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mruby-bigint: Optimize usub with loop unrolling

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Applied 4x loop unrolling to the usub function to improve performance for
multi-limb subtraction operations.

Co-authored-by: Gemini <gemini@google.com>
This commit is contained in:
Yukihiro "Matz" Matsumoto
2025-07-31 09:53:20 +09:00
parent 1d7ef2b85b
commit bcbcef4203
1 changed files with 45 additions and 1 deletions
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mrbgems/mruby-bigint/core/bigint.c
+45 -1
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@@ -415,7 +415,31 @@ usub(mpz_t *z, mpz_t *y, mpz_t *x)
size_t i;
/* Subtract overlapping limbs from both operands */
for (i = 0; i < x->sz; i++) {
/* 4x unrolled loop for better performance */
for (i = 0; i + 4 <= x->sz; i += 4) {
b += (mp_dbl_limb_signed)y->p[i];
b -= (mp_dbl_limb_signed)x->p[i];
z->p[i] = LOW(b);
b = HIGH(b);
b += (mp_dbl_limb_signed)y->p[i+1];
b -= (mp_dbl_limb_signed)x->p[i+1];
z->p[i+1] = LOW(b);
b = HIGH(b);
b += (mp_dbl_limb_signed)y->p[i+2];
b -= (mp_dbl_limb_signed)x->p[i+2];
z->p[i+2] = LOW(b);
b = HIGH(b);
b += (mp_dbl_limb_signed)y->p[i+3];
b -= (mp_dbl_limb_signed)x->p[i+3];
z->p[i+3] = LOW(b);
b = HIGH(b);
}
/* Handle remaining elements */
for (; i < x->sz; i++) {
b += (mp_dbl_limb_signed)y->p[i];
b -= (mp_dbl_limb_signed)x->p[i];
z->p[i] = LOW(b);
@@ -423,6 +447,26 @@ usub(mpz_t *z, mpz_t *y, mpz_t *x)
}
/* Process remaining limbs from minuend with borrow */
/* 4x unrolled loop for better performance */
for (; i + 4 <= y->sz; i += 4) {
b += y->p[i];
z->p[i] = LOW(b);
b = HIGH(b);
b += y->p[i+1];
z->p[i+1] = LOW(b);
b = HIGH(b);
b += y->p[i+2];
z->p[i+2] = LOW(b);
b = HIGH(b);
b += y->p[i+3];
z->p[i+3] = LOW(b);
b = HIGH(b);
}
/* Handle remaining elements */
for (; i < y->sz; i++) {
b += y->p[i];
z->p[i] = LOW(b);