[vlc-devel] [PATCH 4/4] cpu: remove initialization lock

[Steve Lhomme]

Looks OK (apart from the unsigned / uint32_t mismatch)

On 2020-03-29 22:43, Rémi Denis-Courmont wrote:
> In the unlikely event that multiple threads want to check the CPU
> flags concurrently, simply compute them as many times.
> ---
>   src/misc/cpu.c | 20 +++++++++++---------
>   1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/src/misc/cpu.c b/src/misc/cpu.c
> index 7f3a89a3cb..7053a391e7 100644
> --- a/src/misc/cpu.c
> +++ b/src/misc/cpu.c
> @@ -29,6 +29,8 @@
>   # include "config.h"
>   #endif
>   
> +#include <stdatomic.h>
> +
>   #include <vlc_common.h>
>   #include <vlc_cpu.h>
>   #include <vlc_memstream.h>
> @@ -247,18 +249,18 @@ out:
>       return i_capabilities;
>   }
>   
> -static uint32_t cpu_flags;
> -
> -static void vlc_CPU_init(void)
> -{
> -    cpu_flags = vlc_CPU_raw();
> -}
> +static atomic_uint cpu_flags = ATOMIC_VAR_INIT(-1);
>   
>   VLC_WEAK unsigned vlc_CPU(void)
>   {
> -    static vlc_once_t once = VLC_STATIC_ONCE;
> -    vlc_once(&once, vlc_CPU_init);
> -    return cpu_flags;
> +    unsigned flags = atomic_load_explicit(&cpu_flags, memory_order_relaxed);
> +
> +    if (unlikely(flags == -1U)) {
> +        flags = vlc_CPU_raw();
> +        atomic_store_explicit(&cpu_flags, flags, memory_order_relaxed);
> +    }
> +
> +    return flags;
>   }
>   
>   void vlc_CPU_dump (vlc_object_t *obj)
> -- 
> 2.26.0
> 
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