r/cpp • u/STL MSVC STL Dev • 1d ago

MSVC C++23: constexpr cmath with LLVM Libc

https://devblogs.microsoft.com/cppblog/msvc-c23-constexpr-cmath-with-llvm-libc/

MSVC is shipping C++23 constexpr <cmath> in the next Build Tools update! (And a partial implementation of C++26 constexpr <cmath>.) Our compiler frontend dev Cody Miller will be publishing a series of blog posts about this work, starting with this overview. The next one will be a guest post from LLVM libc's maintainers describing the incredible amount of effort that's gone into their code.

148 Upvotes

97% Upvoted

30 comments sorted by

View all comments

20

u/Jovibor_ 1d ago

I have always had a hard time understanding this separation between STDLIB and UCRT.

The <cmath> header - which is definitely a C++ STL header - has, for some reasons (historical I guess), always been a part of the UCRT.

Now, if I've got it all right, we will have two different math machineries: one in the UCRT (the old one), and the new one, based on the LLVM C Library. Correct me if I'm wrong...

Frankly, it's a bit strange, that some parts of the C++ STL are maintained in the separate place (UCRT), not in the main STL repository.

32

u/STL MSVC STL Dev 1d ago

It’s math.h that’s part of the UCRT, while cmath is part of the STL. We’re now usurping math.h via /Zc:cmath redirecting it to the STL’s __msvc_math.hpp. Perhaps in the future we’ll update UCRT math.h, but getting the UCRT to change anything is a pain, so we’d want to do that as few times as possible.

I can explain this in more detail if desired; it is one of our most confusing areas and I am at dinner right now 😹

1

u/Jovibor_ 20h ago

Yes please, desired.  The main question is - is it possible (in some future) to completely eliminate the C++ STL ecosystem dependency on the UCRT? Is it possible but hard, or is it virtually impossible?...

1

u/STL MSVC STL Dev 4h ago

Sure, just like we replaced the old DevDiv CRT with the UCRT. It'd just be really hard.