cross module optimization issues

[Neil Mitchell]

Hi

I've talked to John a bit, and discussed test cases etc. I've tracked
this down a little way.

Given the attached file, compiling witih SHORT_EXPORT_LIST makes the
code go _slower_. By exporting the "print_lines" function the code
doubles in speed. This runs against everything I was expecting, and
that Simon has described.

Taking a look at the .hi files for the two alternatives, there are two
differences:

1) In the faster .hi file, the body of print_lines is exported. This
is reasonable and expected.

2) In the faster .hi file, there are additional specialisations, which
seemingly have little/nothing to do with print_lines, but are omitted
if it is not exported:

"SPEC >>= [GHC.IOBase.IO]" ALWAYS forall @ el
                                         $dMonad :: GHC.Base.Monad GHC.IOBase.IO
  Sound.IterateeM.>>= @ GHC.IOBase.IO @ el $dMonad
  = Sound.IterateeM.a
      `cast`
    (forall el1 a b.
     Sound.IterateeM.IterateeGM el1 GHC.IOBase.IO a
     -> (a -> Sound.IterateeM.IterateeGM el1 GHC.IOBase.IO b)
     -> trans
            (sym ((GHC.IOBase.:CoIO)
                      (Sound.IterateeM.IterateeG el1 GHC.IOBase.IO b)))
            (sym ((Sound.IterateeM.:CoIterateeGM) el1 GHC.IOBase.IO b)))
      @ el
"SPEC Sound.IterateeM.$f2 [GHC.IOBase.IO]" ALWAYS forall @ el
                                                         $dMonad ::
GHC.Base.Monad GHC.IOBase.IO
  Sound.IterateeM.$f2 @ GHC.IOBase.IO @ el $dMonad
  = Sound.IterateeM.$s$f2 @ el
"SPEC Sound.IterateeM.$f2 [GHC.IOBase.IO]" ALWAYS forall @ el
                                                         $dMonad ::
GHC.Base.Monad GHC.IOBase.IO
  Sound.IterateeM.$f2 @ GHC.IOBase.IO @ el $dMonad
  = Sound.IterateeM.$s$f21 @ el
"SPEC Sound.IterateeM.liftI [GHC.IOBase.IO]" ALWAYS forall @ el
                                                           @ a
                                                           $dMonad ::
GHC.Base.Monad GHC.IOBase.IO
  Sound.IterateeM.liftI @ GHC.IOBase.IO @ el @ a $dMonad
  = Sound.IterateeM.$sliftI @ el @ a
"SPEC return [GHC.IOBase.IO]" ALWAYS forall @ el
                                            $dMonad :: GHC.Base.Monad
GHC.IOBase.IO
  Sound.IterateeM.return @ GHC.IOBase.IO @ el $dMonad
  = Sound.IterateeM.a7
      `cast`
    (forall el1 a.
     a
     -> trans
            (sym ((GHC.IOBase.:CoIO)
                      (Sound.IterateeM.IterateeG el1 GHC.IOBase.IO a)))
            (sym ((Sound.IterateeM.:CoIterateeGM) el1 GHC.IOBase.IO a)))
      @ el

My guess is that these cause the slowdown - but is there any reason
that print_lines not being exported should cause them to be omitted?

All these tests were run on GHC 6.10.1 with -O2.

Thanks

Neil


On Fri, Nov 21, 2008 at 10:33 AM, Simon Peyton-Jones
<simonpj at microsoft.com> wrote:
> | This project is based on Oleg's Iteratee code; I started using his
> | IterateeM.hs and Enumerator.hs files and added my own stuff to
> | Enumerator.hs (thanks Oleg, great work as always).  When I started
> | cleaning up by moving my functions from Enumerator.hs to MyEnum.hs, my
> | minimal test case increased from 19s to 43s.
> |
> | I've found two factors that contributed.  When I was cleaning up, I
> | also removed a bunch of unused functions from IterateeM.hs (some of
> | the test functions and functions specific to his running example of
> | HTTP encoding).  When I added those functions back in, and added
> | INLINE pragmas to the exported functions in MyEnum.hs, I got the
> | performance back.
> |
> | In general I hadn't added export lists to the modules yet, so all
> | functions should have been exported.
>
> I'm totally snowed under with backlog from my recent absence, so I can't look at this myself, but if anyone else wants to I'd be happy to support with advice and suggestions.
>
> In general, having an explicit export list is good for performance. I typed an extra section in the GHC performance resource http://haskell.org/haskellwiki/Performance/GHC to explain why.  In general that page is where we should document user advice for performance in GHC.
>
> I can't explain why *adding* unused functions would change performance though!
>
> Simon
>
>
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>
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