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improve initial estimate for division newton raphson iteration

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This commit is contained in:
Jeff Wang 2019-09-23 22:48:06 -04:00
parent 52ddf7dd96
commit eccd89807a
1 changed files with 16 additions and 4 deletions
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20
src/decn/decn.c
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@ -760,11 +760,14 @@ void div_decn(void){
return; return;
} }
#endif #endif
//normalize
remove_leading_zeros(&AccDecn);
remove_leading_zeros(&BDecn);
//store copy of acc for final multiply by 1/x //store copy of acc for final multiply by 1/x
copy_decn(&acc_copy, &AccDecn); copy_decn(&acc_copy, &AccDecn);
//store copy of x //store copy of x
copy_decn(&x_copy, &BDecn); copy_decn(&x_copy, &BDecn);
//get initial estimate for 1/x, by negating exponent, and setting signif. to 1 //get initial exponent of estimate for 1/x
initial_exp = get_exponent(&BDecn); initial_exp = get_exponent(&BDecn);
#ifdef DEBUG_DIV #ifdef DEBUG_DIV
printf("exponent %d", initial_exp); printf("exponent %d", initial_exp);
@ -775,13 +778,22 @@ void div_decn(void){
printf(" -> %d\n", initial_exp); printf(" -> %d\n", initial_exp);
#endif #endif
set_exponent(&curr_recip, initial_exp, (BDecn.exponent < 0)); //set exponent, copy sign set_exponent(&curr_recip, initial_exp, (BDecn.exponent < 0)); //set exponent, copy sign
curr_recip.lsu[0] = 10; //1 with implicit point //get initial estimate for 1/x
if (BDecn.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){
curr_recip.lsu[0] = 30;
} else if (BDecn.lsu[0] < 50){
curr_recip.lsu[0] = 20;
} else {
curr_recip.lsu[0] = 10; //0.1 with implicit point and exponent
}
for (i = 1; i < DEC80_NUM_LSU; i++){ for (i = 1; i < DEC80_NUM_LSU; i++){
curr_recip.lsu[i] = 0; curr_recip.lsu[i] = 0;
} }
copy_decn(&AccDecn, &curr_recip); copy_decn(&AccDecn, &curr_recip);
//do newton raphson iterations //do newton raphson iterations
for (i = 0; i < DEC80_NUM_LSU; i++){ //just fix number of iterations for now for (i = 0; i < 6; i++){ //just fix number of iterations for now
#ifdef DEBUG_DIV #ifdef DEBUG_DIV
decn_to_str_complete(&curr_recip); decn_to_str_complete(&curr_recip);
printf("%2d: %s\n", i, Buf); printf("%2d: %s\n", i, Buf);