Why are the build and installed BMI files not used?

Development
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1 
bash-5.3$ ninja -t deps | egrep -vw '(Cellar|Library)' | sort -u

    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/args.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/base.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/chrono.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/color.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/compile.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/format-inl.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/format.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/os.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/ostream.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/printf.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/ranges.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/std.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/xchar.h
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/module/fmt.cppm
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/module/format.cc
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/module/os.cc
    /Users/clausklein/Workspace/cpp/cxx20/fmt-module/tests/header-only-test.cc
    CMakeFiles/__cmake_cxx23.dir/std.pcm
CMakeFiles/fmt__fmt@synth_439f33eb51e5.dir/f6569b67b774.bmi: #deps 197, deps mtime 1765026197877701690 (VALID)
CMakeFiles/module_header-only-test.dir/Users/clausklein/Workspace/cpp/cxx20/fmt-module/tests/header-only-test.cc.o: #deps 1, deps mtime 1765026197925780567 (VALID)
bash-5.3$ cmake . -L
-- CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES=/usr/local/Cellar/llvm/21.1.6/include/c++/v1;/usr/local/Cellar/llvm/21.1.6/lib/clang/21/include;/Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include
-- CMAKE_CXX_MODULE_STD=ON
-- CMAKE_CXX_COMPILER_IMPORT_STD=23;26
-- Module tests to be built: header-only-test
-- Configuring done (0.1s)
CMake Warning (dev) in CMakeLists.txt:
  CMake's support for `import std;` in C++23 and newer is experimental.  It
  is meant only for experimentation and feedback to CMake developers.
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Generating done (0.0s)
-- Build files have been written to: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/build/dev/tests/find-package-test
-- Cache values
CMAKE_BUILD_TYPE:STRING=Release
CMAKE_CXX_COMPILER_LAUNCHER:STRING=ccache
CMAKE_INSTALL_PREFIX:PATH=/Users/clausklein/.local
CMAKE_MAKE_PROGRAM:FILEPATH=/Users/clausklein/.local/bin/ninja
CMAKE_OSX_ARCHITECTURES:STRING=
CMAKE_OSX_DEPLOYMENT_TARGET:STRING=
CMAKE_OSX_SYSROOT:STRING=
fmt_DIR:PATH=/Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt
bash-5.3$ pwd
/Users/clausklein/Workspace/cpp/cxx20/fmt-module/build/dev/tests/find-package-test
bash-5.3$ ctest
Test project /Users/clausklein/Workspace/cpp/cxx20/fmt-module/build/dev/tests/find-package-test
    Start 1: module_header-only-test
1/1 Test #1: module_header-only-test ..........   Passed    0.00 sec

100% tests passed, 0 tests failed out of 1

Total Test time (real) =   0.01 sec
bash-5.3$ cd ../..

bash-5.3$ find . -name '*.pcm'
./CMakeFiles/fmt.dir/fmt.pcm
./CMakeFiles/__cmake_cxx23.dir/std.compat.pcm
./CMakeFiles/__cmake_cxx23.dir/std.pcm
./tests/find-package-test/CMakeFiles/__cmake_cxx23.dir/std.compat.pcm
./tests/find-package-test/CMakeFiles/__cmake_cxx23.dir/std.pcm
./_CPack_Packages/Darwin/TGZ/fmt-12.1.0-Darwin/lib/cmake/fmt/bmi-Clang_Release/fmt.pcm
bash-5.3$

bash-5.3$ ninja install
[0/2] Re-checking globbed directories...
[0/2] Install the project...
-- Install configuration: "Release"
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/args.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/base.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/chrono.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/color.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/compile.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/core.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/format-inl.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/format.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/os.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/ostream.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/printf.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/ranges.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/std.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/include/fmt/xchar.h
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/libfmt.a
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/module/fmt.cppm
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/bmi-Clang_Release/fmt.pcm  <<<<<<<<
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/module/format.cc
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/module/os.cc
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/fmtConfig.cmake
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/fmtConfigVersion.cmake
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/fmtTargets.cmake
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/fmtTargets-release.cmake
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/./cxx-modules-fmtTargets.cmake
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/./cxx-modules-fmtTargets-Release.cmake
-- Installing: /Users/clausklein/Workspace/cpp/cxx20/fmt-module/stagedir/lib/cmake/fmt/./target-fmt-Release.cmake
bash-5.3$
2

There is currently no mechanism to determine if a prebuilt BMI file is suitable for consumption by a given TU and its command line. Once P2581 (or similar) is available, we can reuse installed BMIs. Note that CMake, today, currently assumes that all BMI usages are compatible. @vito.gamberini is working on fixing that assumption which will fix a number of filed issues.

1 Like
3 
install(TARGETS fmt-header-only EXPORT fmtTargets FILE_SET HEADERS)

if(FMT_USE_MODULES)
    install(
        TARGETS fmt
        EXPORT fmtTargets
        # XXX FILE_SET HEADERS
        FILE_SET public_headers
            DESTINATION
                ${CMAKE_INSTALL_INCLUDEDIR} # Same as default, could be omitted
        # XXX FILE_SET CXX_MODULES
        FILE_SET public_modules DESTINATION ${FMT_INSTALL_CMAKEDIR}/module
        # There's currently no convention for this location
        CXX_MODULES_BMI
            DESTINATION
                ${FMT_INSTALL_CMAKEDIR}/bmi-${CMAKE_CXX_COMPILER_ID}_$<CONFIG>
    )
else()
    install(TARGETS fmt EXPORT fmtTargets FILE_SET public_headers)
endif()
install(FILES module/fmt.cppm DESTINATION ${FMT_INSTALL_CMAKEDIR}/module)
install(FILES ${_fmt_all_sources} DESTINATION ${FMT_INSTALL_CMAKEDIR}/module)

Should the CXX_MODULES_BMI install the std.*.pcm files too?