Re: [committed 1/3] libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221]

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Tue, 10 Jan 2023, 12:26 Jakub Jelinek via Libstdc++, <
libstdc++@gcc.gnu.org> wrote:

> On Tue, Jan 10, 2023 at 11:46:55AM +0000, Jonathan Wakely via Gcc-patches
> wrote:
> > Tested x86_64-linux. Pushed to trunk.
> >
> > -- >8 --
> >
> > The default constructor has a constraint that is always false if
> > arithmetic on size_t values promotes to int. Rewrite the constraint
> > exactly as written in the standard, which works correctly.
> >
> > libstdc++-v3/ChangeLog:
> >
> >       PR libstdc++/108221
> >       * include/std/span (span::span()): Un-simplify constraint to
> >       work for size_t of lesser rank than int.
> > ---
> >  libstdc++-v3/include/std/span | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/libstdc++-v3/include/std/span
> b/libstdc++-v3/include/std/span
> > index 251fed91abf..b336332b190 100644
> > --- a/libstdc++-v3/include/std/span
> > +++ b/libstdc++-v3/include/std/span
> > @@ -145,7 +145,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >
> >        constexpr
> >        span() noexcept
> > -      requires ((_Extent + 1u) <= 1u)
> > +      requires (_Extent == dynamic_extent || _Extent == 0)
> >        : _M_ptr(nullptr), _M_extent(0)
> >        { }
>
> If it would be C++23 only, you could use ((_Extent + 1uz) <= 1uz).
>

That would still promote _Extent and 1uz to int before the addition, and so
would not wrap to zero when _Extent == -1uz

size_t(_Extent + 1) <= 1 would work, but I'm just going to KISS and do what
the standard says.


As this is evaluated at compile time only, it is unfortunate it is
> 3 operations compared to former 2, but not a big deal.  If this was
> in code that would be emitted at runtime, GCC already optimizes
> (x == -1uz || x == 0)
> or
> (x == 0 || x == -1uz)
> to
> ((x + 1uz) <= 1uz)
>
>         Jakub
>
>