[clangd-dev] New LSP language service supporting Swift and C-family languages, using clangd

Sam McCall via clangd-dev clangd-dev at lists.llvm.org
Tue Oct 23 08:22:24 PDT 2018

Hi Argyrios,

Thanks for the update! I think the design makes sense, and having a
cross-language index will allow really nice mixed-language project support.

There is a lot of implementation overlap, the design we ended up with is
actually pretty similar: auto-index (
http://tinyurl.com/clangd-automatic-index) extracts the symbols/refs from
TUs, and then we build a queryable index structure on top of it (Dex). It's
all in-process, but conceptually not too different than the
background-clang-worker variant.
We started with a dynamic index which just has opened TUs, and a static one
built by a mapreduce-like process, and autoindex is these approaches
meeting in the meddle. We'd actually hoped to rely on index-while-build
infrastructure for some of this, but it didn't arrive in time.

That said, the devil is in the details: actually sharing an implementation
of the index layer would probably be hard, as we're all getting pulled in
different directions.
If it's possible to plug your index into the SymbolIndex abstraction in
clangd, then lots of features will "just work", like fast code completion.
Clangd relies on the index for that today rather than the caching
approaches used by libclang.
We've had multiple implementations of this (including a large-scale
out-of-process one internal to Google), but so far they've all mostly
shared indexing logic. So I'm curious whether this is actually
implementable on different data sources like what you'll get from

One question I have is a practical one - I'm sure changes are needed to
clangd, are these likely to happen upstream or in a fork/merge cycle?

Looking forward to seeing more details!
Cheers, Sam

On Tue, Oct 23, 2018 at 8:32 AM Argyrios Kyrtzidis via clangd-dev <
clangd-dev at lists.llvm.org> wrote:

> Hey all,
> We've recently announced that we'll be starting a new open-source project
> for an LSP language service supporting Swift and C-family languages, see
> more details in the announcement post (
> https://forums.swift.org/t/new-lsp-language-service-supporting-swift-and-c-family-languages-for-any-editor-and-platform).
> I wanted to also mention additional details that relate to Clangd.
> Currently, for our C-family support in Xcode (code-completion, clang AST
> queries) we use libclang, but for the new LSP service we will switch to
> using Clangd. We will also open-source a C++ library for global index
> queries, which is built on top of LMDB (https://symas.com/lmdb). The
> functionality of this library is described by Nathan in his
> Index-While-Building design document (
> https://docs.google.com/document/d/1cH2sTpgSnJZCkZtJl1aY-rzy4uGPcrI-6RrUpdATO2Q),
> specifically in the 'Using the index store' section.
> Let me elaborate a bit more on how we use this library. From Clang (and
> Swift) we get raw index data files, either directly from building or from
> invoking clang for background indexing. These data record files are
> designed to be efficient to write and update, ensuring that record files
> for headers are only written once, so that index-while-building has minimal
> overhead. But they are not designed to do efficient global queries (give me
> all symbol occurrences of this symbol USR). To accommodate this we use this
> database library which is a lightweight index layer on top of the raw index
> records. It reads the raw index data files and populates a key-value
> database that enables efficient global queries (it essentially determines
> what raw index record files contain the relevant information and retrieves
> the data).
> In our design for having full cross-language support for Swift and Clang
> languages (e.g. call-hierarchy across languages), we prefer to have a
> language-independent indexing component that is layered on top of the
> compiler-specific support (Clang/Clangd and Swift/sourcekitd). That means
> that our LSP service will contain an indexing and global refactoring engine
> and it will delegate to Clangd for clang-specific document queries, like
> code-completion.
> I understand that Clangd is intended to be a self-contained language
> service, that includes functionality for global index queries along with
> document-specific queries, but we believe we could still collaborate on
> common infrastructure shared by both Clangd and our new cross-language LSP
> service. See AlexL's previous post about how we intend to use Clangd,
> https://lists.llvm.org/pipermail/cfe-dev/2018-April/057668.html and what
> kind of improvements we want to make.
> Once we have the repositories up, you'll be able to check out our overall
> design in more detail, and in the meantime I'd be happy to hear any
> feedback or questions you may have!
> _______________________________________________
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> clangd-dev at lists.llvm.org
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