[FFmpeg-devel] [PATCH v2] cpu: add a function for querying maximum required data alignment

[James Almer]

On 9/27/2017 9:19 PM, wm4 wrote:
> On Wed, 27 Sep 2017 21:15:39 -0300
> James Almer <jamrial at gmail.com> wrote:
> 
>> On 9/4/2017 5:53 PM, Michael Niedermayer wrote:
>>> On Mon, Sep 04, 2017 at 09:27:32PM +0200, wm4 wrote:  
>>>> On Mon, 4 Sep 2017 21:18:35 +0200
>>>> Michael Niedermayer <michael at niedermayer.cc> wrote:
>>>>  
>>>>> On Sat, Sep 02, 2017 at 09:47:38PM -0300, James Almer wrote:  
>>>>>> From: Anton Khirnov <anton at khirnov.net>
>>>>>>
>>>>>> (cherry picked from commit e6bff23f1e11aefb16a2b5d6ee72bf7469c5a66e)
>>>>>> Signed-off-by: James Almer <jamrial at gmail.com>
>>>>>> ---
>>>>>> This is (afaics) the last API introduced to libav before the major bump.
>>>>>>
>>>>>> Now checking all the x86 flags that would require aligment of 16 bytes
>>>>>> or more.
>>>>>>
>>>>>>  doc/APIchanges      |  3 +++
>>>>>>  libavutil/cpu.c     | 35 +++++++++++++++++++++++++++++++++++
>>>>>>  libavutil/cpu.h     | 13 +++++++++++++
>>>>>>  libavutil/version.h |  2 +-
>>>>>>  4 files changed, 52 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/doc/APIchanges b/doc/APIchanges
>>>>>> index 4effbf9364..6a57c210a9 100644
>>>>>> --- a/doc/APIchanges
>>>>>> +++ b/doc/APIchanges
>>>>>> @@ -15,6 +15,9 @@ libavutil:     2015-08-28
>>>>>>  
>>>>>>  API changes, most recent first:
>>>>>>  
>>>>>> +2017-09-xx - xxxxxxx - lavu 55.75.100 / lavu 55.31.0 - cpu.h
>>>>>> +  Add av_cpu_max_align() for querying maximum required data alignment.
>>>>>> +
>>>>>>  2017-09-01 - xxxxxxx - lavf 57.81.100 - avio.h
>>>>>>    Add avio_read_partial().
>>>>>>  
>>>>>> diff --git a/libavutil/cpu.c b/libavutil/cpu.c
>>>>>> index a22da0fa8c..4f04da2460 100644
>>>>>> --- a/libavutil/cpu.c
>>>>>> +++ b/libavutil/cpu.c
>>>>>> @@ -16,9 +16,11 @@
>>>>>>   * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
>>>>>>   */
>>>>>>  
>>>>>> +#include <stddef.h>
>>>>>>  #include <stdint.h>
>>>>>>  #include <stdatomic.h>
>>>>>>  
>>>>>> +#include "attributes.h"
>>>>>>  #include "cpu.h"
>>>>>>  #include "cpu_internal.h"
>>>>>>  #include "config.h"
>>>>>> @@ -299,3 +301,36 @@ int av_cpu_count(void)
>>>>>>  
>>>>>>      return nb_cpus;
>>>>>>  }
>>>>>> +
>>>>>> +size_t av_cpu_max_align(void)
>>>>>> +{
>>>>>> +    int av_unused flags = av_get_cpu_flags();
>>>>>> +
>>>>>> +#if ARCH_ARM || ARCH_AARCH64
>>>>>> +    if (flags & AV_CPU_FLAG_NEON)
>>>>>> +        return 16;
>>>>>> +#elif ARCH_PPC
>>>>>> +    if (flags & AV_CPU_FLAG_ALTIVEC)
>>>>>> +        return 16;
>>>>>> +#elif ARCH_X86
>>>>>> +    if (flags & (AV_CPU_FLAG_AVX2     |
>>>>>> +                 AV_CPU_FLAG_AVX      |
>>>>>> +                 AV_CPU_FLAG_FMA4     |
>>>>>> +                 AV_CPU_FLAG_FMA3))
>>>>>> +        return 32;
>>>>>> +    if (flags & (AV_CPU_FLAG_XOP      |
>>>>>> +                 AV_CPU_FLAG_AESNI    |
>>>>>> +                 AV_CPU_FLAG_SSE42    |
>>>>>> +                 AV_CPU_FLAG_SSE4     |
>>>>>> +                 AV_CPU_FLAG_SSSE3    |
>>>>>> +                 AV_CPU_FLAG_SSE3     |
>>>>>> +                 AV_CPU_FLAG_SSE2     |
>>>>>> +                 AV_CPU_FLAG_SSE      |
>>>>>> +                 AV_CPU_FLAG_AVXSLOW  |
>>>>>> +                 AV_CPU_FLAG_SSE3SLOW |
>>>>>> +                 AV_CPU_FLAG_SSE2SLOW))
>>>>>> +        return 16;
>>>>>> +#endif
>>>>>> +
>>>>>> +    return 8;
>>>>>> +}
>>>>>> diff --git a/libavutil/cpu.h b/libavutil/cpu.h
>>>>>> index de05593446..9e5d40affe 100644
>>>>>> --- a/libavutil/cpu.h
>>>>>> +++ b/libavutil/cpu.h
>>>>>> @@ -21,6 +21,8 @@
>>>>>>  #ifndef AVUTIL_CPU_H
>>>>>>  #define AVUTIL_CPU_H
>>>>>>  
>>>>>> +#include <stddef.h>
>>>>>> +
>>>>>>  #include "attributes.h"
>>>>>>  
>>>>>>  #define AV_CPU_FLAG_FORCE    0x80000000 /* force usage of selected flags (OR) */    
>>>>>
>>>>>  
>>>>>> @@ -113,4 +115,15 @@ int av_parse_cpu_caps(unsigned *flags, const char *s);
>>>>>>   */
>>>>>>  int av_cpu_count(void);
>>>>>>  
>>>>>> +/**
>>>>>> + * Get the maximum data alignment that may be required by FFmpeg.
>>>>>> + *
>>>>>> + * Note that this is affected by the build configuration and the CPU flags mask,
>>>>>> + * so e.g. if the CPU supports AVX, but libavutil has been built with
>>>>>> + * --disable-avx or the AV_CPU_FLAG_AVX flag has been disabled through
>>>>>> + *  av_set_cpu_flags_mask(), then this function will behave as if AVX is not
>>>>>> + *  present.
>>>>>> + */
>>>>>> +size_t av_cpu_max_align(void);    
>>>>>
>>>>> This might interact badly with runtime cpu flags/mask changes
>>>>>
>>>>> If its used to choose the alignment for allocated frames and
>>>>> after some are allocated the cpu flags are changed there could be
>>>>> still frames in queues that may not have sufficient alignment for the
>>>>> new flags  
>>>>
>>>> There's no such thing as runtime CPU flag changes.  
>>>
>>> We even have an API to change the cpu flags at runtime.
>>>
>>> av_set_cpu_flags_mask() and av_force_cpu_flags()
>>>
>>> There is no restriction in the API on when they can be called.
>>>
>>> you can call av_force_cpu_flags(0) then open a decoder then call
>>> av_force_cpu_flags(AV_CPU_FLAG_MMX|AV_CPU_FLAG_SSE)
>>> then open a filter or encoder
>>> then run the code and it could crash as the allocated frames from
>>> earlier are not aligned enough for the 2nd filter or encoder
>>>
>>> There also may be other scenarios where this can occur  
>>
>> Alright, I just arrived to this cherry pick during merges. So my
>> question is, do i apply it and skip the commit that makes use of it to
>> choose alignment within ffmpeg?
>>
>> Even if unused at first, I'd rather not skip its addition.
> 
> API users can rely on it. So you better make sure that can't break.
> Other aspects don't really matter.

What do you mean with this? Do i apply it or not? The function will
return a value based on runtime cpuflags. The value is guaranteed to be
correct, but how the user uses it is beyond our control.