The support includes:
- The standard fetch_add/fetch_sub operations.
- Extra operations: (fetch_/opaque_)negate, (opaque_)add/sub.
- Extra capability macros: BOOST_ATOMIC_FLOAT/DOUBLE/LONG_DOUBLE_LOCK_FREE.
The atomic operations are currently implemented on top of the integer-based
backends and thus are mostly CAS-based. The CAS operations perform binary
comparisons, and as such have different behavior wrt. special FP values like
NaN and signed zero than normal C++.
The support for floating point types is optional and can be disabled by
defining BOOST_ATOMIC_NO_FLOATING_POINT. This can be useful if on a certain
platform parameters of the floating point types cannot be deduced from the
compiler-defined or system macros (in which case the compilation fails).
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0020r6.html
These operations are useful for two reasons. First, they are needed by
atomic<> interface as the pre-increment/decrement and add/subtract operators
need to perform the corresponding arithmetics and return the actual result while
not exhibiting UB in case of overflow. This means that the operation must be
performed on the unsigned storage type in the backend. Second, the (op)_and_test
operations on ARM and PowerPC can be implemented in a more generic way on top of
the operations that return the result. And since we have those operations
internally, why not expose them to users.
Added tests and docs for the new operations. Also, added docs for the recently
added scoped names of the memory_order enum values.
Also, added a specialized "emulated" backend for the extra operations. This
backend makes better use of the fact that the operations are lock-protected
by avoiding any CAS-based loops.
As the names suggest, the methods perform the corresponding operation and test
if the result is not zero.
Also, for the emulated fetch_complement, take care of integral promotion, which
could mess up the storage bits that were not part of the value on backends
where the storage is larger than the value. This could in turn break CAS on
the atomic value as it compares the whole storage.
This makes the result of (op)_and_test more consistent with other
methods such as test_and_set and bit_test_and_set, as well as the
methods used in the C++ standard library.
This is a breaking change. The users are able to define
BOOST_ATOMIC_HIGHLIGHT_OP_AND_TEST macro to generate warnings on each
use of the changed functions. This will help users to port from Boost
1.66 to newer Boost releases.
More info at:
https://github.com/boostorg/atomic/issues/11http://boost.2283326.n4.nabble.com/atomic-op-and-test-naming-
tc4701445.html
Also made a few wording corrections and added is_always_lock_free and
a section about atomic<> typedefs. Clarified the status quo regarding
memory_order_consume. Removed the obsolete preudo-header for doxygen
that was not used for docs (if we want doxygen, it's better to
add comments to the real headers anyway).
