[Haskell-cafe] Re: idea for avoiding temporaries

Simon Marlow simonmarhaskell at gmail.com
Fri Mar 9 11:44:46 EST 2007

Claus Reinke wrote:
>> i never quite understood why DiffArrays are so slow at the moment, so 
>> perhaps this is a good opportunity to ask for enlightenment on this 
>> issue?-)
> it seems i should qualify that. some simple-minded testing shows that 
> DiffArrays do
> become faster than Arrays if the arrays get big enough. for small 
> arrays, though, just copying the Array seems faster than maintaining the 
> DiffArray. copying then discarding old arrays can be cheaper than the 
> overhead associated with avoiding copies.

Looking at the implementation of DiffArrays, there are some obvious 
optimisations that aren't done.  UNPACKing everything in sight would be a good 
start.  I guess this code has never really had a performance evaluation done on 
it.  DiffUArray stores the array unboxed, but the difference elements are stored 
boxed, as far as I can tell.  That means extra unnecessary allocation for each 
update.  Also the indices are stored boxed - there's no reason to do that for 
either kind of DiffArray.  All the operations are wrapped in unsafePerformIO, 
which is far from optimal (unsafePerformIO is not inlined, we should use 
inlinePerformIO if possible).

Would someone like to shake this library into shape?  I bet there's a 
significant performance win to be had.


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