On Sat, 2009-05-23 at 20:42 -0400, Mario Blažević wrote:
> On Sat 23/05/09 2:51 PM , Duncan Coutts duncan.cou...@worc.ox.ac.uk sent:
> > On Sat, 2009-05-23 at 13:31 -0400, Mario Blažević wrote:
> > ...
> >> So the function is not strict, and I don't understand
> >> why GHC should evaluate the arguments before the call.
> >
> > Right, it's lazy in the first and strict in the second argument. As far
> > as I can see we have no evidence that is is evaluating anything before
> > the call.
>
>
> When I look at the Core definition of `test', it begins with
>
>
> \ (n1axl::integer:GHCziIntegerziInternals.Integer)
> (n2axn::integer:GHCziIntegerziInternals.Integer) ->
> %let as1sU :: integer:GHCziIntegerziInternals.Integer =
> base:DataziList.prod1
> (main:Main.factors2
> (base:DataziList.prod1
> (base:GHCziNum.upzulist main:Main.lvl main:Main.lvl n1axl)
> base:DataziList.lvl1))
> base:DataziList.lvl1
> %in %case integer:GHCziIntegerziInternals.Integer
> (ghczmprim:GHCziPrim.parzh
> @
> integer:GHCziIntegerziInternals.Integer
> as1sU)
> %of (dsapq::ghczmprim:GHCziPrim.Intzh)
I recommend using -ddump-simpl, as it produces more readable output.
> To my untrained eyes, this looks like it's evaluating
>
> > product $ factors $ product [1..n1])
>
> which is the first argument to `parallelize'.
That core code is doing:
let blah = product $ factors $ product thelist
in case par# blah of
_ -> ...
So it's calling the primitive par# but nothing is forced to WHNF (except
the unused dummy return value of par#).
> I assume that %case in Core evaluates the argument to WHNF, just like
> case in Haskell.
Yes.
(In fact case in core always reduces to WHNF, where as in Haskell case
(...) of bar -> (...) does not force anything.)
> Then again, I could be completely misinterpreting what Core is, because
> I can't find any call to `parallelize' before or after that. It appears
> to be inlined in Core, regardless of whether the pragma
>
> > {-# INLINE parallelize #-}
>
> is there or not.
Yes, because even without the pragma, ghc decides it's profitable to
inline.
> Actually, I can't see any effect of that pragma in the
> core files whatsoever, but it certainly has effect on run time.
How about diffing the whole core output (and using -ddump-simpl). If
there's a performance difference then there must be a difference in the
core code too.
> Or do you mean to say that *your* installation of GHC behaves
> the same when the function `parallelize' is defined in the same module and
> when
> it's imported?
Yes, exactly. With both ghc-6.10.1 and a very recent build of ghc-6.11
Duncan
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
