https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108974
--- Comment #8 from GCC Commits ---
The master branch has been updated by Jonathan Wakely :
https://gcc.gnu.org/g:ef632273a90657acb45e89df12bce9e96035f52c
commit r16-997-gef632273a90657acb45e89df12bce9e96035f52c
Author: Jonathan Wakely
Date: Fri Jan 10 12:22:21 2025 +
libstdc++: Fix std::barrier for constant initialization [PR118395]
The std::barrier constructor should be constexpr, which means we need to
defer the dynamic allocation if the constructor is called during
constant-initialization. We can defer it to the first call to
barrier::arrive, using compare-and-swap on an atomic (instead of the
unique_ptr currently used).
Also add precondition checks to the constructor and arrive member
function. Also implement the proposed resolution of LWG 3898.
libstdc++-v3/ChangeLog:
PR libstdc++/118395
PR libstdc++/108974
PR libstdc++/98749
* include/std/barrier (__tree_barrier): Use default
member-initializers. Change _M_state member from
unique_ptr<__state_t[]> to atomic<__state_t*>. Add
no_unique_address attribute to _M_completion.
(__tree_barrier::_M_arrive): Load value from _M_state.
(__tree_barrier::_M_invoke_completion): New member function to
ensure a throwing completion function will terminate, as
proposed in LWG 3898.
(__tree_barrier::max): Reduce by one to avoid overflow.
(__tree_barrier::__tree_barrier): Add constexpr. Qualify call to
std::move. Remove mem-initializers made unnecessary by default
member-initializers. Add precondition check. Only allocate state
array if not constant evaluated.
(__tree_barrier::arrive): Add precondition check. Do deferred
initialization of _M_state if needed.
(barrier): Add static_assert, as proposed in LWG 3898.
(barrier::barrier): Add constexpr.
* testsuite/30_threads/barrier/cons.cc: New test.
* testsuite/30_threads/barrier/lwg3898.cc: New test.
