Adding atomic primops

[Ryan Newton]

One last comment -- none of the above is to suggest that I don't think we
should eventually have a memory model (a la Java or C++11).  But I (and
Johan) don't think the addition of the primops Johan listed should wait on
it.  Further, I don't think these primops make the state of affairs any
worse, given that we've *already* had the combination of IORef operations &
parallel IO Threads for a long time, without a memory model.

I think the informal agreement we've been muddling along with is something
like this:

   - IORef operations have the same behavior as the analogous C operations
   -- no implied synchronization
   - all IORef ops are "volatile" wrt GHC (GHC won't reordered)
   - atomicModifyIORef does what its name implies

Though I confess, I'm personally unclear on what the agreement is in at
least two places:

   - What Haskell operations constitute grabbing a "lock" to protect IORef
   reads and writes?  (We often use MVar based strategies for locking, but do
   they give a *guarantee* that they provide the necessary memory fences
   for the previous/subsequent IORef operations?)
   - Is the de-facto "volatile" status I implied before extended to the
   backends (C / LLVM)?  I don't know but assume not.  Note that even if not,
   this doesn't cause a problem for the proposed atomic primops, all of which
   are themselves

Perhaps I and others get away with this level of murkiness because we
depend on IORefs so little, with so much happening in the pure code ;-).

Ah, and last of all -- while we do need to sort out all this stuff -- I
want to point out that adding Johan's proposed primops isn't the key
decision point.  That ship sailed with 7.2 ;-).  This is just about
fleshing out what's already there (e.g. fetch and Xor in addition to fetch
and Add) and improving the implementations by going to in-line primops.

Best,
  -Ryan


On Mon, May 5, 2014 at 12:25 AM, Ryan Newton <rrnewton at gmail.com> wrote:

> For Johan's primops to work, each primop must represent a full memory
>> fence that is respected both by the architecture, and by *both*compilers (GHC & LLVM).  Since I don't think GHC is a problem, let's talk
>> about LLVM.  We need to verify that LLVM understands not to float regular
>> loads and stores past one of its own atomic instructions.  If that is the
>> case (even without anything being marked "volatile"), then I think we are
>> in ok shape, right?
>>
>
> Clarification -- this is assuming we're using the "SequentiallyConsistent"
> setting in the LLVM backend to get full fences on each op, which correspond
> to the gcc-compatible __sync_* builtins:
>
>    http://llvm.org/docs/Atomics.html#sequentiallyconsistent
>
>
>
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