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

[Reimar Döffinger]

On Sat, Apr 21, 2012 at 10:47:55AM +0200, Reimar Döffinger wrote:
> On 21 Apr 2012, at 07:25, Roland Scheidegger <rscheidegger_lists at hispeed.ch> wrote:
> > 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?
> 
> The output looks to me like half the symbols are missing the leading _, i.e. EXTERN_PREFIX?

Just wrapping all the ff_ symbols used in the asm with LABEL_MANGLE()
should work.