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

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We should have been using __builtin_mul_overflow from the start instead
of adding a manual (and error-prone) check for overflow.

Fixes #96196
This commit is contained in:
Louis Dionne 2024-06-25 08:35:23 -05:00 committed by GitHub
parent c69ea04fb9
commit fd62906ddb
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GPG Key ID: B5690EEEBB952194
2 changed files with 31 additions and 4 deletions
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8
libcxx/include/__numeric/gcd_lcm.h
View File

@ -117,11 +117,13 @@ constexpr _LIBCPP_HIDE_FROM_ABI common_type_t<_Tp, _Up> lcm(_Tp __m, _Up __n) {
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

27
libcxx/test/std/numerics/numeric.ops/numeric.ops.lcm/lcm.pass.cpp
View File

@ -16,6 +16,7 @@
#include <cassert>
#include <climits>
#include <cstdint>
#include <limits>
#include <type_traits>
#include "test_macros.h"
@ -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
@ -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;
}