[Bug c++/60272] atomic<>::compare_exchange_weak has spurious store and can cause race conditions

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60272

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |rth at gcc dot gnu.org,
                   |                            |torvald at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Even our __atomic_compare_exchange* documentation states that:
If they are not equal, the current contents of
@code{*@var{ptr}} is written into @code{*@var{expected}}.
But then expand_builtin_atomic_compare_exchange doesn't care:
  oldval = expect;
  if (!expand_atomic_compare_and_swap ((target == const0_rtx ? NULL : &target),
                                       &oldval, mem, oldval, desired,
                                       is_weak, success, failure))
    return NULL_RTX;

  if (oldval != expect)
    emit_move_insn (expect, oldval);

That effectively means that expect will be stored unconditionally.
So, either we'd need to change this function, so that it sets oldval to
NULL_RTX
first, and passes ..., &oldval, mem, expected, ... and needs to also always ask
for target, then conditionally on target store to expected, or perhaps add
extra parameter to expand_atomic_compare_and_swap and do the store only
conditionally in that case.  Richard/Torvald?