They provide relaxed-ordered atomic access semantic. Due to the
FreeBSD memory model, the operations are syntaxical wrappers around
the volatile accesses. The volatile qualifier is used to ensure that
the access not optimized out and in turn depends on the volatile
semantic as implemented by supported compilers.
The motivation for adding the operation is to help people coming from
other systems or knowing the C11/C++ standards where atomics have
special type and require use of the special access operations. It is
still the case that FreeBSD requires plain load and stores of aligned
integer types to be atomic.
Suggested by: jhb
Reviewed by: alc, jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D13534
atomic functions where they are almost identical, or have acquire/release
semantics.
While here clean these function up. The cbnz instruction doesn't change
the condition flags so drop cc, however they should have memory added to the
clobber list.
Reviewed by: kib
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D4318
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
