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[libc++] Fix sub-overflow in std::gcd implementation #117984

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Merged
merged 2 commits into from
Dec 6, 2024
Merged

[libc++] Fix sub-overflow in std::gcd implementation #117984

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1f04c92
[libc++] Fix sub-overflow in std::gcd implementation
serge-sans-paille Nov 28, 2024
fcaad75
fixup! [libc++] Fix sub-overflow in std::gcd implementation
serge-sans-paille Dec 5, 2024
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23 changes: 10 additions & 13 deletions libcxx/include/__numeric/gcd_lcm.h
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Original file line number Diff line number Diff line change
Expand Up @@ -55,7 +55,8 @@ template <class _Tp>
constexpr _LIBCPP_HIDDEN _Tp __gcd(_Tp __a, _Tp __b) {
static_assert(!is_signed<_Tp>::value, "");

// From: https://lemire.me/blog/2013/12/26/fastest-way-to-compute-the-greatest-common-divisor
// Using Binary GCD algorithm https://en.wikipedia.org/wiki/Binary_GCD_algorithm, based on an implementation
// from https://lemire.me/blog/2024/04/13/greatest-common-divisor-the-extended-euclidean-algorithm-and-speed/
//
// If power of two divides both numbers, we can push it out.
// - gcd( 2^x * a, 2^x * b) = 2^x * gcd(a, b)
Expand All @@ -76,21 +77,17 @@ constexpr _LIBCPP_HIDDEN _Tp __gcd(_Tp __a, _Tp __b) {
if (__a == 0)
return __b;

int __az = std::__countr_zero(__a);
int __bz = std::__countr_zero(__b);
int __shift = std::min(__az, __bz);
__a >>= __az;
__b >>= __bz;
_Tp __c = __a | __b;
int __shift = std::__countr_zero(__c);
__a >>= std::__countr_zero(__a);
do {
_Tp __diff = __a - __b;
if (__a > __b) {
__a = __b;
__b = __diff;
_Tp __t = __b >> std::__countr_zero(__b);
if (__a > __t) {
__b = __a - __t;
__a = __t;
} else {
__b = __b - __a;
__b = __t - __a;
}
if (__diff != 0)
__b >>= std::__countr_zero(__diff);
} while (__b != 0);
return __a << __shift;
}
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12 changes: 1 addition & 11 deletions libcxx/test/std/numerics/numeric.ops/numeric.ops.gcd/gcd.pass.cpp
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Expand Up @@ -27,17 +27,7 @@ constexpr struct {
int x;
int y;
int expect;
} Cases[] = {
{0, 0, 0},
{1, 0, 1},
{0, 1, 1},
{1, 1, 1},
{2, 3, 1},
{2, 4, 2},
{36, 17, 1},
{36, 18, 18}
};

} Cases[] = {{0, 0, 0}, {1, 0, 1}, {0, 1, 1}, {1, 1, 1}, {2, 3, 1}, {2, 4, 2}, {11, 9, 1}, {36, 17, 1}, {36, 18, 18}};

template <typename Input1, typename Input2, typename Output>
constexpr bool test0(int in1, int in2, int out)
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