Re: [igt-dev] [PATCH i-g-t v3 1/4] meson: add libatomic dependency

["Ser, Simon" <simon.ser@intel.com>]

On Tue, 2019-06-18 at 13:27 +0100, Guillaume Tucker wrote:
> Add conditional dependency on libatomic in order to be able to use the
> __atomic_* functions instead of the older __sync_* ones.  The
> libatomic library is only needed when there aren't any native support
> on the current architecture, so a linker test is used for this
> purpose.  This enables atomic operations to be on a wider number of
> architectures including MIPS.
> 
> Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
> ---
> 
> Notes:
>     v2: add linker test for libatomic
>     v3: use null_dep
> 
>  meson.build | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/meson.build b/meson.build
> index 6268c58d3634..118ad667ffb5 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -180,6 +180,20 @@ realtime = cc.find_library('rt')
>  dlsym = cc.find_library('dl')
>  zlib = cc.find_library('z')
>  
> +if cc.links('''
> +#include <stdint.h>
> +int main(void) {
> +  uint32_t x32 = 0;
> +  uint64_t x64 = 0;
> +  __atomic_load_n(&x32, __ATOMIC_SEQ_CST);
> +  __atomic_load_n(&x64, __ATOMIC_SEQ_CST);

See my reply for v2. I've looked into this a little bit more and it
looks like __atomic_* functions are a GCC implementation detail. OIn
other words, the C11 standard [1] defines only atomic_* functions, and
GCC implements them with __atomic_* builtins when the platform supports
it, but other compilers might not expose those builtins and still
support atomic_* functions without them. This also seems to be what [2]
explains:

> The first set of library functions are named __atomic_*. This set has
> been “standardized” by GCC, and is described below. (See also GCC’s
> documentation)

(Notice the quotes around “standardized”, meaning they are a GCC
extension)

[1]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf
[2]: https://llvm.org/docs/Atomics.html

> +  return 0;
> +}''', name : 'built-in atomics')
> +	libatomic = null_dep
> +else
> +	libatomic = cc.find_library('atomic')
> +endif
> +
>  if cc.has_header('linux/kd.h')
>  	config.set('HAVE_LINUX_KD_H', 1)
>  endif
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