[cfe-dev] Adding indexing support to Clangd

Doug Schaefer via cfe-dev cfe-dev at lists.llvm.org
Fri Aug 11 07:50:54 PDT 2017

From: Manuel Klimek [mailto:klimek at google.com]
Sent: Friday, August 11, 2017 4:54 AM
To: Doug Schaefer <dschaefer at blackberry.com>; David Chisnall <David.Chisnall at cl.cam.ac.uk>; Marc-André Laperle <marc-andre.laperle at ericsson.com>
Cc: via cfe-dev <cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] Adding indexing support to Clangd

On Thu, Aug 10, 2017 at 5:57 PM Doug Schaefer via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
> -----Original Message-----
> From: Dr D. Chisnall [mailto:dc552 at hermes.cam.ac.uk<mailto:dc552 at hermes.cam.ac.uk>] On Behalf Of David
> Chisnall
> Sent: Thursday, August 10, 2017 6:10 AM
> To: Marc-André Laperle <marc-andre.laperle at ericsson.com<mailto:marc-andre.laperle at ericsson.com>>
> Cc: via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>; zeratul976 at hotmail.com<mailto:zeratul976 at hotmail.com>; Doug
> Schaefer <dschaefer at blackberry.com<mailto:dschaefer at blackberry.com>>
> Subject: Re: [cfe-dev] Adding indexing support to Clangd
> On 8 Aug 2017, at 18:52, Marc-André Laperle via cfe-dev <cfe-
> dev at lists.llvm.org<mailto:dev at lists.llvm.org>> wrote:
> >
> > --ClangdIndexStorage--
> > malloc-like interface that allocates/frees data blocks of variable sizes on
> disk. The blocks can contain ints, shorts, strings, pointers (i.e. file offsets),
> etc. The data is cached in 4K pieces so that local and repeated accesses are all
> done quickly, in memory.
> > Clangd mallocs and writes its index model objects using this.
> >
> > --BTree--
> > A pretty classic BTree implementation. Used to speed up lookup (symbol
> names, file names). It allocates its nodes using ClangdIndexStorage therefore
> it is stored on disk. Keys are actually records in ClangdIndexStorage so you
> can really think of the BTree as a collection of sorted pointers (sorted
> according to a provided comparator).
> This sounds very like bdb.  Is there a reason that we’re reimplementing a
> large chunk of bdb, rather than just using it (or using something like sqlite for
> the index storage)?

It looks like Marc-Andre is following our work on the Eclipse CDT indexer. We found databases to be just too slow.

That is somewhat surprising. Which db's did you benchmark against?

This was many years ago and we were trying Apache Derby (since Eclipse is written in Java). And at the time the biggest issue was write performance totally destroying any gains we made with header caching. But that may have just been Derby.

Though I think the bigger issue is that the language model is a graph which doesn’t fit naturally in tables. You’ll end up spending more time fighting that than building out your own data structures. But that’s just my experience.

Also, forcing the C++ language model to fit into a bunch of tables or key/value pairs just didn't make that much sense in the end. The data is very complex.

Instead it's better to think of this as a big graph data structure that just happens to be backed by memory mapped file store. Clients generally deal directly with pointers into that data structure and follow pointers in the graph. The BTrees help speed up that navigation at various nodes in the graph. It's super fast.

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