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Optimization: first table lookup needs no point addition

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This commit is contained in:
Pieter Wuille
2021-12-26 19:05:49 -05:00
parent 7a33db35cd
commit 15d0cca2a6
1 changed files with 8 additions and 2 deletions
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10
src/ecmult_gen_impl.h
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@@ -56,6 +56,7 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
secp256k1_fe neg; secp256k1_fe neg;
secp256k1_ge_storage adds; secp256k1_ge_storage adds;
secp256k1_scalar d; secp256k1_scalar d;
int first = 1;
memset(&adds, 0, sizeof(adds)); memset(&adds, 0, sizeof(adds));
@@ -177,7 +178,6 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
* secp256k1_ecmult_gen_prec_table[b][index] stores the table(b, m) entries. Index * secp256k1_ecmult_gen_prec_table[b][index] stores the table(b, m) entries. Index
* is the relevant mask(b) bits of m packed together without gaps. */ * is the relevant mask(b) bits of m packed together without gaps. */
secp256k1_gej_set_infinity(r);
/* Outer loop: iterate over comb_off from COMB_SPACING - 1 down to 0. */ /* Outer loop: iterate over comb_off from COMB_SPACING - 1 down to 0. */
comb_off = COMB_SPACING - 1; comb_off = COMB_SPACING - 1;
while (1) { while (1) {
@@ -221,7 +221,13 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
secp256k1_fe_cmov(&add.y, &neg, sign); secp256k1_fe_cmov(&add.y, &neg, sign);
/* Add the looked up and conditionally negated value to r. */ /* Add the looked up and conditionally negated value to r. */
secp256k1_gej_add_ge(r, r, &add); if (EXPECT(first, 0)) {
/* If this is the first table lookup, we can skip addition. */
secp256k1_gej_set_ge(r, &add);
first = 0;
} else {
secp256k1_gej_add_ge(r, r, &add);
}
} }
/* Double the result, except in the last iteration. */ /* Double the result, except in the last iteration. */