[FFmpeg-devel] [PATCH 2/3] avcodec/gdv: Optimize and factorize scaling loops

[Carl Eugen Hoyos]

2019-01-12 16:46 GMT+01:00, Michael Niedermayer <michael at niedermayer.cc>:
> On Sat, Jan 12, 2019 at 04:07:42PM +0100, Carl Eugen Hoyos wrote:
>> 2019-01-04 20:22 GMT+01:00, Michael Niedermayer <michael at niedermayer.cc>:
>>
>> > +static void scaledown(uint8_t *dst, const uint8_t *src, int w)
>> > +{
>> > +    int x;
>> > +    for (x = 0; x < w - 7; x+=8) {
>> > +        dst[x + 0] = src[2*x + 0];
>> > +        dst[x + 1] = src[2*x + 2];
>> > +        dst[x + 2] = src[2*x + 4];
>> > +        dst[x + 3] = src[2*x + 6];
>> > +        dst[x + 4] = src[2*x + 8];
>> > +        dst[x + 5] = src[2*x +10];
>> > +        dst[x + 6] = src[2*x +12];
>> > +        dst[x + 7] = src[2*x +14];
>>
>> Could you add to the commit message the information
>> which compiler is able to optimize this?
>> (Assuming this is a reason for the speedup)
>
> if what you ask for is "which compiler turns this into SIMD"
> i do not know, and i suspect mine does not from the limited
> increase in performance
> I think the speedup is primarly from simply unrolling the trivial loop
>
> is there something you want me to change in the commit message still ?

No, I am a little surprised that unrolling without SIMD makes
a difference.

Thank you for the explanation, Carl Eugen