From: "Daniel Krügler" <daniel.kruegler@gmail.com>
To: Jonathan Wakely <jwakely@redhat.com>
Cc: "libstdc++" <libstdc++@gcc.gnu.org>,
gcc-patches List <gcc-patches@gcc.gnu.org>
Subject: Re: [committed] libstdc++: Fix std::gcd and std::lcm for unsigned integers [PR 92978]
Date: Sat, 29 Aug 2020 00:20:44 +0200 [thread overview]
Message-ID: <CAGNvRgDx_fcyBdzPiV12qbYginpzNvuf+cnaVu=eop3-qQgQGw@mail.gmail.com> (raw)
In-Reply-To: <20200828221126.GA1290141@redhat.com>
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
next prev parent reply other threads:[~2020-08-28 22:20 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-28 22:11 Jonathan Wakely
2020-08-28 22:20 ` Daniel Krügler [this message]
2020-08-28 22:53 ` Jonathan Wakely
2020-08-29 17:24 ` Jonathan Wakely
2020-09-02 16:24 ` Jonathan Wakely
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