[FFmpeg-devel] [PATCHv4] avutil/common: add av_rint64_clip

[Hendrik Leppkes]

On Sat, Nov 14, 2015 at 10:27 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> On Sat, Nov 14, 2015 at 4:03 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
>> On Sat, Nov 14, 2015 at 3:28 PM, Hendrik Leppkes <h.leppkes at gmail.com> wrote:
>>> On Sat, Nov 14, 2015 at 3:51 AM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
>>>> On Fri, Nov 13, 2015 at 7:17 PM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
>>>>> Hi,
>>>>>
>>>>> On Fri, Nov 13, 2015 at 6:16 PM, Ganesh Ajjanagadde <gajjanag at mit.edu>
>>>>> wrote:
>>>>>
>>>>>> On Fri, Nov 13, 2015 at 4:52 PM, Ronald S. Bultje <rsbultje at gmail.com>
>>>>>> wrote:
>>>>>> > Hi,
>>>>>> >
>>>>>> > On Fri, Nov 13, 2015 at 4:28 PM, Ganesh Ajjanagadde <gajjanag at mit.edu>
>>>>>> > wrote:
>>>>>> >
>>>>>> >> On Fri, Nov 13, 2015 at 1:28 PM, Ronald S. Bultje <rsbultje at gmail.com>
>>>>>> >> wrote:
>>>>>> >> > Hi,
>>>>>> >> >
>>>>>> >> > On Fri, Nov 13, 2015 at 12:17 PM, Ganesh Ajjanagadde <
>>>>>> gajjanag at mit.edu>
>>>>>> >> > wrote:
>>>>>> >> >
>>>>>> >> >> On Fri, Nov 13, 2015 at 12:10 PM, Ronald S. Bultje <
>>>>>> rsbultje at gmail.com>
>>>>>> >> >> wrote:
>>>>>> >> >> > Hi Ganesh,
>>>>>> >> >> > On Nov 13, 2015 12:02 PM, "Ganesh Ajjanagadde" <
>>>>>> >> gajjanagadde at gmail.com>
>>>>>> >> >> > wrote:
>>>>>> >> >> >>
>>>>>> >> >> >> The rationale for this function is reflected in the documentation
>>>>>> for
>>>>>> >> >> >> it, and is copied here:
>>>>>> >> >> >>
>>>>>> >> >> >> Clip a double value into the long long amin-amax range.
>>>>>> >> >> >> This function is needed because conversion of floating point to
>>>>>> >> integers
>>>>>> >> >> > when
>>>>>> >> >> >> it does not fit in the integer's representation does not
>>>>>> necessarily
>>>>>> >> >> > saturate
>>>>>> >> >> >> correctly (usually converted to a cvttsd2si on x86) which
>>>>>> saturates
>>>>>> >> >> > numbers
>>>>>> >> >> >> > INT64_MAX to INT64_MIN. The standard marks such conversions as
>>>>>> >> >> undefined
>>>>>> >> >> >> behavior, allowing this sort of mathematically bogus conversions.
>>>>>> >> This
>>>>>> >> >> > provides
>>>>>> >> >> >> a safe alternative that is slower obviously but assures safety and
>>>>>> >> >> better
>>>>>> >> >> >> mathematical behavior.
>>>>>> >> >> >> API:
>>>>>> >> >> >> @param a value to clip
>>>>>> >> >> >> @param amin minimum value of the clip range
>>>>>> >> >> >> @param amax maximum value of the clip range
>>>>>> >> >> >> @return clipped value
>>>>>> >> >> >>
>>>>>> >> >> >> Note that a priori if one can guarantee from the calling side that
>>>>>> >> the
>>>>>> >> >> >> double is in range, it is safe to simply do an explicit/implicit
>>>>>> >> cast,
>>>>>> >> >> >> and that will be far faster. However, otherwise this function
>>>>>> should
>>>>>> >> be
>>>>>> >> >> >> used.
>>>>>> >> >> >>
>>>>>> >> >> >> avutil minor version is bumped.
>>>>>> >> >> >>
>>>>>> >> >> >> Reviewed-by: Ronald S. Bultje <rsbultje at gmail.com>
>>>>>> >> >> >> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
>>>>>> >> >> >> ---
>>>>>> >> >> >>  libavutil/common.h  | 30 ++++++++++++++++++++++++++++++
>>>>>> >> >> >>  libavutil/version.h |  2 +-
>>>>>> >> >> >>  2 files changed, 31 insertions(+), 1 deletion(-)
>>>>>> >> >> >>
>>>>>> >> >> >> diff --git a/libavutil/common.h b/libavutil/common.h
>>>>>> >> >> >> index 6f0f582..f4687ab 100644
>>>>>> >> >> >> --- a/libavutil/common.h
>>>>>> >> >> >> +++ b/libavutil/common.h
>>>>>> >> >> >> @@ -298,6 +298,33 @@ static av_always_inline av_const double
>>>>>> >> >> > av_clipd_c(double a, double amin, double
>>>>>> >> >> >>      else               return a;
>>>>>> >> >> >>  }
>>>>>> >> >> >>
>>>>>> >> >> >> +/**
>>>>>> >> >> >> + * Clip and convert a double value into the long long amin-amax
>>>>>> >> range.
>>>>>> >> >> >> + * This function is needed because conversion of floating point
>>>>>> to
>>>>>> >> >> > integers when
>>>>>> >> >> >> + * it does not fit in the integer's representation does not
>>>>>> >> necessarily
>>>>>> >> >> > saturate
>>>>>> >> >> >> + * correctly (usually converted to a cvttsd2si on x86) which
>>>>>> >> saturates
>>>>>> >> >> > numbers
>>>>>> >> >> >> + * > INT64_MAX to INT64_MIN. The standard marks such conversions
>>>>>> as
>>>>>> >> >> > undefined
>>>>>> >> >> >> + * behavior, allowing this sort of mathematically bogus
>>>>>> conversions.
>>>>>> >> >> > This provides
>>>>>> >> >> >> + * a safe alternative that is slower obviously but assures safety
>>>>>> >> and
>>>>>> >> >> > better
>>>>>> >> >> >> + * mathematical behavior.
>>>>>> >> >> >> + * @param a value to clip
>>>>>> >> >> >> + * @param amin minimum value of the clip range
>>>>>> >> >> >> + * @param amax maximum value of the clip range
>>>>>> >> >> >> + * @return clipped value
>>>>>> >> >> >> + */
>>>>>> >> >> >> +static av_always_inline av_const int64_t av_rint64_clip_c(double
>>>>>> a,
>>>>>> >> >> > int64_t amin, int64_t amax)
>>>>>> >> >> >> +{
>>>>>> >> >> >> +#if defined(HAVE_AV_CONFIG_H) && defined(ASSERT_LEVEL) &&
>>>>>> >> ASSERT_LEVEL
>>>>>> >> >> >>= 2
>>>>>> >> >> >> +    if (amin > amax) abort();
>>>>>> >> >> >> +#endif
>>>>>> >> >> >> +    // INT64_MAX+1,INT64_MIN are exactly representable as IEEE
>>>>>> >> doubles
>>>>>> >> >> >> +    if (a >=  9223372036854775808.0 || llrint(a) >= amax)
>>>>>> >> >> >> +        return amax;
>>>>>> >> >> >> +    if (a <= -9223372036854775808.0 || llrint(a) <= amin)
>>>>>> >> >> >> +        return amin;
>>>>>> >> >> >
>>>>>> >> >> > Doesn't this allow negative overflows in the max check? I think you
>>>>>> >> need
>>>>>> >> >> > both overflow checks before the min/max checks. Try the next float
>>>>>> val
>>>>>> >> >> > smaller than int64_min as input with a small amax, eg 0. I bet it
>>>>>> >> >> returns 0
>>>>>> >> >> > instead of amin.
>>>>>> >> >>
>>>>>> >> >> They are needed. As you and others can clearly see, I am quite bad
>>>>>> >> >> with this stuff.
>>>>>> >> >
>>>>>> >> >
>>>>>> >> > Hm, so, getting back to my computer, I wanted to test this, and I have
>>>>>> >> this
>>>>>> >> > problem: llrint() works correctly for me for the "undefined" cases,
>>>>>> i.e.,
>>>>>> >> > it already does what you're trying to fix in av_rint64_clip_c.
>>>>>> >> >
>>>>>> >> > llrint(-10223372056756029440.000000) returns -9223372036854775808
>>>>>> >> > llrint(10223372056756029440.000000) returns 9223372036854775807
>>>>>> >> >
>>>>>> >> > So, how do I reproduce that llrint() overflows?
>>>>>> >>
>>>>>> >> The link I gave originally
>>>>>> >>
>>>>>> http://blog.frama-c.com/index.php?post/2013/10/09/Overflow-float-integer
>>>>>> >> gives an illustration. Maybe the weird behavior happens only on
>>>>>> >> 9223372036854775808.0. This happens because INT64_MAX+1 is not
>>>>>> >> representable in long long, and hence signed overflow occurs yielding
>>>>>> >> INT64_MIN (of course undefined). Here is a minimal test program:
>>>>>> >> #include <stdio.h>
>>>>>> >> #include <math.h>
>>>>>> >>
>>>>>> >> int main(void) {
>>>>>> >>     printf("%lld\n", llrint(9223372036854775808.0));
>>>>>> >>     return 0;
>>>>>> >> }
>>>>>> >>
>>>>>> >
>>>>>> > 9223372036854775807
>>>>>> >
>>>>>> > Seems apple's libc got one thing right :)
>>>>>>
>>>>>> I personally am not that charitable, looking more carefully at your
>>>>>> asm shows a cmplesd, suggesting slowdown. Here is a source reference:
>>>>>> https://opensource.apple.com/source/Libm/Libm-2026/Source/ARM/llrint.c.
>>>>>> As usual, Apple dumps many implementations of llrint and it is unclear
>>>>>> which is actually being used on OS X at the moment (see e.g other
>>>>>> https://opensource.apple.com/source/Libm/Libm-92/i386.subproj/llrint.c),
>>>>>> but I digress.
>>>>>>
>>>>>> They essentially all put special case code like the patch above. Thus
>>>>>> their function is inherently slower than the conformant GNU libm
>>>>>> implementation. A client may very well want a branch free llrint for
>>>>>> speed. Apple offers no performance choice here, forcing a fast llrint
>>>>>> to use cvt2dsi inline or equivalent. Don't know if FFmpeg is affected
>>>>>> by this slowdown.
>>>>>
>>>>>
>>>>> I think FFmpeg should consider using Apple's version as a x86
>>>>> implementation for av_rint64_clip :)
>>>>
>>>> I don't agree with this: it is a far less readable implementation with
>>>> many more lines of code, and worse yet only handles the llrint aspect
>>>> and not the clipping. Regardless, belongs to a separate patch/thread.
>>>> Pushed. Thanks all for reviews.
>>>>
>>>
>>> This change broke building on VS2012, llrint is apprently not available there.
>>> Note that this is a public header, so our compat headers ala
>>> avutil/libm.h cannot be included there.
>>
>> Hmm, so I could create a local avpriv_llrint with some ifdefry - e.g
>> for GNU_C, use llrint, else use a slower implementation on the lines
>> of Apple's.
>> Any cleaner solutions?
>
> Possibly better idea: use floor(f + 0.5) as a hack (c89) on things
> lacking llrint (via a HAVE_LLRINT check). This won't result in
> identical output past a sufficiently large power of 2, but is still a
> safe API. It is also clearer and smaller. Idea inspired by
> avcodec/mpegaudio_tablegen.h (where this hack may be removed).
>

This code is in a public header, public headers don't have access to
config.h, so no HAVE_* checks.
You could make it non-inline, then you have all the freedom in the world.

- Hendrik