On Sun, May 24, 2026 at 5:21 PM Adrian Johnston via Std-Proposals <std-proposals@lists.isocpp.org> wrote:

For this round of tests I downloaded the latest versions of GCC, Clang and MSVC. I also compared against the current Ubuntu release of GCC and Clang.

Compiler Version Module Build Module Use Include All
g++ 16.1.0 3345ms 249ms 1579ms
g++ 15.2.0 2280ms 172ms 877ms
clang 22.16.6 2014ms 52ms 1545m
clang 20.1.8 1491ms 44ms 1142ms
MSVC 19.38.33145 2058ms 81ms 1295ms
Right now none of the compilers provided submodule support that I saw for the standard library. That means these are single threaded numbers as there is no opportunity for parallelism. These are C++23 unoptimized builds. The module use numbers involve importing the module and then emitting a single trivial function that used the module.

Compiler	Version	Module Build	Module Use	Include All
g++	16.1.0	3345ms	249ms	1579ms
g++	15.2.0	2280ms	172ms	877ms
clang	22.16.6	2014ms	52ms	1545m
clang	20.1.8	1491ms	44ms	1142ms
MSVC	19.38.33145	2058ms	81ms	1295ms

When you say "module build", what does this mean? Is this what happens when you change a command line parameter that forces a rebuild of the C++ standard library module? How often does such a rebuild need to happen?

Also, what "submodule support" are you referring to? The C++ standard doesn't have any submodules; there are just `std` and `std.compat`. And looking at those module use numbers, they seem pretty reasonable; GCC could use some improvement, but the rest are at least an order of magnitude better than includes.