provide a semantic defined by the C11 fences with corresponding
memory_order.
atomic_thread_fence_acq() gives r | r, w, where r and w are read and
write accesses, and | denotes the fence itself.
atomic_thread_fence_rel() is r, w | w.
atomic_thread_fence_acq_rel() is the combination of the acquire and
release in single operation. Note that reads after the acq+rel fence
could be made visible before writes preceeding the fence.
atomic_thread_fence_seq_cst() orders all accesses before/after the
fence, and the fence itself is globally ordered against other
sequentially consistent atomic operations.
Reviewed by: alc
Discussed with: bde
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks
Using plain dsb()/dmb() as full system barriers is usually to much.
Adding proper options to those barriers (instead of full system - sy)
will most likely reduce the cost of the instructions and will benefit
in performance improvement.
This commit adds options to barrier macro definitions.
Obtained from: Semihalf
Reviewed by: andrew, ian
Sponsored by: The FreeBSD Foundation
no need for them to be this strong, we only need to provide one or the
other.
While here replace atomic_load_acq_* and atomic_store_rel_* with a single
instruction version, and fix the definition of atomic_clear_* to point to
the correct functions.
Sponsored by: The FreeBSD Foundation
