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Improving performance of bn_mp_expt_d

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The loop was always iterating DIGIT_BIT times, instead of
halting when possible. This changes makes sure it executes
less operations. This change has also been incorporated
into Rubinius / https://github.com/evanphx/rubinius which
uses libtommath
This commit is contained in:
Dirkjan Bussink 2010-11-26 22:24:23 +01:00
parent 5fc6314643
commit 921be35779
1 changed files with 12 additions and 12 deletions
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24
bn_mp_expt_d.c
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@ -18,7 +18,7 @@
/* calculate c = a**b using a square-multiply algorithm */ /* calculate c = a**b using a square-multiply algorithm */
int mp_expt_d (mp_int * a, mp_digit b, mp_int * c) int mp_expt_d (mp_int * a, mp_digit b, mp_int * c)
{ {
int res, x; int res;
mp_int g; mp_int g;
if ((res = mp_init_copy (&g, a)) != MP_OKAY) { if ((res = mp_init_copy (&g, a)) != MP_OKAY) {
@ -28,23 +28,23 @@ int mp_expt_d (mp_int * a, mp_digit b, mp_int * c)
/* set initial result */ /* set initial result */
mp_set (c, 1); mp_set (c, 1);
for (x = 0; x < (int) DIGIT_BIT; x++) { while (b > 0) {
/* if the bit is set multiply */
if (b & 1) {
if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
mp_clear (&g);
return res;
}
}
/* square */ /* square */
if ((res = mp_sqr (c, c)) != MP_OKAY) { if (b > 1 && (res = mp_sqr (c, c)) != MP_OKAY) {
mp_clear (&g); mp_clear (&g);
return res; return res;
} }
/* if the bit is set multiply */
if ((b & (mp_digit) (((mp_digit)1) << (DIGIT_BIT - 1))) != 0) {
if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
mp_clear (&g);
return res;
}
}
/* shift to next bit */ /* shift to next bit */
b <<= 1; b >>= 1;
} }
mp_clear (&g); mp_clear (&g);