[FFmpeg-devel] [PATCH] dstdec: big-endian compatiblity

[Carl Eugen Hoyos]

2019-01-06 23:55 GMT+01:00, Peter Ross <pross at xvid.org>:
> On Sun, Jan 06, 2019 at 12:57:37PM +0100, Carl Eugen Hoyos wrote:
>> 2019-01-06 12:12 GMT+01:00, Peter Ross <pross at xvid.org>:
>> > for the '127-bit shift left' algorithm to work as intended,
>> > little-endian
>> > reads and writes must be used.
>> >
>> Why not using AV_WL64() and and AV_RL64()?
>
> good question.
>
>> Is there a measurable speed difference?
>
> x86: no difference, compiler output is identical.
>
> other cpus that do not support unaligned reads: big difference, due to the
> compiler inserting additional instructions to check the alignment of the
> data.
>
> mipsel:
> RN64A: bench: utime=105.902s stime=0.040s rtime=105.933s
> RN64:  bench: utime=230.055s stime=0.004s rtime=230.082s
>
> why so much difference? the 127-bit shift left operation must happen for
> each 1-bit DSD sample. in a single channel of DSD audio, there are at
> least 2.8 millions 1-bit samples per second.

Sounds to me as if we would need AV_R[BL]xxA which is aligned
for native endian (only).

But this may be unrelated to your patch, you could add a comment
tough that this makes a difference.

Thank you, Carl Eugen