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authorJason Garrett-Glaser <darkshikari@gmail.com>2010-06-30 23:59:27 +0000
committerJason Garrett-Glaser <darkshikari@gmail.com>2010-06-30 23:59:27 +0000
commit4148855ee4f3506cc1f7341f688e5fa260a017d4 (patch)
tree287a9818256f89a07e3bd5633b5ba632c57cda0a /libavcodec/vp56.h
parent2e79db01414b43e4dee26ad7834ba2f74fc6320a (diff)
downloadffmpeg-4148855ee4f3506cc1f7341f688e5fa260a017d4.tar.gz
Eliminate another redundant instruction in vp56/8 arithcoder
Necessary because of this GCC bug: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44474 To do this, convert some, but not all (!) of the variables in VP56RangeCoder into local variables. If we convert c->high into a local variable, gcc gets the stupids and refuses to use a conditional move for the unpredictable main branch. TODO: dispense with this bullshit and write an asm version. Originally committed as revision 23924 to svn://svn.ffmpeg.org/ffmpeg/trunk
Diffstat (limited to 'libavcodec/vp56.h')
-rw-r--r--libavcodec/vp56.h25
1 files changed, 17 insertions, 8 deletions
diff --git a/libavcodec/vp56.h b/libavcodec/vp56.h
index 6a2e29f898..74a14ef42e 100644
--- a/libavcodec/vp56.h
+++ b/libavcodec/vp56.h
@@ -194,23 +194,32 @@ static inline void vp56_init_range_decoder(VP56RangeCoder *c,
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. */
+ int bits = c->bits;
+ unsigned long code_word = c->code_word;
unsigned int low = 1 + (((c->high - 1) * prob) >> 8);
unsigned int low_shift = low << 8;
- int bit = c->code_word >= low_shift;
+ int bit = code_word >= low_shift;
int 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 ? c->code_word - low_shift : c->code_word;
+ code_word = bit ? code_word - low_shift : code_word;
/* normalize */
shift = ff_h264_norm_shift[c->high] - 1;
- c->high <<= shift;
- c->code_word <<= shift;
- c->bits += shift;
- if(c->bits >= 0 && c->buffer < c->end) {
- c->code_word |= *c->buffer++ << c->bits;
- c->bits -= 8;
+ c->high <<= shift;
+ code_word <<= shift;
+ bits += shift;
+ if(bits >= 0 && c->buffer < c->end) {
+ code_word |= *c->buffer++ << bits;
+ bits -= 8;
}
+ c->bits = bits;
+ c->code_word = code_word;
return bit;
}