[cfe-dev] Adding indexing support to Clangd

[Ilya Biryukov via cfe-dev]

Hi everyone,

The problem with PCHs(either chained or not) is that they only work for the
source file,
i.e. you can only use it when you start the new file from scratch, right?
For header caching we
really want to reuse whatever information we have cached even if it's
included in a different
context(i.e. the order of includes is different in the other translation
unit), which is not possible with PCHs.

My point is that it's not at all straightforward how(or if?) the PCHs can
improve performance of processing
the same header twice.

And as long as building an index is as fast as a recompile and we can reuse
information from the previous
version(slightly outdated) the index while the new version is building, we
can probably get a good enough
UX without any compromises on correctness(and introducing additional
complexity, since I don't think there's
a way to do header caching without significant changes to clang itself).

CLion indeed has a custom parser and serialization format, it's not
clang-based.


On Thu, May 18, 2017 at 10:33 PM, Marc-André Laperle via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Yeah, it sounds like a good approach to tackle header caching a bit later.
> "Precompiled preamble" looks promising so we can keep this in mind as we
> go.
>
>
> Cheers,
>
> Marc-André
> ------------------------------
> *From:* Doug Schaefer <dschaefer at blackberry.com>
> *Sent:* Thursday, May 18, 2017 10:44:18 AM
> *To:* Marc-André Laperle; via cfe-dev
> *Cc:* zeratul976 at hotmail.com; zaks.anna at gmail.com; Dániel Krupp; Zoltan
> Porkoláb; Marton Csordas
> *Subject:* Re: Adding indexing support to Clangd
>
> On 2017-05-17, 6:38 PM, "Marc-André Laperle"
> <marc-andre.laperle at ericsson.com> wrote:
> >  - For header caching, I wonder if it is possible to reuse the
> >precompiled header support in Clang. There would be some logic that would
> >decide whether or not a precompiled header could be used depending on the
> >preprocessing context (same macro definitions, etc).
>
> I¹m not certain header cacheing is needed right away. We did it in the CDT
> because our parsers were fairly slow and indexing a project took a very
> long time. I have hope that clang would be faster. At the very least, you
> would want cacheing to be optional so you need to be able to work without
> it. But make sure you have the architecture to graft it in later.
>
> In CDT we cheated a lot to gain performance and the cost of accuracy. The
> results are still very good so its an interesting balancing act.
>
> Doug Schaefer
> Eclipse CDT Project Lead
>
>
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>


-- 
Regards,
Ilya Biryukov
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