[FFmpeg-devel] [PATCH] h264: assembly version of get_cabac for x86_64 with PIC (v4)

[Roland Scheidegger]

Am 21.04.2012 07:25, schrieb Roland Scheidegger:
> Am 21.04.2012 02:15, schrieb Michael Niedermayer:
>> On Sat, Apr 21, 2012 at 02:10:46AM +0200, Michael Niedermayer wrote:
>>> On Sat, Apr 21, 2012 at 01:26:54AM +0200, Michael Niedermayer wrote:
>>>> On Fri, Apr 20, 2012 at 02:10:57AM +0200, Roland Scheidegger wrote:
>>>>> This adds a hand-optimized assembly version for get_cabac much like the
>>>>> existing one, but it works if the table offsets are RIP-relative.
>>>>> Compared to the non-RIP-relative version this adds 2 lea instructions
>>>>> and it needs one extra register.
>>>>> There is a surprisingly large performance improvement over the c version (more
>>>>> so than the generated assembly seems to suggest) just in get_cabac, I measured
>>>>> roughly 40% faster for get_cabac on a K8. However, overall the difference is
>>>>> not that big, I measured roughly 5% on a test clip on a K8 and a Core2.
>>>>> Hopefully it still compiles on x86 32bit...
>>>>> v2: incorporated feedback from Loren Merritt to avoid rip-relative movs
>>>>> for every table, and got rid of unnecessary @GOTPCREL.
>>>>> v3: apply similar fixes to the the decode_significance functions, and use
>>>>> same macro arguments for non-pic case.
>>>>> v4: prettify inline asm arguments, add a non-fast-cmov version (as I expect
>>>>> the c code to be faster otherwise since both cmov and sbb suck hard on a
>>>>> Prescott, even can't construct the mask with a 64bit shift as that's just as
>>>>> terrible - it's quite difficult to find usable instructions on that chip...).
>>>>> This is tested to work but not on a P4, in theory it _should_ be fast there.
>>>>
>>>> applied
>>>>
>>>> if someone has more ideas on how to improve it, it can easily be done
>>>
>>>> lets hope it doesnt fail on any odd platforms ...
>>>
>>> fails on darwin
>>> http://fate.ffmpeg.org/log.cgi?time=20120420235051&log=compile&slot=x86_64-darwin-llvm-gcc-4.2.1
>>
>> ill revert it in a moment, dont want to leave compile broken until
>> another solution is proposed
> 
> Oops. Any ideas what went wrong? Somehow the trick to access the table
> via the difference of the table symbol and the label doesn't work? Is
> that something just not supposed to work on darwin (PIC code should work
> the same there right for x64?) or is this fixable? I think the only
> thing I could do about is is just not enable this on darwin but surely
> there has to be some way to make it work?

Oh forgot to mention when searching for that error I found quite lots of
hits but no real explanation. There were some references even to ffmpeg
code where the same errors were generated:
https://trac.macports.org/ticket/33735

Roland