On 29/08/20 00:20 +0200, Daniel Krügler wrote:
Am Sa., 29. Aug. 2020 um 00:12 Uhr schrieb Jonathan Wakely via Libstdc++ <libstd...@gcc.gnu.org>:This fixes a bug with mixed signed and unsigned types, where converting a negative value to the unsigned result type alters the value. The solution is to obtain the absolute values of the arguments immediately and to perform the actual GCD or LCM algorithm on two arguments of the same type. In order to operate on the most negative number without overflow when taking its absolute, use an unsigned type for the result of the abs operation. For example, -INT_MIN will overflow, but -(unsigned)INT_MIN is (unsigned)INT_MAX+1U which is the correct value. libstdc++-v3/ChangeLog: PR libstdc++/92978 * include/std/numeric (__abs_integral): Replace with ... (__detail::__absu): New function template that returns an unsigned type, guaranteeing it can represent the most negative signed value. (__detail::__gcd, __detail::__lcm): Require arguments to be unsigned and therefore already non-negative. (gcd, lcm): Convert arguments to absolute value as unsigned type before calling __detail::__gcd or __detail::__lcm. * include/experimental/numeric (gcd, lcm): Likewise. * testsuite/26_numerics/gcd/gcd_neg.cc: Adjust expected errors. * testsuite/26_numerics/lcm/lcm_neg.cc: Likewise. * testsuite/26_numerics/gcd/92978.cc: New test. * testsuite/26_numerics/lcm/92978.cc: New test. * testsuite/experimental/numeric/92978.cc: New test. Tested powerpc64le-linux. Committed to trunk.Shouldn't the overload of __absu void __absu(bool) = delete; still also be a template or is just the diff presentation confusing me?
Good point! It's called as __absu<U>(v) so it needs to be a function template for the deleted one to be a candidate. I'm not sure we really need it, since all the callers have a static_assert ensuring it's not a bool. But if we have it, it should be correct.
