[FFmpeg-devel] [PATCH] vp9: add 32x32 idct AVX2 implementation.

[Ronald S. Bultje]

Hi,

On Sat, Jul 16, 2016 at 5:55 AM, Henrik Gramner <henrik at gramner.com> wrote:

> On Wed, Jul 13, 2016 at 6:37 PM, Ronald S. Bultje <rsbultje at gmail.com>
> wrote:
> > +cglobal vp9_idct_idct_32x32_add, 4, 9, 16, 2048, dst, stride, block, eob
> [...]
> > +    movd               xm0, [blockq]
> > +    mova                m1, [pw_11585x2]
> > +    pmulhrsw            m0, m1
> > +    pmulhrsw            m0, m1
> > +    vpbroadcastw        m0, xm0
> > +    pmulhrsw            m0, [pw_512]
>
> [..]

Is it mathematically possible to merge consecutive pmulhrsw
> instructions into a single one using a different constant? I'm
> guessing no, but I'm not sure.


To my knowledge: no. The intermediate rounding step gets rid of the least
significant bits before the second mul, and merging the muls would remove
this which would change the integer result.

Ronald