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big ints: only write xor overflow if required

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This commit is contained in:
Robin Voetter 2021-09-23 06:22:18 +02:00
parent 351e4f07ce
commit cd3dcc225b
1 changed files with 8 additions and 1 deletions
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9
lib/std/math/big/int.zig
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@ -2495,6 +2495,8 @@ fn llsignedand(r: []Limb, a: []const Limb, a_positive: bool, b: []const Limb, b_
// r may alias.
// a and b must not be -0.
// Returns `true` when the result is positive.
// If the sign of a and b is equal, then r requires at least `max(a.len, b.len)` limbs are required.
// Otherwise, r requires at least `max(a.len, b.len) + 1` limbs.
fn llsignedxor(r: []Limb, a: []const Limb, a_positive: bool, b: []const Limb, b_positive: bool) bool {
@setRuntimeSafety(debug_safety);
assert(a.len != 0 and b.len != 0);
@ -2538,7 +2540,12 @@ fn llsignedxor(r: []Limb, a: []const Limb, a_positive: bool, b: []const Limb, b_
r_carry = @boolToInt(@addWithOverflow(Limb, r[i], r_carry, &r[i]));
}
r[i] = r_carry;
// If both inputs don't share the same sign, an extra limb is required.
if (a_positive != b_positive) {
r[i] = r_carry;
} else {
assert(r_carry == 0);
}
assert(a_borrow == 0);
assert(b_borrow == 0);