[Bug c++/98632] New: Warn about unspecified expression ordering for atomics with non-relaxed memory ordering.

["tilps at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98632

            Bug ID: 98632
           Summary: Warn about unspecified expression ordering for atomics
                    with non-relaxed memory ordering.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tilps at hotmail dot com
  Target Milestone: ---

C++ defines that execution ordering for expressions is largely unspecified.  In
cases where there are multiple atomic operations in an expression for which
there is no standard required ordering, and if those atomic operations are
marked with a non-relaxed memory ordering, it would be useful to have a
warning. Since the compiler is technically free to reorder them in-spite of the
memory ordering indicating that the user cares about the specific ordering.

While it might be able to be argued that the warning should fire for any
expression involving just a single atomic and some other expression component
that would be unable to be reordered if the sub-expressions had been assigned
to locals first, it seems that would be likely to have vastly more false
positives than expressions that involve multiple atomic operations. So I would
suggest only triggering for expressions involving multiple atomic operations.