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

Argyrios Kyrtzidis via clangd-dev clangd-dev at lists.llvm.org
Mon Oct 22 18:53:45 PDT 2018

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 <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 <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 <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 <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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