Re: Gnulib's use of _Atomic, and hamt

Hi Paul,

Paul Eggert wrote:
> Still, it appears that using _Atomic is dicey, at least in the Gnulib world which ports to older platforms. It's not clear to me whether or when Gnulib-using code should use _Atomic instead of Gnulib's simple-atomic module, for example.

On the contrary, I think we're not far from the point where we can bump the
minimum requirement from C99 to C11, and assume _Atomic (at least for small
types).

_Atomic is supported in
  - gcc >= 4.9
  - clang >= 4
  - MSVC since 2026 [1],
  - Fil-C (for small types only [2])
but
  - requires a special compiler option with Sun C: -xatomic=studio,
  - is ignored in TinyCC (tcc).

These are the distros with older GCC and clang versions:

GCC

3.3.2   Fedora 1
3.3.3   FreeBSD 5.2.1
4.1.2   CentOS 5
4.1.3   NetBSD 5
4.2.1   OpenBSD 6.x
4.4.7   CentOS 6
4.5.2   Slackware 13.37
4.7.2   kFreeBSD 7
4.6.2   openSUSE 12.1
4.7.1   Slackware 14
4.8.2   Ubuntu 14.04
4.8.5   CentOS 7

clang

3.8.0   FreeBSD 11

These are all end-of-life / out-of-support by now.

By the way, C11 atomics are nicely explained here: [3]

> First, it misspelled __STDC_NO_ATOMICS__, a bug that nobody has reported
> since hamt was added to Gnulib in 2021.

This is in a place where is does not matter, because the previous parts of
the #if condition already make sure that atomics are supported.

> Come to think of it, should we remove the hamt module from Gnulib?

I think we should keep it, because
  - It's a general-purpose module.
  - It's well documented [4].
  - As you have just shown, we can use it as a "guinea pig" module for _Atomic
    support, going forward.

Bruno

[1] https://learn.microsoft.com/en-us/cpp/overview/what-s-new-for-msvc?view=msvc-170
[2] https://github.com/pizlonator/fil-c/issues/299
[3] https://beej.us/guide/bgc/html/split/chapter-atomics.html
[4] https://www.gnu.org/software/gnulib/manual/html_node/Specialized-containers.html