[Haskell-cafe] FFI: Overhead of foreign unsafe imports

[Jason Dagit]

On Sun, Feb 26, 2012 at 10:36 AM, Yves Parès <yves.pares at gmail.com> wrote:
> Hello,
> When I was using C code from Python, the overhead put on calling C code by
> Python was significant.
> To simplify, say I have on C-side two procedures f and g, I do all the stuff
> to call them in a row from Python, well I'm better off factorizing: adding
> on C side a wrapper h procedure that calls them both, and then call h from
> Python, because then I will have half as much overhead:
>
> Instead of SwitchToC -> call f -> SwitchToPython -> SwitchToC -> call g ->
> SwitchToPython,
> the factorization leads to SwitchToC -> call f -> call g -> SwitchToPython,
> which gives the same result yet is different performance-wise because each
> switching has a cost.
>
> This is painful, because if another time I have to call f and j (another
> function), then I have to make another wrapper.
>
> In Haskell world, now, given that my functions f and g would have been
> imported using unsafe:
>
> foreign import unsafe "f" f :: Thing -> Foo -> IO ()
> foreign import unsafe "g" g :: Stuff -> Bar -> IO ()
> foreign import unsafe "h" h :: Thing -> Foo -> Stuff -> Bar -> IO ()
>
> Are
> doStuff = f x y >> g z w
> and
> doStuff = h x y z w
> equivalent, or is there an overhead (e.g. due to IO monad, or due to the way
> the FFI does the calls) when compiled (if need be with optimizations) with
> GHC?

I would expect the second doStuff to be more efficient, but you
shouldn't care what I expect! To check what is going on you can use a
tool like ghc-core (available on hackage) to see what code GHC is
generating. To see the difference in performance you can use a micro
benchmark tool like criterion (also on hackage) to quantify
performance differences.

Once you have a conclusion based on some good evidence, please report
back here :)

I hope that helps,
Jason