[cpp-threads] Draft Plan for Threads in C++

[Ben Hutchings]

Lawrence.Crowl at Sun.com wrote:
> Ben Hutchings <ben at decadentplace.org.uk> writes:
>  >Lawrence.Crowl at Sun.com wrote:
>  >> "Boehm, Hans" <hans.boehm at hp.com> writes:
>  >>  >Should it provide specific "atomic" classes like the Java one?
>  >>  >Should it be a template on arbitrary types, which can just say "not
>  >>  >implemented" if you ask for atomic 276 byte structs with overloaded
>  >>  >assignment?  Do we agree that the operations should be (op,
>  >>  >ordering_constraint) pairs?  Is the ordering_constraint a template
>  >>  >parameter?  What about optional hardware-dependent functionality?
>  >> 
>  >> All good questions.  My current thinking is to keep it simple, e.g. a
>  >> few lock objects, an atomic integer with atomic += operations, a
>  >> compare-and-swap operation.
>  ><snip>
>  >
>  >With respect, I think this is naive.  Where available, atomic += may be
>  >highly constrained; for example on IA64 it can add or subtract 1, 4, 8
>  >or 16.  CAS isn't universally available and on the ARM architecture it
>  >probably still won't be commonly available in 2009 without OS support.
>  >If I'm not mistaken, most lockless algorithms require ordering
>  >constraints.
> 
> Implicitly, I was assuming that the ability to write lockless algorithms
> was a goal for the work.

Exactly, and I don't think a "simple" interface will support that well.
Maybe I misunderstood what you said, but it sounded like you were
advocating that a common set of atomic operations should be required
across all implementations (or all that support multiple threads).  I
believe that would result in the standard being ignored either by
implementers, because they can't practically support the whole set on
some target hardware, or by users, because they find the set inadequate
for high performance on multiprocessor systems, or even both groups.  If
you already agree that the availability of some operations could be
implementation-defined, ignore me.

Ben.

-- 
Ben Hutchings
Time is nature's way of making sure that everything doesn't happen at once.