phase.
>> I.e. during initial phase you'd probably like to have navigation for file
>> opened in editor and can work in functions bodies.
>>
> We definitely want diagnostics/completions for the currently open file to
> be available. Good point, we definitely want to explicitly name the
> available features in the docs/discussions.
>
> As to initial indexing:
>> Using PTH (not PCH) gave significant speedup.
>>
> Skipping bodies gave significant speedup, but you miss the references and
>> later have to reindex bodies on demand.
>> Using chainged PCH gave the next visible speedup.
>>
> Of course we had to made some hacks for PCHs to be more often "reusable"
>> (comparing to strict compiler rule) and keep multiple versions. In average
>> 2: one for C and one for C++ parse context.
>> Also there is a difference between system headers and projects headers,
>> so systems' can be cached more aggressively.
>>
> Is this work open-source? The interesting part is how to "reuse" the PCH
> for a header that's included in a different order.
> I.e. is there a way to reuse some cached information(PCH, or anything
> else) for <map> and <vector> when parsing these two files:
> ```
> // foo.cpp
> #include <vector>
> #include <map>
> ...
>
> // bar.cpp
> #include <map>
> #include <vector>
> ....
> ```
>
> --
> Regards,
> Ilya Biryukov
>
>
>


-- 
Regards,
Ilya Biryukov
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