Re: [config] First steps towards C++20 modules

From: Daniela Engert (dani_at_[hidden])
Date: 2024-08-12 18:17:44

Am 12.08.2024 um 19:38 schrieb Antony Polukhin via Boost:
> ср, 10 июл. 2024 г. в 00:01, Antony Polukhin<antoshkka_at_[hidden]>:
>> понедельник, 8 июля 2024 г. пользователь Ruben Perez<rubenperez038_at_[hidden]> написал:
>> <...>
>>> I'm happy to hear your opinion.
>> My opinion is that I have not enough experience with modules yet :)
>>
>> To fix that I'll try to modularise Boost.PFR in a few months. Many thanks for the links, especially with CMake scripts for modules
> Here's my first attempthttps://github.com/boostorg/pfr/pull/177/files
>
> I've only tested it on clang-15. Looks like the compiler has issues
> with mixing global module fragment and includes (just as was noted
> above), so I've ended up with a toy sample that uses only std::puts.
> And it works!
>
> The compilation time change is qute impressive:
>
> $ time clang++-15 -std=c++20 -I /data/code/boost/
> example/motivating_example0.cpp
> real    0m2,051s
> user    0m1,844s
> sys    0m0,144s
>
> $ time clang++-15 -std=c++20 -fmodule-file=pfr.pcm pfr.pcm
> example/module_usage.cpp
> real    0m0,375s
> user    0m0,249s
> sys    0m0,107s
>
> No idea how to deal with dependencies between diffrent Boost
> libraries, so I'd probably try to modularise Boost.Core +
> Boost.TypeIndex to get the idea.
>
Cool!

Please be aware that Clang-15 is riddled with module-related bugs 
(according to one of the compiler devlopers I've been speaking to in 
Varna last year). With Clang-16, this is still true, Clang-17 is quite 
usable if you know how to deal with the ABI problems. And yet, Clang-18 
still has troubles with merging definitions coming from multiple TUs.

Dani

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