[FFmpeg-devel] [PATCH] Avoid creating unecessary dependencies on thread libraries.

[wm4]

On Mon, 21 Nov 2016 09:58:33 -0500
"Gregory J. Wolfe" <gregory.wolfe at kodakalaris.com> wrote:

> (1) Multi-threading support requires knowing the number of CPUs available.
> When building with MinGW on a Windows system, both gcc and Windows run
> time functions are available to get this information.  However, when Windows
> threading has been selected, the Windows function should be used, not the
> gcc function.  This avoids creating an unnecessary dependency on the gcc
> thread library.
> 
> (2) When ALL threading support is disabled, the build should not create a
> dependency on ANY thread library.
> 
> Signed-off-by: Gregory J. Wolfe <gregory.wolfe at kodakalaris.com>
> ---
>  libavutil/cpu.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/libavutil/cpu.c b/libavutil/cpu.c
> index f5785fc..3843778 100644
> --- a/libavutil/cpu.c
> +++ b/libavutil/cpu.c
> @@ -258,10 +258,15 @@ int av_cpu_count(void)
>      static volatile int printed;
>  
>      int nb_cpus = 1;
> +#if HAVE_THREADS
>  #if HAVE_WINRT
>      SYSTEM_INFO sysinfo;
>  #endif
> -#if HAVE_SCHED_GETAFFINITY && defined(CPU_COUNT)
> +    // if HAVE_W32THREADS and HAVE_GETPROCESSAFFINITYMASK, we will use
> +    // Windows GetProcessAffinityMask() instead of gcc library function
> +    // sched_getaffinity().  This avoids creating a dependency on the gcc
> +    // thread library that we don't need/want.
> +#if HAVE_SCHED_GETAFFINITY && defined(CPU_COUNT) && !(HAVE_W32THREADS && HAVE_GETPROCESSAFFINITYMASK)
>      cpu_set_t cpuset;
>  
>      CPU_ZERO(&cpuset);
> @@ -286,6 +291,7 @@ int av_cpu_count(void)
>      GetNativeSystemInfo(&sysinfo);
>      nb_cpus = sysinfo.dwNumberOfProcessors;
>  #endif
> +#endif
>  
>      if (!printed) {
>          av_log(NULL, AV_LOG_DEBUG, "detected %d logical cores\n", nb_cpus);

Wouldn't it be better to move the HAVE_GETPROCESSAFFINITYMASK case
above HAVE_SCHED_GETAFFINITY, and avoid making the ifdeffery mess
worse? GetProcessAffinityMask is available on every supported Windows
and could as well be replaced by "ifdef _WIN32" instead.

Also, technically you might have to stop the configure script to stop
linking to libpthread. I'm not sure what exactly stops it from doing
that in your case.