Skip to content

[libc++] Fix incorrect overflow checking in std::lcm #96310

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 4 commits into from
Jun 25, 2024
Merged

[libc++] Fix incorrect overflow checking in std::lcm #96310

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
4 commits
Select commit Hold shift + click to select a range
04cf65e
[libc++] Fix incorrect overflow checking in std::lcm
ldionne Jun 21, 2024
f0d63a0
Fix modules issue
ldionne Jun 21, 2024
0e5972b
Use maybe_unused attribute
ldionne Jun 25, 2024
95bb475
NFC fix #endif comment:
ldionne Jun 25, 2024
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
8 changes: 5 additions & 3 deletions libcxx/include/__numeric/gcd_lcm.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -117,11 +117,13 @@ _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI common_type_t<_Tp, _Up> lcm(_Tp __m, _Up
using _Rp = common_type_t<_Tp, _Up>;
_Rp __val1 = __ct_abs<_Rp, _Tp>()(__m) / std::gcd(__m, __n);
_Rp __val2 = __ct_abs<_Rp, _Up>()(__n);
_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN((numeric_limits<_Rp>::max() / __val1 > __val2), "Overflow in lcm");
return __val1 * __val2;
_Rp __res;
[[maybe_unused]] bool __overflow = __builtin_mul_overflow(__val1, __val2, &__res);
_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(!__overflow, "Overflow in lcm");
return __res;
}

#endif // _LIBCPP_STD_VER
#endif // _LIBCPP_STD_VER >= 17

_LIBCPP_END_NAMESPACE_STD

Expand Down
27 changes: 26 additions & 1 deletion libcxx/test/std/numerics/numeric.ops/numeric.ops.lcm/lcm.pass.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@
#include <cassert>
#include <climits>
#include <cstdint>
#include <limits>
#include <type_traits>
#include "test_macros.h"

Expand Down Expand Up @@ -89,6 +90,13 @@ constexpr bool do_test(int = 0)
return accumulate;
}

template <class T>
constexpr bool test_limits() {
assert(std::lcm(std::numeric_limits<T>::max() - 1, std::numeric_limits<T>::max() - 1) == std::numeric_limits<T>::max() - 1);
assert(std::lcm(std::numeric_limits<T>::max(), std::numeric_limits<T>::max()) == std::numeric_limits<T>::max());
return true;
}

int main(int argc, char**)
{
int non_cce = argc; // a value that can't possibly be constexpr
Expand Down Expand Up @@ -141,5 +149,22 @@ int main(int argc, char**)
assert(res1 == 1324997410816LL);
}

return 0;
// https://github.com/llvm/llvm-project/issues/96196
{
assert(test_limits<unsigned int>());
assert(test_limits<std::uint32_t>());
assert(test_limits<std::uint64_t>());
assert(test_limits<int>());
assert(test_limits<std::int32_t>());
assert(test_limits<std::int64_t>());

static_assert(test_limits<unsigned int>(), "");
static_assert(test_limits<std::uint32_t>(), "");
static_assert(test_limits<std::uint64_t>(), "");
static_assert(test_limits<int>(), "");
static_assert(test_limits<std::int32_t>(), "");
static_assert(test_limits<std::int64_t>(), "");
}

return 0;
}
Loading