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Increase VpMult batch size #511

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Merged
tompng merged 1 commit into from
Apr 1, 2026
+16 −13
Merged

Increase VpMult batch size #511

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25 changes: 14 additions & 11 deletions ext/bigdecimal/bigdecimal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -35,7 +35,9 @@

#define BIGDECIMAL_VERSION "4.1.0"

#define NTT_MULTIPLICATION_THRESHOLD 350
/* Make sure VPMULT_BATCH_SIZE*BASE*BASE does not overflow DECDIG_DBL */
#define VPMULT_BATCH_SIZE 16
#define NTT_MULTIPLICATION_THRESHOLD 450
#define NEWTON_RAPHSON_DIVISION_THRESHOLD 100
#define SIGNED_VALUE_MAX INTPTR_MAX
#define SIGNED_VALUE_MIN INTPTR_MIN
Expand Down Expand Up @@ -4842,7 +4844,7 @@ VP_EXPORT size_t
VpMult(Real *c, Real *a, Real *b)
{
ssize_t a_batch_max, b_batch_max;
DECDIG_DBL batch[15];
DECDIG_DBL batch[VPMULT_BATCH_SIZE * 2 - 1];

if (!VpIsDefOP(c, a, b, OP_SW_MULT)) return 0; /* No significant digit */

Expand Down Expand Up @@ -4882,27 +4884,28 @@ VpMult(Real *c, Real *a, Real *b)
c->Prec = a->Prec + b->Prec; /* set precision */
memset(c->frac, 0, c->Prec * sizeof(DECDIG)); /* Initialize c */

// Process 8 decdigits at a time to reduce the number of carry operations.
a_batch_max = (a->Prec - 1) / 8;
b_batch_max = (b->Prec - 1) / 8;
// Process VPMULT_BATCH_SIZE decdigits at a time to reduce the number of carry operations.
a_batch_max = (a->Prec - 1) / VPMULT_BATCH_SIZE;
b_batch_max = (b->Prec - 1) / VPMULT_BATCH_SIZE;
for (ssize_t ibatch = a_batch_max; ibatch >= 0; ibatch--) {
int isize = ibatch == a_batch_max ? (a->Prec - 1) % 8 + 1 : 8;
int isize = ibatch == a_batch_max ? (a->Prec - 1) % VPMULT_BATCH_SIZE + 1 : VPMULT_BATCH_SIZE;
for (ssize_t jbatch = b_batch_max; jbatch >= 0; jbatch--) {
int jsize = jbatch == b_batch_max ? (b->Prec - 1) % 8 + 1 : 8;
int jsize = jbatch == b_batch_max ? (b->Prec - 1) % VPMULT_BATCH_SIZE + 1 : VPMULT_BATCH_SIZE;
memset(batch, 0, (isize + jsize - 1) * sizeof(DECDIG_DBL));

// Perform multiplication without carry calculation.
// 999999999 * 999999999 * 8 < 2**63 - 1, so DECDIG_DBL can hold the intermediate sum without overflow.
// BASE * BASE * VPMULT_BATCH_SIZE < 2**64 should be satisfied so that
// DECDIG_DBL can hold the intermediate sum without overflow.
for (int i = 0; i < isize; i++) {
for (int j = 0; j < jsize; j++) {
batch[i + j] += (DECDIG_DBL)a->frac[ibatch * 8 + i] * b->frac[jbatch * 8 + j];
batch[i + j] += (DECDIG_DBL)a->frac[ibatch * VPMULT_BATCH_SIZE + i] * b->frac[jbatch * VPMULT_BATCH_SIZE + j];
}
}

// Add the batch result to c with carry calculation.
DECDIG_DBL carry = 0;
for (int k = isize + jsize - 2; k >= 0; k--) {
size_t l = (ibatch + jbatch) * 8 + k + 1;
size_t l = (ibatch + jbatch) * VPMULT_BATCH_SIZE + k + 1;
DECDIG_DBL s = c->frac[l] + batch[k] + carry;
c->frac[l] = (DECDIG)(s % BASE);
carry = (DECDIG_DBL)(s / BASE);
Expand All @@ -4911,7 +4914,7 @@ VpMult(Real *c, Real *a, Real *b)
// Adding carry may exceed BASE, but it won't cause overflow of DECDIG.
// Exceeded value will be resolved in the carry operation of next (ibatch + jbatch - 1) batch.
// WARNING: This safety strongly relies on the current nested loop execution order.
c->frac[(ibatch + jbatch) * 8] += (DECDIG)carry;
c->frac[(ibatch + jbatch) * VPMULT_BATCH_SIZE] += (DECDIG)carry;
}
}

Expand Down
4 changes: 2 additions & 2 deletions test/bigdecimal/test_vp_operation.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,7 @@ def setup

def test_vpmult
# Max carry case
[*32...40].repeated_permutation(2) do |n, m|
[*32...48].repeated_permutation(2) do |n, m|
x = BigDecimal('9' * BASE_FIG * n)
y = BigDecimal('9' * BASE_FIG * m)
assert_equal(x.to_i * y.to_i, x.vpmult(y))
Expand All @@ -30,7 +30,7 @@ def test_vpmult

def test_nttmult
# Max carry case
[*32...40].repeated_permutation(2) do |n, m|
[*32...48].repeated_permutation(2) do |n, m|
x = BigDecimal('9' * BASE_FIG * n)
y = BigDecimal('9' * BASE_FIG * m)
assert_equal(x.to_i * y.to_i, x.nttmult(y))
Expand Down