Add support for C++23 std modules by Diyou · Pull Request #27065 · emscripten-core/emscripten

Diyou · 2026-06-07T15:43:48Z

This adds the libcxx/modules folder from https://github.com/emscripten-core/llvm-project/tree/emscripten-libs-21 with its generated files in system/lib/libcxx/modules/prebuilt

Prebuilt module interface files installed in sysroot via update_libcxx.py + install_system_headers
Added support in cmake/Modules/Emscripten.cmake for CMake > 3.30
Added Test

Resolves #21143

Additional Note:

To use the std or std.compat modules without CMake one would need to precompile the module interface like:

em++ -std=c++23 -stdlib=libc++ \
  -Wno-reserved-module-identifier \
  --precompile <EMSCRIPTEN_SYSROOT>/share/libc++/v1/std.cppm \
  -o std.pcm

And then compile the source files with:

em++ -std=c++23 -stdlib=libc++ \
  -fmodule-file=std=std.pcm \
  main.cpp -o main.js

sbc100

Nice!

To be clear are these files taken from the emscripten-libs-21 branch?

We normally use the system/lib/update_libcxx.py script to update these files. Perhaps you could try using that (maybe it needs modifying to include the modules directory somehow?)

Also made copy_tree conditionally exclude files because we need the CMakeLists.txt for the modules

sbc100

lgtm!

But maybe we could add a test for this ?

This reverts commit 1125e15.

Diyou · 2026-06-08T20:07:38Z

But maybe we could add a test for this ?

Might need to get Clang-CXX-CXXImportStd working if the test environment doesn't support cmake 4.2 for CMAKE_CXX_STDLIB_MODULES_JSON .

In this case we'd need to patch emcc.py to allow using a relative search path for -print-file-name with CMAKE_CXX_COMPILER_ID_ARG1

This is explicitly used by CMake's Clang-CXX-CXXImportStd.cmake

sbc100 · 2026-06-08T20:52:16Z

But maybe we could add a test for this ?

Might need to get Clang-CXX-CXXImportStd working if the test environment doesn't support cmake 4.2 for CMAKE_CXX_STDLIB_MODULES_JSON .

In this case we'd need to patch emcc.py to allow using a relative search path for -print-file-name with CMAKE_CXX_COMPILER_ID_ARG1

If we can get cmake 4.2 installed on at least one of the CI machines would that address the issue?

I would be fine landing this change standalone and following up with a test too.

…o CMAKE_INSTALL_PREFIX

This reverts commit 8b5827f.

needed for out of tree builds i.e. tests

Also adjust the toolchain file to use those Q: Shouldn't these rather be installed when building libc++ ?

This creates 2 separate test: - test_cxx_import_std23 - test_cxx_import_std26

sbc100

lgtm!

sbc100

lgtm! (with one last nit)

Thanks for working on this

Diyou · 2026-06-13T22:41:23Z

Pleasure. Hope I can get clangd for this :D . Differing versions still have trouble reading the .pcm files.

sbc100 · 2026-06-14T20:49:54Z

I think you need to rebase or merge to fix the chrome tests.

sbc100 · 2026-06-14T21:40:30Z

+  endif()
+endif()
+
+set_property(SOURCE


Does it make sense to keep this outside of the above condition?

Because cmake caches CMAKE_CXX_STDLIB_MODULES_JSON the above condition would not run on subsequent configuration runs and remove the flags

sbc100 · 2026-06-14T21:43:53Z

+if(CMAKE_VERSION VERSION_GREATER_EQUAL 4.3.0)
+  set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD 451f2fe2-a8a2-47c3-bc32-94786d8fc91b)
+else()
+  set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD d0edc3af-4c50-42ea-a356-e2862fe7a444)


Are normal end users executed to write lines like this?

For now yes. This is still an Opt-in feature and users might also not have the backported values and would look that up for their installation.
Ref: https://cmake.org/cmake/help/latest/manual/cmake-cxxmodules.7.html#import-std-support

Note
This support is provided only when experimental support for import std has been enabled by the CMAKE_EXPERIMENTAL_CXX_IMPORT_STD gate.

Removes the dependency on python3-packaging in test_other.py which was introduced in #27065

aheejin · 2026-07-01T02:58:43Z

What version of LLVM was this taken from? We normally sync library updates in specific LLVM version and it's confusing to mix different versions within the same library. The last update was 21.1.8 (#26058).

sbc100 · 2026-07-01T03:10:59Z

I'm pretty sure it was taking from the emscripten-libs-21 branch.. or at least I hope it was and seem to remember point it out at some point.

@Diyou maybe you could confirm and also update the PR description to mention this branch name.

Diyou · 2026-07-01T06:32:04Z

Yes I changed it to the emscripten-libs-21 branch in this commit after you pointed this out which fixed the compatibility issues we initially had.

These should stay in sync with the changes in update_libcxx.py

Diyou added 3 commits June 7, 2026 17:16

Adds libcxx/modules subdirectory

f235e78

Add c++23 stl module support to the Emscripten toolchain

922e9aa

Prevent overwriting CMAKE_CXX_STDLIB_MODULES_JSON

fc98e2b

sbc100 approved these changes Jun 8, 2026

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Comment thread cmake/Modules/Platform/Emscripten.cmake Outdated

Comment thread cmake/Modules/Platform/Emscripten.cmake Outdated

Diyou added 3 commits June 8, 2026 19:50

Use libcxx/modules from the emscripten-libs-21 branch

0518db0

Fix indentation in Emscripten.cmake

8bc7bd8

Include modules in update_libcxx.py

1125e15

Also made copy_tree conditionally exclude files because we need the CMakeLists.txt for the modules

sbc100 reviewed Jun 8, 2026

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Comment thread system/lib/update_libcxx.py Outdated

Diyou added 2 commits June 8, 2026 22:03

Revert "Include modules in update_libcxx.py"

2a95f40

This reverts commit 1125e15.

Remove CMakeLists.txt from the excludes

79d3d70

Diyou added 2 commits June 8, 2026 22:10

Add back modules in update_libcxx.py

135e83f

Allow passing a search path with -print-file-name

8b5827f

This is explicitly used by CMake's Clang-CXX-CXXImportStd.cmake

sbc100 reviewed Jun 8, 2026

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Comment thread emcc.py Outdated

sbc100 reviewed Jun 8, 2026

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Comment thread emcc.py Outdated

sbc100 reviewed Jun 8, 2026

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Comment thread cmake/Modules/Platform/Emscripten.cmake Outdated

sbc100 reviewed Jun 8, 2026

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Comment thread cmake/Modules/Platform/Emscripten.cmake Outdated

Flip LIBCXX_* definitions so LIBCXX_INSTALL_* paths can be relative t…

d881828

…o CMAKE_INSTALL_PREFIX

sbc100 reviewed Jun 8, 2026

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Comment thread system/lib/update_libcxx.py Outdated

Diyou added 3 commits June 9, 2026 01:54

Make 'excludes' into a set

29d7923

Revert "Allow passing a search path with -print-file-name"

d9c2803

This reverts commit 8b5827f.

Specifcy binary_dir for the modules subdirectory

53c6e9c

needed for out of tree builds i.e. tests

sbc100 reviewed Jun 9, 2026

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Comment thread cmake/Modules/Platform/Emscripten.cmake Outdated

Diyou mentioned this pull request Jun 10, 2026

Allow using the search path from -resource-dir with emcc -print-file-name #27082

Merged

Diyou added 4 commits June 11, 2026 05:24

Generate/Install modules alongside system headers

fa09f3e

Also adjust the toolchain file to use those Q: Shouldn't these rather be installed when building libc++ ?

Add import std test

85ebfb3

Add missing import in system_libs

8b97952

CS fixes

8575e83

Diyou added 6 commits June 12, 2026 18:25

Exit update_libcxx.py early if CMake is not found

3c73acd

Lower requirement to CMake 3.30 by utilizing Clang-CXX-CXXImportStd

c5a5ee0

Avoid using if( ... IN_LIST) and use CMAKE_CXX_COMPILER_LOADED instead

490f66d

Skip test_cmake_cxx_import_std specifically on version check

578f8e3

Add a CMake-less test

c6b1eda

Some reformatting

f552d85

sbc100 reviewed Jun 13, 2026

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Comment thread test/test_other.py Outdated

sbc100 reviewed Jun 13, 2026

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Comment thread test/test_other.py Outdated

sbc100 reviewed Jun 13, 2026

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Comment thread test/test_other.py Outdated

Simplified test

f0784c1

This creates 2 separate test: - test_cxx_import_std23 - test_cxx_import_std26

sbc100 approved these changes Jun 13, 2026

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Comment thread test/test_other.py Outdated

sbc100 reviewed Jun 13, 2026

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Comment thread system/lib/libcxx/modules/prebuilt/lib/emscripten/libc++.modules.json

sbc100 reviewed Jun 13, 2026

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Comment thread system/lib/update_libcxx.py Outdated

Diyou added 2 commits June 14, 2026 00:17

Remove dupes and copy directly from std and std.compat

32e1eaa

Semantical to the test and to update_libcxx.py's early sexit

3cdd0e6

sbc100 reviewed Jun 13, 2026

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Comment thread test/test_other.py Outdated

Lowercase'd cflags

1ee986a

sbc100 changed the title ~~C++23 stl modules~~ Add support for C++23 std modules Jun 14, 2026

Merge branch 'main' into cxx23_stl_modules

713cd59

sbc100 approved these changes Jun 14, 2026

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Simplified update_libcxx and removed dependency of tempdir

d318d12

sbc100 merged commit d913170 into emscripten-core:main Jun 15, 2026
39 checks passed

kleisauke reviewed Jun 18, 2026

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Comment thread test/test_other.py

Diyou mentioned this pull request Jun 19, 2026

Removed packaging.version as a dependency in test_other.py #27152

Merged

sbc100 pushed a commit that referenced this pull request Jun 19, 2026

Removed packaging.version as a dependency in test_other.py (#27152)

e944019

Removes the dependency on python3-packaging in test_other.py which was introduced in #27065

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