Adding atomic primops

[Ryan Newton]

Oh, just the first CAS primop -- the initial decision to include these
kinds of ops.

Sent from my phone.
On May 5, 2014 12:59 AM, "Carter Schonwald" <carter.schonwald at gmail.com>
wrote:

> what sailed in ghc 7.2?
>
>
> On Mon, May 5, 2014 at 12:46 AM, Ryan Newton <rrnewton at gmail.com> wrote:
>
>> 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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>
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