Proposal: provide cas and barriers symbols even without -threaded

[Ryan Newton]

Just to keep you all up to date...  I'm adding the primops in question and
validating the individual commits before putting them here:

    https://github.com/rrnewton/ghc/commits/atomicPrimOps

The basic idea for using these extensions is:

   - the atomic-primops library will work in 7.6 or 7.7+.  It will use
   ifdefs to decide whether to use its own primops or GHC-builtin
   - future versions will simply get faster, as Carter replaces out-of-line
   primops that *also* use C calls, with inline primops / LLVM equivalents

Shall I stick a patch on a ticket, or will someone volunteer to pull?
 What's the protocol for requesting commit access anyway?  (By the way, can
someone share the reason that pull-requests to the github ghc mirror are
such a no-no?  They seem no worse than a patch in an email which the
big warning
sign <https://github.com/ghc/ghc> recommends.)

Best,
  -Ryan

P.S. FYI, I'm periodically getting these:

    0 caused framework failures
    0 unexpected passes
    1 unexpected failures

     Unexpected failures:
perf/compiler  T1969 [stat not good enough] (normal)

Can that just be because of running on a loaded machine?  How narrow are
these windows?


On Thu, Aug 1, 2013 at 12:32 PM, Ryan Newton <rrnewton at gmail.com> wrote:

> On Sun, Jul 21, 2013 at 3:32 AM, Carter Schonwald <
> carter.schonwald at gmail.com> wrote:
>
>> ok, could you add those comments (about additional operations to
>> consider) to the ticket?
>>
>
> Sure.  Just did that.
>
>
>> relatedly: if we want these atomic ops to use the sequential analogues
>> when we're not using the threaded run time system, does that mean
>> we need to have a symbol / constant variable exposed in the RTS we link
>> in, so that the inline code branches on a linktime constant value / symbol
>> (something like "isThreadedRTS:: Bool", )  or some sort of analogue
>> thereof?
>>
>
> I think it will take some care to mimic the semantics perfectly.  Why not
> just leave the real atomic ops even in non-threaded mode, at least at
> first?  Later we can optimize it if we find that people are using
> concurrent data structures heavily in non-threaded mode ;-).
>
>
>> one nice thing about doing such, is that if at some point link time
>> optimization is added, the branch would go away! On the other hand, it
>> could be argued that the cost of the call to the CAS primops in their
>> current form isn't that much more expensive than such a branch.
>>
>
> Indeed, I'm much more concerned about performance in the threaded case and
> making sure they're correct.
>
>
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