[Haskell-cafe] Re: Why can't Haskell be faster?
stefanor at cox.net
Wed Oct 31 21:38:26 EDT 2007
On Wed, Oct 31, 2007 at 03:37:12PM +0000, Neil Mitchell wrote:
> I've been working on optimising Haskell for a little while
> (http://www-users.cs.york.ac.uk/~ndm/supero/), so here are my thoughts
> on this. The Clean and Haskell languages both reduce to pretty much
> the same Core language, with pretty much the same type system, once
> you get down to it - so I don't think the difference between the
> performance is a language thing, but it is a compiler thing. The
> uniqueness type stuff may give Clean a slight benefit, but I'm not
> sure how much they use that in their analyses.
> Both Clean and GHC do strictness analysis - I don't know which one
> does better, but both do quite well. I think Clean has some
> generalised fusion framework, while GHC relies on rules and short-cut
> deforestation. GHC goes through C-- to C or ASM, while Clean has been
> generating native code for a lot longer. GHC is based on the STG
> machine, while Clean is based on the ABC machine - not sure which is
> better, but there are differences there.
> My guess is that the native code generator in Clean beats GHC, which
> wouldn't be too surprising as GHC is currently rewriting its CPS and
> Register Allocator to produce better native code.
I don't think the register allocater is being rewritten so much as it is
stefan at stefans:/tmp$ cat X.hs
module X where
memset :: Ptr Int32 -> Int32 -> Int -> IO ()
memset p v i = p `seq` v `seq` case i of
0 -> return ()
_ -> poke p v >> memset (p `plusPtr` sizeOf v) v (i - 1)
stefan at stefans:/tmp$ ghc -fbang-patterns -O2 -c -fforce-recomp -ddump-asm X.hs
Admittedly that's better than it used to be (I recall 13 memory
references last time I tested it), but still... the reason for your
performance woes should be quite obvious in that snippet.
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