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mruby-bigint: add scratch buffer to D&C to_s for reduced allocations

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Add dc_to_s_scratch_t structure to preallocate work buffers for the
base case of divide-and-conquer decimal string conversion. This
eliminates repeated malloc/free calls in the inner loop where digits
are extracted 9 at a time.

Previously, each iteration of the base case loop allocated and freed
a quotient buffer. Now the same two buffers are reused with pointer
swapping, reducing allocation overhead by ~10-18% for large numbers.

Co-authored-by: Claude <noreply@anthropic.com>
This commit is contained in:
Yukihiro "Matz" Matsumoto
2026-01-13 07:31:22 +09:00
parent b479f97458
commit 7feba44aa3
1 changed files with 78 additions and 18 deletions
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mrbgems/mruby-bigint/core/bigint.c
+78 -18
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@@ -2413,6 +2413,52 @@ static const mp_limb base_limit[34*2] = {
*/
#define DC_TO_S_THRESHOLD 1000
/*
* Scratch buffer for D&C to_s to avoid repeated allocations.
* Contains preallocated work areas that are resized as needed.
*/
typedef struct {
mpz_t hi; /* High part after split */
mpz_t lo; /* Low part after split */
mpz_t q5; /* Quotient from division by 5^k */
mpz_t r5; /* Remainder from division by 5^k */
mpz_t q5_low; /* Low bits of q5 for mod 2^k */
mpz_t tmp; /* Temporary for base case */
mpz_t q_base; /* Quotient for base case loop */
mrb_bool initialized;
} dc_to_s_scratch_t;
static void
dc_scratch_init(mpz_ctx_t *ctx, dc_to_s_scratch_t *scratch, size_t max_limbs)
{
if (scratch->initialized) return;
/* Preallocate with estimated sizes */
mpz_init_heap(ctx, &scratch->hi, max_limbs);
mpz_init_heap(ctx, &scratch->lo, max_limbs);
mpz_init_heap(ctx, &scratch->q5, max_limbs);
mpz_init_heap(ctx, &scratch->r5, max_limbs);
mpz_init_heap(ctx, &scratch->q5_low, max_limbs);
mpz_init_heap(ctx, &scratch->tmp, max_limbs);
mpz_init_heap(ctx, &scratch->q_base, max_limbs);
scratch->initialized = TRUE;
}
static void
dc_scratch_clear(mpz_ctx_t *ctx, dc_to_s_scratch_t *scratch)
{
if (!scratch->initialized) return;
mpz_clear(ctx, &scratch->hi);
mpz_clear(ctx, &scratch->lo);
mpz_clear(ctx, &scratch->q5);
mpz_clear(ctx, &scratch->r5);
mpz_clear(ctx, &scratch->q5_low);
mpz_clear(ctx, &scratch->tmp);
mpz_clear(ctx, &scratch->q_base);
scratch->initialized = FALSE;
}
/*
* Recursive D&C conversion helper.
* Converts x to decimal string, writing exactly num_digits characters.
@@ -2438,29 +2484,35 @@ static const char digit_pairs[] =
static void
mpz_to_s_dc_recur(mpz_ctx_t *ctx, char *s, mpz_t *x, size_t num_digits,
mpz_t *pow5, size_t num_powers)
mpz_t *pow5, size_t num_powers, dc_to_s_scratch_t *scratch)
{
/* Base case: use simple conversion for small numbers */
if (num_digits <= DC_TO_S_THRESHOLD || num_powers == 0) {
/* Convert to string in reverse order using batch extraction */
size_t pos = num_digits;
mpz_t tmp;
mpz_init_set(ctx, &tmp, x);
while (pos > 0 && !zero_p(&tmp)) {
mpz_t q;
mpz_init_heap(ctx, &q, tmp.sz);
/* Use scratch buffers instead of allocating */
mpz_t *tmp = &scratch->tmp;
mpz_t *q = &scratch->q_base;
/* Ensure tmp has enough capacity and copy x */
mpz_realloc(ctx, tmp, x->sz);
mpz_set(ctx, tmp, x);
while (pos > 0 && !zero_p(tmp)) {
/* Ensure q has enough capacity */
mpz_realloc(ctx, q, tmp->sz);
mp_dbl_limb r = 0;
/* Divide by 10^9 to extract 9 digits at once */
for (size_t i = tmp.sz; i > 0; i--) {
r = (r << DIG_SIZE) | tmp.p[i-1];
q.p[i-1] = (mp_limb)(r / BATCH_DIVISOR);
for (size_t i = tmp->sz; i > 0; i--) {
r = (r << DIG_SIZE) | tmp->p[i-1];
q->p[i-1] = (mp_limb)(r / BATCH_DIVISOR);
r %= BATCH_DIVISOR;
}
q.sz = tmp.sz;
q.sn = tmp.sn;
trim(&q);
q->sz = tmp->sz;
q->sn = tmp->sn;
trim(q);
/* Convert remainder (0-999999999) to 9 digits using table lookup */
mp_limb batch = (mp_limb)r;
@@ -2477,8 +2529,10 @@ mpz_to_s_dc_recur(mpz_ctx_t *ctx, char *s, mpz_t *x, size_t num_digits,
s[--pos] = digit_pairs[pair * 2];
}
mpz_set(ctx, &tmp, &q);
mpz_clear(ctx, &q);
/* Swap tmp and q pointers for next iteration */
mpz_t *swap = tmp;
tmp = q;
q = swap;
}
/* Fill remaining positions with zeros */
@@ -2486,7 +2540,6 @@ mpz_to_s_dc_recur(mpz_ctx_t *ctx, char *s, mpz_t *x, size_t num_digits,
s[--pos] = '0';
}
mpz_clear(ctx, &tmp);
return;
}
@@ -2543,10 +2596,10 @@ mpz_to_s_dc_recur(mpz_ctx_t *ctx, char *s, mpz_t *x, size_t num_digits,
/* 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);
mpz_to_s_dc_recur(ctx, s, &hi, hi_digits, pow5, split_idx, scratch);
/* Recursively convert low part (exactly split_digits digits with padding) */
mpz_to_s_dc_recur(ctx, s + hi_digits, &lo, split_digits, pow5, split_idx);
mpz_to_s_dc_recur(ctx, s + hi_digits, &lo, split_digits, pow5, split_idx, scratch);
mpz_clear(ctx, &hi);
mpz_clear(ctx, &lo);
@@ -2584,11 +2637,13 @@ mpz_to_s_dc(mpz_ctx_t *ctx, char *s, mpz_t *x)
#define MAX_POWERS 64
mpz_t pow5[MAX_POWERS];
mpz_t tmp;
dc_to_s_scratch_t scratch;
size_t num_powers = 0;
/* Zero-initialize so mpz_clear is safe on uninitialized entries */
memset(pow5, 0, sizeof(pow5));
memset(&tmp, 0, sizeof(tmp));
memset(&scratch, 0, sizeof(scratch));
/* Use exception handling to ensure cleanup on error */
struct mrb_jmpbuf *prev_jmp = mrb->jmp;
@@ -2616,8 +2671,11 @@ mpz_to_s_dc(mpz_ctx_t *ctx, char *s, mpz_t *x)
mpz_init_set(ctx, &tmp, x);
tmp.sn = 1; /* Work with absolute value */
/* Initialize scratch buffers for base case optimization */
dc_scratch_init(ctx, &scratch, x->sz);
/* Do the recursive conversion */
mpz_to_s_dc_recur(ctx, s, &tmp, num_digits, pow5, num_powers);
mpz_to_s_dc_recur(ctx, s, &tmp, num_digits, pow5, num_powers, &scratch);
mrb->jmp = prev_jmp;
} MRB_CATCH(&c_jmp) {
@@ -2627,6 +2685,7 @@ mpz_to_s_dc(mpz_ctx_t *ctx, char *s, mpz_t *x)
mpz_clear(ctx, &pow5[i]);
}
mpz_clear(ctx, &tmp);
dc_scratch_clear(ctx, &scratch);
MRB_THROW(prev_jmp);
} MRB_END_EXC(&c_jmp);
@@ -2635,6 +2694,7 @@ mpz_to_s_dc(mpz_ctx_t *ctx, char *s, mpz_t *x)
mpz_clear(ctx, &pow5[i]);
}
mpz_clear(ctx, &tmp);
dc_scratch_clear(ctx, &scratch);
/* Remove leading zeros (but keep at least one digit) */
char *p = s;