[boost] [atomic] Support for specialized instructions

Subject: [boost] [atomic] Support for specialized instructions
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2013-11-21 01:00:33

Hi,

I've been contemplating the idea of adding support for some
specialized atomic instructions to Boost.Atomic. Since I'm only
familiar with x86 architecture, my current candidates are:

* Increment/decrement with a check for zero result (lock inc/dec). The
decrement is the most essential of the two because it would be useful
for reference counters of various kinds. The increment is mostly
beneficial in terms of code size, and it probably doesn't need the
check for zero in most cases.

* In-place logical operations (lock and/or/xor). These are useful in
cases when the code needs to operate on flags or bit sets but doesn't
need the result of the operation. Optional variants can be provided
with a test for zero result.

* Bit set/reset operations (lock bts/btr). The use cases are similar
to the logical operations but the advantage is that the previous value
of the altered bit can be returned. BTW, these operations are needed
in Boost.Sync to implement mutexes on Windows.

All these operations can be implemented through the standard atomic<>
interface, and that's what would be done on platforms without support
for the specialized instructions.

For now the idea is to add special member functions to atomic<>, when
used with arithmetic types. I was thinking about providing free
functions as well, although I think that the complete set of the
standard atomic functions would be required for that.

Comments? Opinions?