[Haskell-cafe] Haskell version of ray tracer code ismuchslowerthan the original ML

Claus Reinke claus.reinke at talk21.com
Sun Jun 24 10:53:20 EDT 2007

>>also try replacing that (foldl' intersect') with (foldr (flip intersect'))!
> OK, next question: Given that I'm using all the results from
> intersect', why is the lazy version better than the strict one? Is ghc
> managing to do some loop fusion?

haskell tends to prefer foldr where mls prefer foldl, be it for 
lazyness and short-circuiting operators, or because a tail-recursive
function with a lazy accumulator is only an efficient way to construct
inefficient expressions.

so, the very first thing i tend to look for when someone ports a 
program from ml to haskell are tail recursions with non-strict 
accumulators. even using foldl', when constructing pairs in the
accumulator, there's no guarantee that the pair components will 
be evaluated early even if the pairs themselves are forced. so
replacing foldl with foldr when porting from ml to haskell tends
to be a useful habit, unless there are good reasons for foldl.

however, things seem to be a little bit more involved in this 
example: intersect' forces the first component, and ignores
the second, never building nasty delayed combinations of old 
accumulator and list head in the new accumulator. but if you 
compare the outputs of -ddump-simpl, or if you export all 
definitions from main and compare the outputs of --show-iface, 
you'll find differences related to the the result of intersect': 
whether or not that result can be passed unboxed. 

    Constructed Product Result Analysis for Haskell (2000)

i don't know the details, but in the foldr case, the result of
intersect' seems to be passed unboxed, in the foldl' case, it 
isn't. i'll leave it to the experts to explain whether that has to
be the case or whether it is an omission in the optimizer.

>>using a recent ghc head instead of ghc-6.6.1 also seems to
>>make a drastic difference 

$ uname -a
CYGWIN_NT-5.1 cr3-lt 1.5.19(0.150/4/2) 2006-01-20 13:28 i686 Cygwin

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.6.1

$ gcc --version
gcc.exe (GCC) 3.4.2 (mingw-special)
Copyright (C) 2004 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO

$ ghc --make Ray1.hs
[1 of 1] Compiling Main             ( Ray1.hs, Ray1.o )
Linking Ray1.exe ...

$ time ./Ray1.exe >out

real    0m55.705s
user    0m0.015s
sys     0m0.031s

$ /cygdrive/c/fptools/ghc/ghc-6.7.20070613/bin/ghc --make Ray1.hs -o Ray1head.exe
[1 of 1] Compiling Main             ( Ray1.hs, Ray1.o )
Linking Ray1head.exe ...

$ time ./Ray1head.exe >out.head

real    0m24.989s
user    0m0.031s
sys     0m0.015s

$ diff -q --binary out out.head


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