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llvm-project/libcxx/modules/README.md
Mark de Wever 4d4c70c91c [libc++][modules] Removes the module partitions. 
This patch is based on the suggestion by @ChuanqiXu on discourse
(https://discourse.llvm.org/t/alternatives-to-the-implementation-of-std-modules/71958)

Instead of making a module partition per header every header gets an inc
file which contains the exports per header. The std module then includes
all public headers and these inc files. The one file per header is
useful for testing purposes. The CI tests whether the exports of a
header's module partition matches the "public" named declarations in the
header. With one file per header this can still be done.

The patch improves compilation time of files using "import std;" and the
size of the std module.

A comparision of the compilation speed using a libc++ test
  build/bin/llvm-lit -a -Dstd=c++23 -Denable_modules=std libcxx/test/std/modules/std.pass.cpp

Which boils down to
  import std;

  int main(int, char**) {
    std::println("Hello modular world");
    return 0;
  }
and has -ftime-report enabled

Before
===-------------------------------------------------------------------------===
                          Clang front-end time report
===-------------------------------------------------------------------------===
  Total Execution Time: 8.6585 seconds (8.6619 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
   4.5041 ( 57.2%)   0.4264 ( 54.4%)   4.9305 ( 56.9%)   4.9331 ( 57.0%)  Clang front-end timer
   3.2037 ( 40.7%)   0.2408 ( 30.7%)   3.4445 ( 39.8%)   3.4452 ( 39.8%)  Reading modules
   0.1665 (  2.1%)   0.1170 ( 14.9%)   0.2835 (  3.3%)   0.2837 (  3.3%)  Loading .../build/test/__config_module__/CMakeFiles/std.dir/std.pcm
   7.8744 (100.0%)   0.7842 (100.0%)   8.6585 (100.0%)   8.6619 (100.0%)  Total

After
===-------------------------------------------------------------------------===
                          Clang front-end time report
===-------------------------------------------------------------------------===
  Total Execution Time: 1.2420 seconds (1.2423 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
   0.8892 ( 84.6%)   0.1698 ( 88.8%)   1.0590 ( 85.3%)   1.0590 ( 85.2%)  Clang front-end timer
   0.1533 ( 14.6%)   0.0168 (  8.8%)   0.1701 ( 13.7%)   0.1704 ( 13.7%)  Reading modules
   0.0082 (  0.8%)   0.0047 (  2.5%)   0.0129 (  1.0%)   0.0129 (  1.0%)  Loading .../build/test/__config_module__/CMakeFiles/std.dir/std.pcm
   1.0507 (100.0%)   0.1913 (100.0%)   1.2420 (100.0%)   1.2423 (100.0%)  Total

Using "include <print>" instead of "import module;"
===-------------------------------------------------------------------------===
                          Clang front-end time report
===-------------------------------------------------------------------------===
  Total Execution Time: 2.1507 seconds (2.1517 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
   1.9714 (100.0%)   0.1793 (100.0%)   2.1507 (100.0%)   2.1517 (100.0%)  Clang front-end timer
   1.9714 (100.0%)   0.1793 (100.0%)   2.1507 (100.0%)   2.1517 (100.0%)  Total

It's possible to use the std module in external projects
(https://libcxx.llvm.org/Modules.html#using-in-external-projects)

Tested this with a private project to validate the size of the generated files:

Before
$ du -sch std-*
448M	std-build
508K	std-src
120K	std-subbuild
449M	total

After
$ du -sch std-*
29M	std-build
1004K	std-src
132K	std-subbuild
30M	total

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D156907
2023-08-09 19:39:08 +02:00

1.1 KiB
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The "module partitions" for the std module

The files in this directory contain the exported named declarations per header. These files are used for the following purposes:

  • During testing exported named declarations are tested against the named declarations in the associated header. This excludes reserved names; they are not exported.
  • Generate the module std.

These use cases require including the required headers for these "partitions" at different locations. This means the user of these "partitions" are responsible for including the proper header and validating whether the header can be loaded in the current libc++ configuration. For example "include " fails when locales are not available. The "partitions" use the libc++ feature macros to export the declarations available in the current configuration. This configuration is available if the user includes the `__config' header.

We use .inc files that we include from the top-level module instead of using real C++ module partitions. This is a lot faster than module partitions, see this for details.