#4474: 3 ways to write a function (unexpected performance difference and
regression)
---------------------------------+------------------------------------------
Reporter: claus | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.1 | Keywords:
Testcase: | Blockedby:
Os: Windows | Blocking:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
---------------------------------+------------------------------------------
Consider the attached `comparison.hs`, a small performance test for
difference lists. I was somewhat surprised by the performance differences
between `flatListCons` and `flatDList`, and I tried to reproduce the issue
by writing 3 equivalent versions of `flatListCons`.
{{{
flatListCons t = flat t []
where
flat (Leaf n) ns = n:ns
flat (Fork a b) ns = flat a (flat b ns)
flatListCons2 t = flat t []
where
flat (Leaf n) = \ns -> n:ns
flat (Fork a b) = \ns -> flat a (flat b ns)
flatListCons3 t = flat t []
where
flat (Leaf n) = (n:)
flat (Fork a b) = flat a . flat b
}}}
Not only does GHC not give equal performance for the 3 versions (factor of
2), but GHC-6.12.3 and GHC-7.1.20101022 differ in optimizations. With
GHC-6.12.3, `flatListCons` and `flatListCons2` are fast, `flatListCons3`
is slow. With GHC-7.1.20101022, only `flatListCons` is fast,
`flatListCons2` and `flatListCons3` are slow:
{{{
$ time ./comparison-6.12.3.exe c 26
67108864
real 0m4.574s
user 0m0.015s
sys 0m0.358s
$ time ./comparison-6.12.3.exe c2 26
67108864
real 0m4.442s
user 0m0.046s
sys 0m0.312s
$ time ./comparison-6.12.3.exe c3 26
67108864
real 0m10.694s
user 0m0.046s
sys 0m0.328s
$ time ./comparison-7.1.20101022.exe c 26
67108864
real 0m4.473s
user 0m0.046s
sys 0m0.327s
$ time ./comparison-7.1.20101022.exe c2 26
67108864
real 0m10.791s
user 0m0.046s
sys 0m0.327s
$ time ./comparison-7.1.20101022.exe c3 26
67108864
real 0m10.729s
user 0m0.078s
sys 0m0.280s
}}}
Ideally, I'd like all three versions (as well as `flatDList`) to be
equally fast.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4474>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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