[vlc-devel] [PATCH 09/12] network: io: Remove VLA usages

[Rémi Denis-Courmont]

Le mercredi 9 décembre 2020, 11:38:42 EET Thomas Guillem a écrit :
> MSCV is a recent compiler, and we should support it

No?

The pragmatic reality is that development/debug builds must be done with GCC 
or Clang to get all the diagnostics, test all the optimisations and simply to 
keep your sanity and not keep hitting accidental compatibility regressions.

Likewise, production builds must be done with GCC or Clang again for the 
optimisations, as well as for licensing reasons (which we've covered multiple 
times in the past already).

Considering that different people have different, and somtimes conflicting, goals 
and opinions, I do like to use the full C11 specification as an *objective* 
reference. I also think people should be allowed to use other complete C11 
compilers for research purpose. But I do note that some developers here 
ostensibly don't even agree with that (see below).

However, I don't want to make the code slower, buggier and more complicated 
just to support "incomplete" C11 compilers. It's open-source so you are free 
to fork if you want to but I don't want to have to deal with it.

> (lot of  open-source projects support mingw and msvc).

Not only don't I see why we should care. But I do recall several active VLC 
developers arguing that we should just cross the rubicon back again and use 
Clang-compatible GCCisms, or even use Clangisms. Indeed, as much as I am 
loathe to drop C11, I can only acknowledge that it would help with macros, for 
instance.

And we've also had developers push patches that plainly contradict C11. TO be 
fair, VLC never supported a pure C11 environment; it has always required not 
only "optional" C11 features, but plenty of POSIX or OS-specific stuff on top.

tl;dr: we can't have it both ways.

> Some other projects tend to remove VLA usage :
> https://www.phoronix.com/scan.php?page=news_item&px=Linux-Kills-The-VLA

The Linux kernel is running on bare metal and written in free-standing GNU 
C89, with a completely custom memory model and atomic variable API. VLC is 
application code, written in hosted ISO C11, with the standard memory model 
and atomic variables. The comparison is as vain as it gets.

And as for VLAs, the Linux kernel uses typically 8 KiB stacks in physical 
memory, and has hard real-time use cases. I can see why you wouldn't want VLA 
in such environment. Meanwhile, VLC has, what, 2 MiB stacks in virtual memory. 
Last time I tried couldn't even run with stacks less than 128 KiB on 386, 
regardless of VLAs. It's also nowhere near real-time-capable.

-- 
Rémi Denis-Courmont