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

[James Almer <jamrial@gmail.com>]

On 2/18/2024 1:27 PM, Andreas Rheinhardt wrote:
> James Almer:
>> 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))
>> +        ptr = aligned_malloc(size, ALIGN);
>> +    else
>> +        ptr = malloc(size);
>>   #endif
>>       if(!ptr && !size) {
>>           size = 1;
> 
> 1. The function is called aligned_alloc (how did you test this?).

By compiling on a target with _aligned_malloc(), which hid my mistake.

> 2. C11: "The value of alignment shall be a valid alignment supported by
> the implementation and the value of size shall be an integral multiple
> of alignment."

Well, that sure is not very useful...

> a) To use this, you would have to round size upwards; but this will make
> sanitiziers more lenient.
> b) If ALIGN is just not supported by the implementation, then everything
> is UB in C11.
> 3. What's the advantage of this patch anyway?

Removing all the different target specific allocation functions in favor 
of the standard one. But your second point makes it moot, so patch 
withdrawn.

> 
> - Andreas
> 
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