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Inline asm for VP56 arith coder

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This is a lot more reliable to get cmov rather than trying to trick gcc into
generating it, useful since it's 2% faster overall.

Patch by Eli Friedman <eli.friedman at gmail>

Originally committed as revision 24471 to svn://svn.ffmpeg.org/ffmpeg/trunk
This commit is contained in:
Eli Friedman 2010-07-23 21:46:30 +00:00 committed by David Conrad
parent ca18a478e3
commit 3611e7a309
2 changed files with 61 additions and 5 deletions
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12
libavcodec/vp56.h
View File

@ -208,23 +208,25 @@ static av_always_inline unsigned int vp56_rac_renorm(VP56RangeCoder *c)
return code_word;
}
#if ARCH_X86
#include "x86/vp56_arith.h"
#endif
#ifndef vp56_rac_get_prob
#define vp56_rac_get_prob vp56_rac_get_prob
static inline int vp56_rac_get_prob(VP56RangeCoder *c, uint8_t prob)
{
/* Don't put c->high in a local variable; if we do that, gcc gets
* the stupids and turns the code below into a branch again. */
unsigned int code_word = vp56_rac_renorm(c);
unsigned int low = 1 + (((c->high - 1) * prob) >> 8);
unsigned int low_shift = low << 8;
int bit = code_word >= low_shift;
/* Incantation to convince GCC to turn these into conditional moves
* instead of branches -- faster, as this branch is basically
* unpredictable. */
c->high = bit ? c->high - low : low;
c->code_word = bit ? code_word - low_shift : code_word;
return bit;
}
#endif
// branchy variant, to be used where there's a branch based on the bit decoded
static av_always_inline int vp56_rac_get_prob_branchy(VP56RangeCoder *c, int prob)

54
libavcodec/x86/vp56_arith.h Normal file
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@ -0,0 +1,54 @@
/**
* VP5 and VP6 compatible video decoder (arith decoder)
*
* Copyright (C) 2006 Aurelien Jacobs <aurel@gnuage.org>
* Copyright (C) 2010 Eli Friedman
*
* This file is part of FFmpeg.
*
* FFmpeg is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* FFmpeg is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with FFmpeg; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
#ifndef AVCODEC_X86_VP56_ARITH_H
#define AVCODEC_X86_VP56_ARITH_H
#if HAVE_FAST_CMOV
#define vp56_rac_get_prob vp56_rac_get_prob
static inline int vp56_rac_get_prob(VP56RangeCoder *c, uint8_t prob)
{
unsigned int code_word = vp56_rac_renorm(c);
unsigned int high = c->high;
unsigned int low = 1 + (((high - 1) * prob) >> 8);
unsigned int low_shift = low << 8;
int bit = 0;
__asm__(
"subl %4, %1 \n\t"
"subl %3, %2 \n\t"
"leal (%2, %3), %3 \n\t"
"setae %b0 \n\t"
"cmovb %4, %1 \n\t"
"cmovb %3, %2 \n\t"
: "+q"(bit), "+r"(high), "+r"(code_word), "+r"(low_shift)
: "r"(low)
);
c->high = high;
c->code_word = code_word;
return bit;
}
#endif
#endif /* AVCODEC_X86_VP56_ARITH_H */