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