Why write out-of-line PrimOps in Cmm?
marlowsd at gmail.com
Thu Feb 14 13:32:50 CET 2013
On 14/02/13 11:55, Alexander Kjeldaas wrote:
> I was looking at the eventlog code, and I wanted to move processing of a
> full eventlog buffer into Haskell, instead of the now fixed behavior of
> writing the data to a file.
> To do this, I though that having a haskell process blocked on an MVar,
> or Chan would be nice, and then some way to signal the process.
> But the low-level PrimOps for MVars, takeMVar, tryTakeMVar etc are in
> PrimOps.cmm and I don't know how to call them from the RTS.
You can't call them directly. The problem is that to call Haskell code
you need a Haskell thread to run it in.
There are a couple of ways that we call Haskell code from the RTS:
- rts_lock()/rts_evalIO()/rts_unlock(). This is what calling a
foreign export does, and it's quite heavyweight.
- create a new Haskell thread and add it to the run queue, like we do
> This lead me to question what the point of out-of-line PrimOps in
> PrimOps.cmm is. I don't think the commentary covers this.
> So why aren't all the stuff in PrimOps.cmm just "ccall" wrappers around
> C implementations? Wouldn't that in general be more flexible for the RTS?
They need to do things like block, which you can't do from a ccall.
There's also the overhead - a primop can allocate memory directly from
the nursery, whereas a ccall would have to call allocate() (and couldn't
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