Best way to get to CAS/fetch-and-add on unboxed vectors?

[Ryan Newton]

We do expose CAS for boxed data, and we depend on that for various
concurrent data structures (e.g. Michael-scott queues).

But it's a messy business:

<explanation> You have to fight hard against the GHC optimizer to prevent
it from performing optimizations (e.g. unbox and rebox an Int) that
completely destroy pointer equality (in all runs of the program).  The way
that we've codified our solution to this is to use abstract "Ticket"s
(based on the Any kind) that GHC is not supposed to be able to see through.
 These are used to abstractly represent old reads when you come back to do
a new CAS operation:


http://hackage.haskell.org/package/atomic-primops-0.4/docs/Data-Atomics.html#g:2

Even then, there was a bunch of careful NOINLINE's and other business
necessary to achieve something workable, and it's highly GHC specific.  If
we were to expose CAS on boxed "Data.Vector", I think it would only make
sense as the ticketed interface above (raw CAS is useless, and will just
condemn others to the same problems we had).
</explanation>

As a result of all this the CAS for Data.Vector would have a different
signature (unless we wanted to force Storable/Unbox to use Tickets), and
thus couldn't go in the "MVector" type class along with all the other basic
operations that are uniform across representations.

That said, it's easy to provide CAS from a separate "vector-atomics"
package, because Data.Vector exposes the MVector constructor that lets you
get at the underlying MutableArray.  So we certainly can provide the
functionality somewhere, somehow.

  -Ryan



On Wed, Nov 20, 2013 at 10:21 AM, Jan-Willem Maessen
<jmaessen at alum.mit.edu>wrote:

> On Wed, Nov 20, 2013 at 10:06 AM, Ryan Newton <rrnewton at gmail.com> wrote:
>
>> (3) The most invasive (but best for the user) change would be to extend
>> the basic MVector interface to include notions of concurrency-related
>> operations right inside the vector package (CAS, etc).  But they should
>> still probably go in a separate class (not class MVector), just because
>> they will be specific to Unboxed and Storable vectors rather than boxed
>> ones....
>>
>
> Any reason we shouldn't be able to CAS on boxed vectors?  Every
> architecture supports a pointer-sized CAS.  What "equality" means for CAS,
> of course, has to be rather carefully defined, but that's equally true for
> the unboxed types.
>
> -Jan
>
>
>>
>>   -Ryan
>>
>>
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