Improving performance of bn_mp_expt_d
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
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@ -18,7 +18,7 @@
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/* calculate c = a**b using a square-multiply algorithm */
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int mp_expt_d (mp_int * a, mp_digit b, mp_int * c)
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{
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int res, x;
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int res;
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mp_int g;
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if ((res = mp_init_copy (&g, a)) != MP_OKAY) {
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@ -28,23 +28,23 @@ int mp_expt_d (mp_int * a, mp_digit b, mp_int * c)
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/* set initial result */
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mp_set (c, 1);
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for (x = 0; x < (int) DIGIT_BIT; x++) {
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while (b > 0) {
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/* if the bit is set multiply */
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if (b & 1) {
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if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
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mp_clear (&g);
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return res;
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}
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}
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/* square */
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if ((res = mp_sqr (c, c)) != MP_OKAY) {
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if (b > 1 && (res = mp_sqr (c, c)) != MP_OKAY) {
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mp_clear (&g);
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return res;
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}
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/* if the bit is set multiply */
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if ((b & (mp_digit) (((mp_digit)1) << (DIGIT_BIT - 1))) != 0) {
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if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
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mp_clear (&g);
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return res;
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}
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}
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/* shift to next bit */
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b <<= 1;
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b >>= 1;
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}
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mp_clear (&g);
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