[all-commits] [llvm/llvm-project] fe6c24: [clangd] [C++20] [Modules] Introduce initial suppo...

[Chuanqi Xu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: fe6c24000f2d7316899d4ec4c12273892326ed47
      https://github.com/llvm/llvm-project/commit/fe6c24000f2d7316899d4ec4c12273892326ed47
  Author: Chuanqi Xu <yedeng.yd at linux.alibaba.com>
  Date:   2024-07-18 (Thu, 18 Jul 2024)

  Changed paths:
    M clang-tools-extra/clangd/CMakeLists.txt
    M clang-tools-extra/clangd/ClangdLSPServer.cpp
    M clang-tools-extra/clangd/ClangdLSPServer.h
    M clang-tools-extra/clangd/ClangdServer.cpp
    M clang-tools-extra/clangd/ClangdServer.h
    M clang-tools-extra/clangd/Compiler.h
    M clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
    M clang-tools-extra/clangd/GlobalCompilationDatabase.h
    A clang-tools-extra/clangd/ModulesBuilder.cpp
    A clang-tools-extra/clangd/ModulesBuilder.h
    M clang-tools-extra/clangd/ParsedAST.cpp
    M clang-tools-extra/clangd/Preamble.cpp
    M clang-tools-extra/clangd/Preamble.h
    A clang-tools-extra/clangd/ProjectModules.h
    A clang-tools-extra/clangd/ScanningProjectModules.cpp
    A clang-tools-extra/clangd/ScanningProjectModules.h
    M clang-tools-extra/clangd/test/CMakeLists.txt
    A clang-tools-extra/clangd/test/modules.test
    M clang-tools-extra/clangd/tool/Check.cpp
    M clang-tools-extra/clangd/tool/ClangdMain.cpp
    M clang-tools-extra/clangd/unittests/CMakeLists.txt
    A clang-tools-extra/clangd/unittests/PrerequisiteModulesTest.cpp
    M clang-tools-extra/clangd/unittests/TestFS.h
    M clang-tools-extra/docs/ReleaseNotes.rst

  Log Message:
  -----------
  [clangd] [C++20] [Modules] Introduce initial support for C++20 Modules (#66462)

Alternatives to https://reviews.llvm.org/D153114.

Try to address https://github.com/clangd/clangd/issues/1293.

See the links for design ideas and the consensus so far. We want to have
some initial support in clang18.

This is the initial support for C++20 Modules in clangd.
As suggested by sammccall in https://reviews.llvm.org/D153114,
we should minimize the scope of the initial patch to make it easier
 to review and understand so that every one are in the same page:

> Don't attempt any cross-file or cross-version coordination: i.e. don't
    > try to reuse BMIs between different files, don't try to reuse BMIs
> between (preamble) reparses of the same file, don't try to persist the
> module graph. Instead, when building a preamble, synchronously scan
> for the module graph, build the required PCMs on the single preamble
> thread with filenames private to that preamble, and then proceed to
    > build the preamble.

This patch reflects the above opinions.

# Testing in real-world project

I tested this with a modularized library:
https://github.com/alibaba/async_simple/tree/CXX20Modules. This library
has 3 modules (async_simple, std and asio) and 65 module units. (Note
that a module consists of multiple module units). Both `std` module and
`asio` module have 100k+ lines of code (maybe more, I didn't count). And
async_simple itself has 8k lines of code. This is the scale of the
project.

The result shows that it works pretty well, ..., well, except I need to
wait roughly 10s after opening/editing any file. And this falls in our
expectations. We know it is hard to make it perfect in the first move.

# What this patch does in detail

- Introduced an option `--experimental-modules-support` for the support
for C++20 Modules. So that no matter how bad this is, it wouldn't affect
current users. Following off the page, we'll assume the option is
enabled.
- Introduced two classes `ModuleFilesInfo` and
`ModuleDependencyScanner`. Now `ModuleDependencyScanner` is only used by
`ModuleFilesInfo`.
- The class `ModuleFilesInfo` records the built module files for
specific single source file. The module files can only be built by the
static member function `ModuleFilesInfo::buildModuleFilesInfoFor(PathRef
File, ...)`.
- The class `PreambleData` adds a new member variable with type
`ModuleFilesInfo`. This refers to the needed module files for the
current file. It means the module files info is part of the preamble,
which is suggested in the first patch too.
- In `isPreambleCompatible()`, we add a call to
`ModuleFilesInfo::CanReuse()` to check if the built module files are
still up to date.
- When we build the AST for a source file, we will load the built module
files from ModuleFilesInfo.

# What we need to do next

Let's split the TODOs into clang part and clangd part to make things
more clear.

The TODOs in the clangd part include:
1. Enable reusing module files across source files. The may require us
to bring a ModulesManager like thing which need to handle `scheduling`,
`the possibility of BMI version conflicts` and `various events that can
invalidate the module graph`.
2. Get a more efficient method to get the `<module-name> ->
<module-unit-source>` map. Currently we always scan the whole project
during `ModuleFilesInfo::buildModuleFilesInfoFor(PathRef File, ...)`.
This is clearly inefficient even if the scanning process is pretty fast.
I think the potential solutions include:
- Make a global scanner to monitor the state of every source file like I
did in the first patch. The pain point is that we need to take care of
the data races.
- Ask the build systems to provide the map just like we ask them to
provide the compilation database.
3. Persist the module files. So that we can reuse module files across
clangd invocations or even across clangd instances.

TODOs in the clang part include:
1. Clang should offer an option/mode to skip writing/reading the bodies
of the functions. Or even if we can requrie the parser to skip parsing
the function bodies.

And it looks like we can say the support for C++20 Modules is initially
workable after we made (1) and (2) (or even without (2)).



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