I'm afraid I don't know off the top of my head but hopefully Simon Marlow will be able to shed some more light here.
When he does it would be great if you could write a nice comment explaining this logic and either open a Phabricator Diff or send the text to me so I can merge it.
Here is the barrier:
(h) = ccall cas(mv + SIZEOF_StgHeader + OFFSET_StgMutVar_var, x, y);
cas() is an external C function defined in includes/stg/SMP.h using inline assembly.
Wow, I really did miss it. Thanks for pointing it out. I also have a question about the sort of relaxed behaviours permitted. The Data.IORef docs say:
The implementation is required to ensure that reordering of memory operations cannot cause type-correct code to go wrong. In particular, when inspecting the value read from an IORef, the memory writes that created that value must have occurred from the point of view of the current thread.
but I'm not sure that's enough. This also seems clearly wrong, but is permitted by that restriction: λ> newIORef False >>= \ref -> writeIORef ref True >> readIORef ref False Is there a write-up anywhere of the memory model the GHC primitives give rise to? Or is it just one of those cases where the code is the specification? Thanks -- Michael Walker (http://www.barrucadu.co.uk)