On Wed, Aug 27, 2025 at 4:03 PM Patrick Palka <ppa...@redhat.com> wrote:
> On Wed, 27 Aug 2025, Jonathan Wakely wrote:
>
> > On Wed, 27 Aug 2025 at 11:05, Tomasz Kamiński <tkami...@redhat.com>
> wrote:
> > >
> > > For any minimum value of a signed type, its negation (with wraparound)
> results
> > > in the same value, behaving like zero. Representing the unordered
> result with
> > > this minimum value, along with 0 for equal, 1 for greater, and -1 for
> less
> > > in partial_ordering, allows its value to be reversed using unary
> negation.
> > >
> > > The operator<=(partial_order, 0) now checks if the reversed value is
> positive.
> > > This works correctly because the unordered value remains unchanged and
> thus
> > > negative.
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > > * libsupc++/compare (_Ncmp::_Unordered): Rename and change the
> value
> > > to minimum value of signed char.
> > > (_Ncomp::unordered): Renamed from _Unordered, the name is
> reserved
> > > by partial_ordered::unordered.
> > > (partial_ordering::_M_reverse()): Define.
> > > (operator<=(partial_ordering, __cmp_cat::__unspec))
> > > (operator>=(__cmp_cat::__unspec, partial_ordering)):
> Implemented
> > > in terms of negated _M_value.
> > > (operator>=(partial_ordering, __cmp_cat::__unspec))
> > > (operator<=(__cmp_cat::__unspec, partial_ordering)): Directly
> > > compare _M_value, as unordered value is negative.
> > > (partial_ordering::unordered): Handle _Ncmp::unoredred rename.
> > > ---
> > > Changes in v3:
> > > * rename and simplify defintion of _Ncmp::unordered
> >
> > Ah yes, we don't need to use an _Ugly name name for unordered.
>
> FWIW it wasn't 100% obvious to me that renaming _Unordered to unordered
> is safe here, but it does seem to be, since AFAICT there's no way for a
> user to refer to the enumeration __cmp_cat::_Ncmp (e.g. via decltype).
>
> Otherwise it'd potentially be an observable change:
>
> struct A {
> using enum decltype(some-expr-of-type-__cmp_cat::_Ncmp);
> int unordered; // now errors due to conflict with _Ncmp::unordered
> };
>
Same reasoning apply to equivalent, less, greater for _Ord enumerator,
so we should either make all uglified or all pretty consistently.
>
> >
> > > * simplify defintion of _M_reverse
> > >
> > > Testing on x86_64-linux and powerpc64le-linux. OK for trunk when test
> > > passes?
> >
> > OK for trunk with the pretty printer change included.
> >
> >
