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fix division: make recip operate on accum

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also avoids extra copy when only taking recip
This commit is contained in:
Jeff Wang 2019-10-07 00:08:08 -04:00
parent 8e9f37c7d2
commit a3b00dd815
1 changed files with 9 additions and 11 deletions
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20
src/decn/decn.c
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@ -769,12 +769,9 @@ void recip_decn(void){
#define X_COPY Tmp3Decn //holds copy of original x
uint8_t i;
exp_t initial_exp;
//denominator is already in BDecn for division, but x is in Accum for 1/x
// copy accum to be for 1/x case
copy_decn(&BDecn, &AccDecn);
//check divide by zero
#ifdef EXTRA_CHECKS
if (decn_is_zero(&BDecn)){
if (decn_is_zero(&AccDecn)){
set_dec80_NaN(&AccDecn);
#ifdef DESKTOP
printf("error division by 0\n");
@ -784,11 +781,10 @@ void recip_decn(void){
#endif
//normalize
remove_leading_zeros(&AccDecn);
remove_leading_zeros(&BDecn);
//store copy of x
copy_decn(&X_COPY, &BDecn);
copy_decn(&X_COPY, &AccDecn);
//get initial exponent of estimate for 1/x
initial_exp = get_exponent(&BDecn);
initial_exp = get_exponent(&AccDecn);
#ifdef DEBUG_DIV
printf("exponent %d", initial_exp);
#endif
@ -797,13 +793,13 @@ void recip_decn(void){
#ifdef DEBUG_DIV
printf(" -> %d\n", initial_exp);
#endif
set_exponent(&CURR_RECIP, initial_exp, (BDecn.exponent < 0)); //set exponent, copy sign
set_exponent(&CURR_RECIP, initial_exp, (AccDecn.exponent < 0)); //set exponent, copy sign
//get initial estimate for 1/x
if (BDecn.lsu[0] < 20){ //mantissa between 1 and 2
if (AccDecn.lsu[0] < 20){ //mantissa between 1 and 2
CURR_RECIP.lsu[0] = 50; //0.50 with implicit point and exponent
} else if (BDecn.lsu[0] < 33){
} else if (AccDecn.lsu[0] < 33){
CURR_RECIP.lsu[0] = 30;
} else if (BDecn.lsu[0] < 50){
} else if (AccDecn.lsu[0] < 50){
CURR_RECIP.lsu[0] = 20;
} else {
CURR_RECIP.lsu[0] = 10; //0.1 with implicit point and exponent
@ -856,6 +852,7 @@ inline void div_decn(void){
#define ACC_COPY Tmp4Decn //holds copy of original acc
//store copy of acc for final multiply by 1/x
copy_decn(&ACC_COPY, &AccDecn);
copy_decn(&AccDecn, &BDecn);
recip_decn();
//Accum now holds 1/x, multiply by original acc to complete division
copy_decn(&BDecn, &ACC_COPY);
@ -866,6 +863,7 @@ inline void div_decn(void){
}
//constants used for ln(x) and exp(x)
#define NUM_A_ARR 9
static const dec80 LN_A_ARR[NUM_A_ARR] = {