I know that in the case of the prefetch operations they need to be treated
like writes so that other operations don't get reordered before / after
them.
For Mvars, concurrency probably comes into play. Or at least, before we
did a readMvar primop it was made out of a take and put, so even through
the read prim doesn't write and doesn't block other readers, we still want
to treat it's ordering as through it has a write like side effect (Edward
yang can probably better opine than I on that one )
On Fri, Mar 10, 2017 at 5:17 PM David Feuer
Note [PrimOp can_fail and has_side_effects] in prelude/PrimOp.hs says
A primop "has_side_effects" if it has some *write* effect, visible elsewhere
- writing to the world (I/O) - writing to a mutable data structure (writeIORef) - throwing a synchronous Haskell exception
[...]
* NB3: *Read* effects (like reading an IORef) don't count here,
because it doesn't matter if we don't do them, or do them more than once. *Sequencing* is maintained by the data dependency of the state token.
But this does not actually seem to match what goes on in primops.txt.pp. The following, among many other seemingly read-only operations, have has_side_effects = True:
readMutVar# (the very example cited!), readArray#, unsafeFreezeArray#, unsafeThawArray#, tryReadMVar#, deRefWeak#
So what's the correct story? Do we want to change the note, or change the reality? The reason I happen to be looking at this is that I think the current arrangement allows us to define unsafeInterleaveIO in a particularly simple fashion:
unsafeInterleaveIO = pure . unsafePerformIO
but that's only safe as long as the interleaved IO won't float out and get performed before it's forced by normal IO.
But the unsafeInterleaveIO story seems much less important, in the grand scheme of things, than making everything else run fast. If indeed it's otherwise safe to mark these read-only ops has_side_effects=False, then I imagine we probably should do that.
David Feuer _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs