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

Jonathan Wakely jwakely@redhat.com
Sat Aug 29 17:24:49 GMT 2020

On 28/08/20 23:53 +0100, Jonathan Wakely wrote:
>On 29/08/20 00:20 +0200, Daniel Krügler wrote:
>>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.
>>>        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.

I've just pushed this fix, after testing on powerpc64le-linux.

Thanks for the review, Daniel.

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