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mruby-bigint: fix carry calculation in MSVC 64-bit multiplication

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The MSVC-specific _umul128 code path had incorrect carry propagation
when adding three values (rp[i] + lo + carry). The original code:

  carry = hi + (sum < lo);

only detected overflow between sum and lo, missing overflow in the
first addition rp[i] + lo. This caused incorrect bigint calculations
on VC 64-bit builds.

Fixed by splitting three-way addition into two two-way additions
with proper overflow detection for each step:

  temp = rp_val + lo;
  sum = temp + carry;
  carry = hi + (temp < rp_val) + (sum < temp);

Co-authored-by: Claude <noreply@anthropic.com>
This commit is contained in:
Yukihiro "Matz" Matsumoto
2025-08-23 10:49:18 +09:00
parent e7925141fb
commit 589f256ffa
1 changed files with 40 additions and 20 deletions
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mrbgems/mruby-bigint/core/bigint.c
+40 -20
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@@ -794,34 +794,46 @@ limb_addmul_1(mp_limb *rp, const mp_limb *s1p, size_t n, mp_limb limb)
unsigned long long hi, lo, sum;
lo = _umul128((unsigned long long)s1p[i], (unsigned long long)limb, &hi);
sum = (unsigned long long)rp[i] + lo + carry;
unsigned long long rp_val = (unsigned long long)rp[i];
unsigned long long temp = rp_val + lo;
sum = temp + carry;
rp[i] = (mp_limb)sum;
carry = hi + (sum < lo);
carry = hi + (temp < rp_val) + (sum < temp);
lo = _umul128((unsigned long long)s1p[i+1], (unsigned long long)limb, &hi);
sum = (unsigned long long)rp[i+1] + lo + carry;
unsigned long long rp_val1 = (unsigned long long)rp[i+1];
unsigned long long temp1 = rp_val1 + lo;
sum = temp1 + carry;
rp[i+1] = (mp_limb)sum;
carry = hi + (sum < lo);
carry = hi + (temp1 < rp_val1) + (sum < temp1);
lo = _umul128((unsigned long long)s1p[i+2], (unsigned long long)limb, &hi);
sum = (unsigned long long)rp[i+2] + lo + carry;
unsigned long long rp_val2 = (unsigned long long)rp[i+2];
unsigned long long temp2 = rp_val2 + lo;
sum = temp2 + carry;
rp[i+2] = (mp_limb)sum;
carry = hi + (sum < lo);
carry = hi + (temp2 < rp_val2) + (sum < temp2);
lo = _umul128((unsigned long long)s1p[i+3], (unsigned long long)limb, &hi);
sum = (unsigned long long)rp[i+3] + lo + carry;
unsigned long long rp_val3 = (unsigned long long)rp[i+3];
unsigned long long temp3 = rp_val3 + lo;
sum = temp3 + carry;
rp[i+3] = (mp_limb)sum;
carry = hi + (sum < lo);
carry = hi + (temp3 < rp_val3) + (sum < temp3);
lo = _umul128((unsigned long long)s1p[i+4], (unsigned long long)limb, &hi);
sum = (unsigned long long)rp[i+4] + lo + carry;
unsigned long long rp_val4 = (unsigned long long)rp[i+4];
unsigned long long temp4 = rp_val4 + lo;
sum = temp4 + carry;
rp[i+4] = (mp_limb)sum;
carry = hi + (sum < lo);
carry = hi + (temp4 < rp_val4) + (sum < temp4);
lo = _umul128((unsigned long long)s1p[i+5], (unsigned long long)limb, &hi);
sum = (unsigned long long)rp[i+5] + lo + carry;
unsigned long long rp_val5 = (unsigned long long)rp[i+5];
unsigned long long temp5 = rp_val5 + lo;
sum = temp5 + carry;
rp[i+5] = (mp_limb)sum;
carry = hi + (sum < lo);
carry = hi + (temp5 < rp_val5) + (sum < temp5);
}
/* 3x unrolled loop for medium operands */
@@ -829,28 +841,36 @@ limb_addmul_1(mp_limb *rp, const mp_limb *s1p, size_t n, mp_limb limb)
unsigned long long hi, lo, sum;
lo = _umul128((unsigned long long)s1p[i], (unsigned long long)limb, &hi);
sum = (unsigned long long)rp[i] + lo + carry;
unsigned long long rp_val = (unsigned long long)rp[i];
unsigned long long temp = rp_val + lo;
sum = temp + carry;
rp[i] = (mp_limb)sum;
carry = hi + (sum < lo);
carry = hi + (temp < rp_val) + (sum < temp);
lo = _umul128((unsigned long long)s1p[i+1], (unsigned long long)limb, &hi);
sum = (unsigned long long)rp[i+1] + lo + carry;
unsigned long long rp_val1 = (unsigned long long)rp[i+1];
unsigned long long temp1 = rp_val1 + lo;
sum = temp1 + carry;
rp[i+1] = (mp_limb)sum;
carry = hi + (sum < lo);
carry = hi + (temp1 < rp_val1) + (sum < temp1);
lo = _umul128((unsigned long long)s1p[i+2], (unsigned long long)limb, &hi);
sum = (unsigned long long)rp[i+2] + lo + carry;
unsigned long long rp_val2 = (unsigned long long)rp[i+2];
unsigned long long temp2 = rp_val2 + lo;
sum = temp2 + carry;
rp[i+2] = (mp_limb)sum;
carry = hi + (sum < lo);
carry = hi + (temp2 < rp_val2) + (sum < temp2);
}
/* Handle remaining elements */
for (; i < n; i++) {
unsigned long long hi, lo;
lo = _umul128((unsigned long long)s1p[i], (unsigned long long)limb, &hi);
unsigned long long sum = (unsigned long long)rp[i] + lo + carry;
unsigned long long rp_val = (unsigned long long)rp[i];
unsigned long long temp = rp_val + lo;
unsigned long long sum = temp + carry;
rp[i] = (mp_limb)sum;
carry = hi + (sum < lo);
carry = hi + (temp < rp_val) + (sum < temp);
}
return (mp_limb)carry;