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

[wm4]

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.