C++11 atomic library notes

[Andrew MacLeod]

On 10/01/2011 02:55 AM, Marc Glisse wrote:
>
>
> "The compiler must ensure that for any given object, it either ALWAYS 
> inlines lock free routines, OR calls the external routines. For any 
> given object, these cannot be intermixed."
>
> Why? You give an example explaining why it is fine to link 386 and 486 
> objects, and I cant see the difference. Not that I'm advocating mixing 
> them, just wondering whether it really matters if it happens (by 
> accident).

If we have an architecture which we cannot generate one of the functions 
for, say __atomic_load_16, then it will have to use whatever the library 
supplies. If you continues to generate all the rest of the __atomic 
builtins for 16 bytes using lock free instructions, and the call to the 
library turns out to be a locked implementation at runtime, then atomic 
support for 16 byte objects is broken. The load thinks its getting a 
lock, but none of the other routines pay any attention to locks. So if 
one atomic operations requires then library, they all do in order to get 
consistent behaviour.

>
> I assume that locks are supposed to be implemented in terms of those 
> functions too (it sounds like lock uses atomic which uses lock ;-)
>
> For the atomic version of a user-defined "small" POD type, do you 
> intend to query the compiler about the presence of a volatile member 
> to dispatch to the right function?
if it is sorted out that volatile does require something different, then 
we'd need to dispatch differently for volatile.  At the moment Im not 
doing anything different for them, just asking "the experts" if we need 
to :-)  I suspect we might need to, but Its easier to add something 
later than to remove something thats already in a compiler.
>
> The design looks a lot like:
> http://libcxx.llvm.org/atomic_design_a.html
> which is good since the main point seems to be to share it between 
> implementations. Are there others on board?

Im not aware that anyone has done anything with an external library 
yet.   Hopefully this will make us all aware of each other and get 
consistent :-).  I hadn's seen that link, I guess the name is a good 
choice :-)

Andrew