[FFmpeg-devel] [PATCH] lavc/aacenc_utils: unroll abs_pow34_v loop

[Ganesh Ajjanagadde]

On Sat, Mar 19, 2016 at 9:09 AM, Reimar Döffinger
<Reimar.Doeffinger at gmx.de> wrote:
> On Sat, Mar 19, 2016 at 12:42:09PM +0100, Clément Bœsch wrote:
>> On Fri, Mar 18, 2016 at 10:12:14PM -0700, Ganesh Ajjanagadde wrote:
>> > -static inline void abs_pow34_v(float *av_restrict out, const float *av_restrict in, const int size)
>> > -{
>> > -    int i;
>> > -    for (i = 0; i < size; i++) {
>> > -        float a = fabsf(in[i]);
>> > -        out[i] = sqrtf(a * sqrtf(a));
>> > -    }
>> > -}
>> > -
>> >  static inline float pos_pow34(float a)
>> >  {
>> >      return sqrtf(a * sqrtf(a));
>> >  }
>> >
>> > +static inline void abs_pow34_v(float *av_restrict out, const float *av_restrict in, const int size)
>> > +{
>> > +    av_assert2(!(size % 4));
>> > +    for (int i = 0; i < size; i+=4) {
>> > +        float a0 = fabsf(in[i]);
>> > +        float a1 = fabsf(in[i+1]);
>> > +        float a2 = fabsf(in[i+2]);
>> > +        float a3 = fabsf(in[i+3]);
>> > +        out[i  ] = pos_pow34(a0);
>> > +        out[i+1] = pos_pow34(a1);
>> > +        out[i+2] = pos_pow34(a2);
>> > +        out[i+3] = pos_pow34(a3);
>> > +    }
>> > +}
>> > +
>>
>> I'm curious (and lazy), is GCC able to unroll by itself if you hint it
>> with a loop such as:
>>
>>     int i;
>>     for (i = 0; i < size & ~3; i++) {
>>         float a = fabsf(in[i]);
>>         out[i] = sqrtf(a * sqrtf(a));
>>     }

Does not help, yields ~ 140 decicycles like earlier.

>
> I haven't been able to to figure out for
> sure for this one, but at least the other one
> Debian gcc 5.3.1 already unrolls and vectorizes
> for me, though it has a bit of extra code to
> handle cases where size is not a multiple of 4.

I suspect the speed change in that case is coming from the removal of
such extra code, as I am running gcc 5.3.0 on Arch.

> So I suspect "which gcc?" is probably an important
> question.
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