Re: [FFmpeg-devel] [PATCH] avutil/mem: use C11 aligned_malloc()

["Rémi Denis-Courmont" <remi@remlab.net>]

Le sunnuntaina 18. helmikuuta 2024, 18.16.36 EET James Almer a écrit :
> Save for the Microsoft C Runtime library, where free() can't handle aligned
> buffers, aligned_malloc() should be available and working on all supported
> targets.
> Also, malloc() alone may be sufficient if alignment requirement is low, so
> add a check for it.
> 
> Signed-off-by: James Almer <jamrial@gmail.com>
> ---
>  configure       |  2 --
>  libavutil/mem.c | 42 ++++++------------------------------------
>  2 files changed, 6 insertions(+), 38 deletions(-)
> 
> diff --git a/configure b/configure
> index 7c45ac25c8..8fd2895ac2 100755
> --- a/configure
> +++ b/configure
> @@ -6450,8 +6450,6 @@ if test -n "$custom_allocator"; then
>  fi
> 
>  check_func_headers malloc.h _aligned_malloc     && enable aligned_malloc
> -check_func  ${malloc_prefix}memalign            && enable memalign
> -check_func  ${malloc_prefix}posix_memalign      && enable posix_memalign
> 
>  check_func  access
>  check_func_headers stdlib.h arc4random_buf
> diff --git a/libavutil/mem.c b/libavutil/mem.c
> index 36b8940a0c..a72981d1ab 100644
> --- a/libavutil/mem.c
> +++ b/libavutil/mem.c
> @@ -100,44 +100,14 @@ void *av_malloc(size_t size)
>      if (size > atomic_load_explicit(&max_alloc_size, memory_order_relaxed))
> return NULL;
> 
> -#if HAVE_POSIX_MEMALIGN
> -    if (size) //OS X on SDK 10.6 has a broken posix_memalign implementation
> -    if (posix_memalign(&ptr, ALIGN, size))
> -        ptr = NULL;
> -#elif HAVE_ALIGNED_MALLOC
> +#if HAVE_ALIGNED_MALLOC
>      ptr = _aligned_malloc(size, ALIGN);
> -#elif HAVE_MEMALIGN
> -#ifndef __DJGPP__
> -    ptr = memalign(ALIGN, size);
> -#else
> -    ptr = memalign(size, ALIGN);
> -#endif
> -    /* Why 64?
> -     * Indeed, we should align it:
> -     *   on  4 for 386
> -     *   on 16 for 486
> -     *   on 32 for 586, PPro - K6-III
> -     *   on 64 for K7 (maybe for P3 too).
> -     * Because L1 and L2 caches are aligned on those values.
> -     * But I don't want to code such logic here!
> -     */
> -    /* Why 32?
> -     * For AVX ASM. SSE / NEON needs only 16.
> -     * Why not larger? Because I did not see a difference in benchmarks ...
> -     */
> -    /* benchmarks with P3
> -     * memalign(64) + 1          3071, 3051, 3032
> -     * memalign(64) + 2          3051, 3032, 3041
> -     * memalign(64) + 4          2911, 2896, 2915
> -     * memalign(64) + 8          2545, 2554, 2550
> -     * memalign(64) + 16         2543, 2572, 2563
> -     * memalign(64) + 32         2546, 2545, 2571
> -     * memalign(64) + 64         2570, 2533, 2558
> -     *
> -     * BTW, malloc seems to do 8-byte alignment by default here.
> -     */
>  #else
> -    ptr = malloc(size);
> +    // malloc may already allocate sufficiently aligned buffers
> +    if (ALIGN > _Alignof(max_align_t))

If you ever try to reintroduce something like this, you would need 
<stdalign.h> here, and thus you should use alignof rather than _Alignof (which 
was already deprecated by C23 deprecated).

> +        ptr = aligned_malloc(size, ALIGN);
> +    else
> +        ptr = malloc(size);
>  #endif
>      if(!ptr && !size) {
>          size = 1;


-- 
Rémi Denis-Courmont
http://www.remlab.net/



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