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mruby-bigint: refactor mpz_pow to reduce memory allocations

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Co-authored-by: Gemini <gemini@google.com>
This commit is contained in:
Yukihiro "Matz" Matsumoto
2025-07-28 15:47:42 +09:00
parent ec6359408e
commit 8825225e01
1 changed files with 29 additions and 31 deletions
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mrbgems/mruby-bigint/core/bigint.c
+29 -31
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@@ -1663,46 +1663,44 @@ mpz_xor(mpz_ctx_t *ctx, mpz_t *z, mpz_t *x, mpz_t *y) /* not the most efficient
static void
mpz_pow(mpz_ctx_t *ctx, mpz_t *zz, mpz_t *x, mrb_int e)
{
mrb_uint mask = 1ULL<<(sizeof(mrb_int)*8-1);
/* Initialize result with better size estimation for binary exponentiation */
size_t bits_in_e = 0;
mrb_int temp_e = e;
while (temp_e > 0) {
bits_in_e++;
temp_e >>= 1;
}
/* Each iteration can roughly double the result size, so estimate conservatively */
size_t estimated_size = x->sz * bits_in_e;
if (estimated_size < x->sz) estimated_size = x->sz; /* Prevent underflow */
mpz_init_auto(ctx, zz, estimated_size);
if (e==0) {
mpz_set_int(ctx, zz, 1L);
if (e == 0) {
mpz_init_set_int(ctx, zz, 1L);
return;
}
/* Set initial value to x for exponentiation */
mpz_set(ctx, zz, x);
mrb_uint mask = 1ULL << (sizeof(mrb_int) * 8 - 1);
while (mask != 0 && !(mask & e)) {
mask >>= 1;
}
/* Set initial value to x for exponentiation */
mpz_init_set(ctx, zz, x);
if (mask == 0) { /* e is 0 or 1 */
if (e == 0) mpz_set_int(ctx, zz, 1L);
return;
}
mask >>= 1;
/* Pre-allocate a single temporary variable */
mpz_t temp;
mpz_init(ctx, &temp);
for (; mask != 0; mask >>= 1) {
/* squaring: temp = zz * zz */
mpz_mul(ctx, &temp, zz, zz);
for (;!(mask &e); mask>>=1)
;
mask>>=1;
for (;mask!=0; mask>>=1) {
mpz_t temp;
mpz_init(ctx, &temp);
mpz_mul(ctx, &temp, zz, zz); /* temp = zz^2 */
if (e & mask) {
mpz_t temp2;
mpz_init(ctx, &temp2);
mpz_mul(ctx, &temp2, &temp, x); /* temp2 = temp * x */
mpz_move(ctx, zz, &temp2);
mpz_clear(ctx, &temp); /* temp is not moved, so clear it */
/* multiplication: zz = temp * x */
mpz_mul(ctx, zz, &temp, x);
}
else {
mpz_move(ctx, zz, &temp); /* temp is moved to zz, ownership transferred */
/* move result: zz = temp */
mpz_move(ctx, zz, &temp);
}
}
mpz_clear(ctx, &temp);
}
static void