[committed] libstdc++: Fix std::gcd and std::lcm for unsigned integers [PR 92978]

Daniel Krügler daniel.kruegler@gmail.com
Fri Aug 28 22:20:44 GMT 2020


Am Sa., 29. Aug. 2020 um 00:12 Uhr schrieb Jonathan Wakely via
Libstdc++ <libstdc++@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?

Thanks,

- Daniel


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