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 <[email protected]> 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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