[PATCH] Clarify __atomic_compare_exchange_n docs

[Andrew Haley]

On 09/29/2015 04:21 PM, Sandra Loosemore wrote:
> What is "weak compare_exchange", and what is "the strong variation", and 
> how do they differ in terms of behavior?

It's in C++11 29.6.5:

Remark: The weak compare-and-exchange operations may fail spuriously,
that is, return false while leaving the contents of memory pointed to
by expected before the operation is the same that same as that of the
object and the same as that of expected after the operation. [ Note:
This spurious failure enables implementation of compare-and-exchange
on a broader class of machines, e.g., load- locked store-conditional
machines. A consequence of spurious failure is that nearly all uses of
weak compare-and-exchange will be in a loop.  When a
compare-and-exchange is in a loop, the weak version will yield better
performance on some platforms. When a weak compare-and-exchange would
require a loop and a strong one would not, the strong one is
preferable. — end note ]

The classic use of this is for shared counters: you don't care if you
miss an occasional count but you don't want the counter to go
backwards.

Whether we should replicate all of the C++11 language is perhaps
something we should discuss.

Andrew.