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Move renormalization of the VP56 arith decoder to before decoding a bit

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No difference at the moment, but allows a future branchy variant
of vp56_rac_get_prob to be significantly faster

Originally committed as revision 24467 to svn://svn.ffmpeg.org/ffmpeg/trunk
This commit is contained in:
David Conrad 2010-07-23 21:46:14 +00:00
parent b3d755ec8b
commit 5474ec2ac8
1 changed files with 11 additions and 15 deletions
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26
libavcodec/vp56.h
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@ -191,10 +191,11 @@ static inline void vp56_init_range_decoder(VP56RangeCoder *c,
c->code_word = bytestream_get_be16(&c->buffer); c->code_word = bytestream_get_be16(&c->buffer);
} }
static av_always_inline void vp56_rac_renorm(VP56RangeCoder *c, unsigned int code_word) static av_always_inline unsigned int vp56_rac_renorm(VP56RangeCoder *c)
{ {
int shift = ff_h264_norm_shift[c->high] - 1; int shift = ff_h264_norm_shift[c->high] - 1;
int bits = c->bits; int bits = c->bits;
unsigned int code_word = c->code_word;
c->high <<= shift; c->high <<= shift;
code_word <<= shift; code_word <<= shift;
@ -204,14 +205,14 @@ static av_always_inline void vp56_rac_renorm(VP56RangeCoder *c, unsigned int cod
bits -= 8; bits -= 8;
} }
c->bits = bits; c->bits = bits;
c->code_word = code_word; return code_word;
} }
static inline int vp56_rac_get_prob(VP56RangeCoder *c, uint8_t 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 /* 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. */ * the stupids and turns the code below into a branch again. */
unsigned int code_word = c->code_word; unsigned int code_word = vp56_rac_renorm(c);
unsigned int low = 1 + (((c->high - 1) * prob) >> 8); unsigned int low = 1 + (((c->high - 1) * prob) >> 8);
unsigned int low_shift = low << 8; unsigned int low_shift = low << 8;
int bit = code_word >= low_shift; int bit = code_word >= low_shift;
@ -220,31 +221,26 @@ static inline int vp56_rac_get_prob(VP56RangeCoder *c, uint8_t prob)
* instead of branches -- faster, as this branch is basically * instead of branches -- faster, as this branch is basically
* unpredictable. */ * unpredictable. */
c->high = bit ? c->high - low : low; c->high = bit ? c->high - low : low;
code_word = bit ? code_word - low_shift : code_word; c->code_word = bit ? code_word - low_shift : code_word;
vp56_rac_renorm(c, code_word);
return bit; return bit;
} }
static inline int vp56_rac_get(VP56RangeCoder *c) static inline int vp56_rac_get(VP56RangeCoder *c)
{ {
unsigned int code_word = vp56_rac_renorm(c);
/* equiprobable */ /* equiprobable */
int low = (c->high + 1) >> 1; int low = (c->high + 1) >> 1;
unsigned int low_shift = low << 8; unsigned int low_shift = low << 8;
int bit = c->code_word >= low_shift; int bit = code_word >= low_shift;
if (bit) { if (bit) {
c->high = (c->high - low) << 1; c->high -= low;
c->code_word -= low_shift; code_word -= low_shift;
} else { } else {
c->high = low << 1; c->high = low;
} }
/* normalize */ c->code_word = code_word;
c->code_word <<= 1;
if (++c->bits == 0 && c->buffer < c->end) {
c->bits = -8;
c->code_word |= *c->buffer++;
}
return bit; return bit;
} }