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mruby-bigint: reuse scratch buffers for D&C to_s recursive temporaries

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Use scratch buffers for q5, r5, and q5_low in the recursive case of
D&C decimal string conversion. These temporaries are only needed
during the computation of hi and lo values, not during the recursive
calls, so they can be safely reused at each recursion level.

This eliminates 3 allocations per recursion level (approximately
log2(digits/1000) levels for large numbers), providing an additional
2-3% performance improvement on top of the base case optimization.

Co-authored-by: Claude <noreply@anthropic.com>
This commit is contained in:
Yukihiro "Matz" Matsumoto
2026-01-13 07:32:54 +09:00
parent 7feba44aa3
commit 8b0641f97a
1 changed files with 12 additions and 15 deletions
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mrbgems/mruby-bigint/core/bigint.c
+12 -15
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@@ -2566,34 +2566,31 @@ mpz_to_s_dc_recur(mpz_ctx_t *ctx, char *s, mpz_t *x, size_t num_digits,
* hi = q5 >> k (right shift by k bits)
* lo = (q5 & ((1<<k)-1)) * 5^k + r5
*/
/* hi and lo must persist through recursive calls, so allocate fresh */
mpz_t hi, lo;
mpz_init(ctx, &hi);
mpz_init(ctx, &lo);
mpz_t q5, r5;
mpz_init(ctx, &q5);
mpz_init(ctx, &r5);
/* q5, r5, q5_low are only needed for computing hi/lo, reuse from scratch */
mpz_t *q5 = &scratch->q5;
mpz_t *r5 = &scratch->r5;
mpz_t *q5_low = &scratch->q5_low;
/* Step 1: Divide by 5^k (cheaper than dividing by 10^k) */
mpz_mdivmod(ctx, &q5, &r5, x, &pow5[split_idx]);
r5.sn = (r5.sn < 0) ? -r5.sn : r5.sn;
mpz_mdivmod(ctx, q5, r5, x, &pow5[split_idx]);
r5->sn = (r5->sn < 0) ? -r5->sn : r5->sn;
/* Step 2: hi = q5 >> split_digits (divide by 2^k using bit shift) */
mpz_div_2exp(ctx, &hi, &q5, (mrb_int)split_digits);
mpz_div_2exp(ctx, &hi, q5, (mrb_int)split_digits);
/* Step 3: lo = (q5 mod 2^k) * 5^k + r5 */
mpz_t q5_low;
mpz_init(ctx, &q5_low);
mpz_mod_2exp(ctx, &q5_low, &q5, (mrb_int)split_digits);
mpz_mod_2exp(ctx, q5_low, q5, (mrb_int)split_digits);
mpz_mul(ctx, &lo, &q5_low, &pow5[split_idx]);
mpz_add(ctx, &lo, &lo, &r5);
mpz_mul(ctx, &lo, q5_low, &pow5[split_idx]);
mpz_add(ctx, &lo, &lo, r5);
lo.sn = (lo.sn < 0) ? -lo.sn : lo.sn;
mpz_clear(ctx, &q5);
mpz_clear(ctx, &r5);
mpz_clear(ctx, &q5_low);
/* Recursively convert high part */
size_t hi_digits = num_digits - split_digits;
mpz_to_s_dc_recur(ctx, s, &hi, hi_digits, pow5, split_idx, scratch);