[FFmpeg-devel] [RFC] optimize ff_emulated_edge_mc

[Ronald S. Bultje]

Hi,

On Sun, Jan 2, 2011 at 5:59 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Sun, Jan 02, 2011 at 01:05:43PM -0500, Ronald S. Bultje wrote:
>> On Thu, Dec 30, 2010 at 5:26 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
>> > On Wed, Dec 29, 2010 at 10:03:04PM -0500, Ronald S. Bultje wrote:
>> >> On Wed, Dec 29, 2010 at 8:06 PM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
>> >> > emu_edge_mc looks optimizable and shows up in my profilings. A simple
>> >> > loop->memcpy makes things a lot faster already (see attached):
>> >> [..]
>> >> > after
>> >> [..]
>> >> > 6165 dezicycles in ff_emulated_edge_mc, 1048040 runs, 536 skips
>> >> > 6115 dezicycles in ff_emulated_edge_mc, 1048044 runs, 532 skips
>> >> > 6087 dezicycles in ff_emulated_edge_mc, 1048158 runs, 418 skips
>> >> >
>> >> > before
>> >> [..]
>> >> > 9104 dezicycles in ff_emulated_edge_mc, 1047805 runs, 771 skips
>> >> > 9131 dezicycles in ff_emulated_edge_mc, 1047866 runs, 710 skips
>> >> > 9097 dezicycles in ff_emulated_edge_mc, 1047874 runs, 702 skips
>> >> [..]
>> >>
>> >> Another few more changes attached, doing memcpy() on top/bottom edge
>> >> brings it to 540 cycles:
>> >>
>> >> 5414 dezicycles in ff_emulated_edge_mc, 1048331 runs, 245 skips
>> >>
>> >> and then reordering the left/right edge loop a little brings it to 520:
>> >>
>> >> 5186 dezicycles in ff_emulated_edge_mc, 1048288 runs, 288 skips
>> >>
>> >> I'm too lazy to run this multiple times.
>> >>
>> >> For the left/right edge fills, I tried using memset(), but that slows
>> >> it down considerably, it appears it doesn't inline it. Jason said he
>> >> saw the same on some compilers withthe memcpy() trick. Which makes me
>> >> think, maybe we can emulate the inline memset() trick with some more
>> >> elaborate C code? What I'm thinking is basically edge_val *=
>> >> 0x01010101U; while (to_write >= 4) write(edge_val); if (to_write&2)
>> >> write(edge_val); if (to_write & 1) write(edge_val); or so. Also, since
>> >> most time is spent in copying the blocks quite literally, the main
>> >> copy block could certainly use some optimizations, especially since
>> >> width is generally something like 16...
>> >>
>> >> Ronald
>> >
>> >> ?dsputil.c | ? 22 ++++++++++------------
>> >> ?1 file changed, 10 insertions(+), 12 deletions(-)
>> >> 6b5be1a69247178dd53af1f622a49750d231045d ?emu_edge_mc.patch
>> >
>> > feel free to commit whatever makes ff_emulated_edge_mc() faster
>>
>> Attached is a more reviewable version. It contains basically similar
>> changes as above to the C version, plus I've added the function to
>> DSPContext and have all decoders use it. It's now (for VP8) down from
>> >1000 cycles (see above) to ~259 cycles, or 4x as fast as original and
>> about 2x as fast as the faster C variant in my original post. All this
>> on a Core i7, Elephants Dream sample on a Macbook Pro / OSX 10.6.
>>
>> Here's what it does different than the C version:
>> - memcpy-style copy of top/bottom edge and body uses movdqu and then
>> only mov for the remaining 8/4/2/1 bytes
>> - left/right edge writing decision is made once, and then the loop is
>> largely branchless - this could be done for the C version also perhaps
>> - the left/right edges are written two bytes at a time (makes a little
>> bit of a difference, I tried 4/8 bytes also but that's slower,
>> probably because we now need to ensure we write the correct amount of
>> bytes, whereas for 2, we can overwrite by one into the edge pixel
>> itself and then it doesn't matter I like how you can mov %al, %ah
>> without destroying the lower 8bits, unfortunate that that's not
>> possible for any part of the general registers (or xmm/mmx
>> registers)...
>
> mov al,ah with *ax being used afterwards
> has speed issues on some cpus
> what about some *mul by 257 ?

That was a lot slower than the mov al, ah. Do you know which CPUs it's
supposed to be slower at so I can test & compare and possibly set up a
compile-time variant of both?

Ronald