newtypes and optimization

[Scott Dillard]

You can find an example at:

graphics.cs.ucdavis.edu/~sdillard/newtype-maybe-bug.tar.gz

Here's my session with it:

ghc --make -O2 -DFAST Test
time ./Test +RTS -tstderr

<<ghc: 21120 bytes, 2 GCs, 32768/32768 avg/max bytes residency (1
samples), 1M in use, 0.00 INIT (0.00 elapsed), 7.52 MUT (8.70
elapsed), 0.00 GC (0.00 elapsed) :ghc>>

real	0m8.827s
user	0m7.520s
sys	0m1.224s



ghc --make -O2 -DSLOW Test -no-recomp
time ./Test +RTS -tstderr

<Ctrl-C>

Test: interrupted
<<ghc: 13476104656 bytes, 25762 GCs, 36864/36864 avg/max bytes
residency (1 samples), 1M in use, 0.00 INIT (0.00 elapsed), 91.19 MUT
(92.85 elapsed), 0.22 GC (0.36 elapsed) :ghc>>

real	1m33.232s
user	1m31.410s
sys	0m1.020s


If you do

  ghc -c -O2 -DFAST -ddump-simpl Test.hs | grep VecMath.V

you won't find any occurrences/pattern matches on the constructor, but with

  ghc -c -O2 -DSLOW -ddump-simpl Test.hs | grep VecMath.V

you'll see lots, due to a call to peek that is not inlined.

This is with ghc-6.8.1 and ghc-6.8.2 (no difference)


Thanks for any light you can shed on this,
Scott




On Dec 12, 2007 1:48 PM, Don Stewart <dons at galois.com> wrote:
> stefanor:
>
> > On Wed, Dec 12, 2007 at 11:02:15AM -0700, Scott Dillard wrote:
> > > with strictness annotations and INLINEs for everything. I also tried automatic
> > > newtype deriving, with no luck. Why does a newtype defeat so much of the
> > > optimization?
> > >
> > > Thanks,
> > > Scott
> >
> > (Not a GHC developer, but someone fairly familiar with how the Simons
> > work)
> >
> > What version of GHC are you using?  The implementation of newtypes was
> > completely redone in the 6.7.x period.
> >
> > Do you have a fairly small complete working example?  If so, link to or
> > attach a tarball - will make their jobs much easier.
> >
> > Stefan
>
> Yeah, this sounds like maybe a bug, or maybe something wrong. We need to
> investigate! An example please.
>
> -- Don
>