https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105844
--- Comment #13 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-10 branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:14175f2262c94868e26f01eef1e14418f2b5e6a9 commit r10-11388-g14175f2262c94868e26f01eef1e14418f2b5e6a9 Author: Jonathan Wakely <jwak...@redhat.com> Date: Fri Jun 10 14:39:13 2022 +0100 libstdc++: Make std::lcm and std::gcd detect overflow [PR105844] When I fixed PR libstdc++/92978 I introduced a regression whereby std::lcm(INT_MIN, 1) and std::lcm(50000, 49999) would no longer produce errors during constant evaluation. Those calls are undefined, because they violate the preconditions that |m| and the result can be represented in the return type (which is int in both those cases). The regression occurred because __absu<unsigned>(INT_MIN) is well-formed, due to the explicit casts to unsigned in that new helper function, and the out-of-range multiplication is well-formed, because unsigned arithmetic wraps instead of overflowing. To fix 92978 I made std::gcm and std::lcm calculate |m| and |n| immediately, yielding a common unsigned type that was used to calculate the result. That was partly correct, but there's no need to use an unsigned type. Doing so only suppresses the overflow errors so the compiler can't detect them. This change replaces __absu with __abs_r that returns the common type (not its corresponding unsigned type). This way we can detect overflow in __abs_r when required, while still supporting the most-negative value when it can be represented in the result type. To detect LCM results that are out of range of the result type we still need explicit checks, because neither constant evaluation nor UBsan will complain about unsigned wrapping for cases such as std::lcm(500000u, 499999u). We can detect those overflows efficiently by using __builtin_mul_overflow and asserting. libstdc++-v3/ChangeLog: PR libstdc++/105844 * include/experimental/numeric (experimental::gcd): Simplify assertions. Use __abs_r instead of __absu. (experimental::lcm): Likewise. Remove use of __detail::__lcm so overflow can be detected. * include/std/numeric (__detail::__absu): Rename to __abs_r and change to allow signed result type, so overflow can be detected. (__detail::__lcm): Remove. (gcd): Simplify assertions. Use __abs_r instead of __absu. (lcm): Likewise. Remove use of __detail::__lcm so overflow can be detected. * testsuite/26_numerics/gcd/gcd_neg.cc: Adjust dg-error lines. * testsuite/26_numerics/lcm/lcm_neg.cc: Likewise. * testsuite/26_numerics/gcd/105844.cc: New test. * testsuite/26_numerics/lcm/105844.cc: New test. (cherry picked from commit 671970a5621e18e7079b4ca113e56434c858db66)
