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More simplifications to the decoder renormalization loop.

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This removes an XOR, an ADD, and an AND, and replaces them with
 an AND NOT in ec_dec_normalize().
Also, simplify the loop structure of ec_dec_cdf() and eliminate a
 CMOV.
This commit is contained in:
Timothy B. Terriberry 2010-12-18 04:51:55 -08:00 • committed by Jean-Marc Valin
parent e86fb268b0
commit 53454f4910
1 changed files with 9 additions and 13 deletions
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22
libcelt/rangedec.c
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@ -121,16 +121,13 @@ static inline void ec_dec_normalize(ec_dec *_this){
int sym;
_this->rng<<=EC_SYM_BITS;
/*Use up the remaining bits from our last symbol.*/
sym=_this->rem<<EC_CODE_EXTRA&EC_SYM_MAX;
sym=_this->rem<<EC_CODE_EXTRA;
/*Read the next value from the input.*/
_this->rem=ec_dec_in(_this);
/*Take the rest of the bits we need from this new symbol.*/
sym|=_this->rem>>EC_SYM_BITS-EC_CODE_EXTRA;
_this->dif=(_this->dif<<EC_SYM_BITS)-sym+((1<<EC_SYM_BITS)-1)&EC_CODE_MASK;
/*dif must be smaller than EC_CODE_TOP.
This is equivalent to the slightly more readable:
if(_this->dif>=EC_CODE_TOP)_this->dif-=EC_CODE_TOP;*/
_this->dif^=_this->dif&EC_CODE_TOP;
/*And subtract them from dif, capped to be less than EC_CODE_TOP.*/
_this->dif=(_this->dif<<EC_SYM_BITS)+(EC_SYM_MAX&~sym)&EC_CODE_TOP-1;
}
}
@ -225,20 +222,19 @@ int ec_dec_cdf(ec_dec *_this,const int *_cdf,unsigned _ftb){
ec_uint32 s;
ec_uint32 t;
int val;
r=_this->rng>>_ftb;
s=_this->rng;
d=_this->dif;
_cdf++;
r=s>>_ftb;
val=0;
t=0;
s=IMUL32(r,(1<<_ftb)-_cdf[0]);
while(d<s){
do{
t=s;
s=IMUL32(r,(1<<_ftb)-_cdf[++val]);
}
while(d<s);
_this->dif=d-s;
_this->rng=(val?t:_this->rng)-s;
_this->rng=t-s;
ec_dec_normalize(_this);
return val;
return val-1;
}
ec_uint32 ec_dec_tell(ec_dec *_this,int _b){