Re: [PATCH] libstdc++: Make atomic notify_one and notify_all non-const

[Thomas Rodgers <trodgers@redhat.com>]

Committed to trunk, backported to releases/gcc-11.

On Fri, Feb 11, 2022 at 12:22 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> On Fri, 11 Feb 2022 at 17:40, Thomas Rodgers via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > <recording this here for future reference>
> > PR102994 "atomics: std::atomic<ptr>::wait is not marked const" raises the
> > issue that the current libstdc++ implementation marks the notify members
> > const, the implementation strategy used by libstdc++, as well as libc++
> > and the Microsoft STL, do not require the atomic to be mutable (it is
> hard
> > to conceive of a desirable implementation approach that would require
> it).
> > The original paper proposing the wait/notify functionality for atomics
> > (p1185) also had these members marked const for the first three
> revisions,
> > but that was changed without explanation in r3 and subsequent revisions
> of
> > the paper.
> >
> > After raising the issue to the authors of p1185 and the author of the
> > libc++ implementation, the consensus seems to be "meh, it's harmless" so
> > there seems little appetite for an LWG issue to revisit the subject.
> >
> > This patch changes the libstdc++ implementation to be in agreement with
> > the standard by removing const from those notify_one/notify_all members.
> >
> > libstdc++-v3/ChangeLog:
>
> Might as well add a "PR libstdc++/102994" here to the bug gets updated
> automatically.
>
> OK for trunk with that change.
>
> >         * include/bits/atomic_base.h (atomic_flag::notify_one,
> >         notify_all): Remove const qualification.
> >         (__atomic_base::notify_one, notify_all): Likewise.
> >         * include/std/atomic (atomic<bool>::notify_one, notify_all):
> >         Likewise.
> >         (atomic::notify_one, notify_all): Likewise.
> >         (atomic<T*>::notify_one, notify_all): Likewise.
> >         (atomic_notify_one, atomic_notify_all): Likewise.
> >         * testsuite/29_atomics/atomic/wait_notify/102994.cc: Adjust test
> >         to account for change in notify_one/notify_all signature.
> >
> > Tested x86_64-pc-linux-gnu.
>
>