[Haskell-cafe] Problems with strictness analysis?

[Lennart Augustsson]

I tried your example in GHC 6.10 and isum appears to work fine.
The type of 10000000 gets defaulted to Integer, a specialized version
of isum for Integer is then created, the strictness analyzer
determines that isum is strict in s, and the code generator produces a
loop.  (If you want to look at the assembly code, it's easier if you
make the type Int.)
Ghc 6.8 had a bug (if I remember right) so when defaulting was
involved it didn't always optimize things properly.

  -- Lennart

On Mon, Nov 3, 2008 at 12:31 PM, Patai Gergely
<patai_gergely at fastmail.fm> wrote:
> Hi everyone,
>
> I was experimenting with simple accumulator functions, and found that an
> apparently tail recursive function can easily fill the stack. Playing
> around with ghc and jhc gave consistently unpleasant results. Look at
> this program:
>
> %%%%%%%%%%%
>
> -- ghc: no, ghc -O3: yes, jhc: no
> isum 0 s = s
> isum n s = isum (n-1) (s+n)
>
> -- ghc: no, ghc -O3: no, jhc: yes (because gcc -O3 is clever)
> rsum 0 = 0
> rsum n = n + rsum (n-1)
>
> main = case isum 10000000 0 {- rsum 10000000 -} of
>         0 -> print 0
>         x -> print x
>
> %%%%%%%%%%%
>
> I would expect the analysis to find out that the result of the function
> is needed in every case (since we are matching a pattern to it), yet I
> need to add a $! to the recursive call of isum to get a truly iterative
> function. It's interesting how ghc and jhc seem to diverge, one
> favouring the "iterative" version, the other the "recursive" one
> (although it only works because gcc figures out that the function can be
> expressed in an iterative way).
>
> Of course this is a real problem when I'm trying to write an actual
> program, since it means I can be bitten by the laziness bug any time...
> Is there any solution besides adding strictness annotations? Can I
> expect the compiler to handle this situation better in the foreseeable
> future?
>
> Gergely
>
> --
> http://www.fastmail.fm - IMAP accessible web-mail
>
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