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cabac: Ensure 2-byte cabac loads are on 2-byte boundry

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Ensure that cabac init sets the bitstream pointer to an even value.
It is often faster to load from an aligned boundry

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
This commit is contained in:
John Cox 2016-01-20 17:48:30 +00:00 committed by Michael Niedermayer
parent 17399f6a9f
commit 48f80831ba
1 changed files with 10 additions and 1 deletions
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11
libavcodec/cabac.c
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@ -183,10 +183,19 @@ int ff_init_cabac_decoder(CABACContext *c, const uint8_t *buf, int buf_size){
#if CABAC_BITS == 16
c->low = (*c->bytestream++)<<18;
c->low+= (*c->bytestream++)<<10;
// Keep our fetches on a 2-byte boundry as this should avoid ever having to
// do unaligned loads if the compiler (or asm) optimises the double byte
// load into a single instruction
if(((uintptr_t)c->bytestream & 1) == 0) {
c->low += (1 << 9);
}
else {
c->low += ((*c->bytestream++) << 2) + 2;
}
#else
c->low = (*c->bytestream++)<<10;
#endif
c->low+= ((*c->bytestream++)<<2) + 2;
#endif
c->range= 0x1FE;
if ((c->range<<(CABAC_BITS+1)) < c->low)
return AVERROR_INVALIDDATA;