[Haskell-cafe] Re: What I learned from my first serious attempt low-level Haskell programming

[Simon Marlow]

Stefan O'Rear wrote:
 > 2. Parameters are very expensive.  Our type of functions that build
 >    (ignoring CPS for the time being) was MBA# -> Int# -> [ByteString],
 >    where the Int# is the current write pointer.  Adding an extra Int#
 >    to cache the size of the array (rather than calling sMBA# each
 >    time) slowed the code down ~2x.  Conversely, moving the write
 >    pointer into the byte array (storing it in bytes 0#, 1#, 2#, and
 >    3#) sped the code by 4x.

If you were measuring on x86 then parameters are passed on the stack, which may 
be expensive.  On x86_64 the first 3 arguments are passed in registers, which is 
usually a win, but if the function immediately does an eval they need to be 
saved on the stack anyway.  Still, 4x sounds like a lot, perhaps you managed to 
avoid a stack check in the inner loop or something.

 > 3. MBA# is just as fast as Addr#, and garbage collected to boot.

Not really surprising, that.

 > 4. You can't keep track of which version of the code is which, what is
 >    a regression, and what is an enhancement.  Don't even try.  Next
 >    time I try something like this I will make as much use of darcs as
 >    possible.

Absolutely - if you'd used darcs, then we could peer in more detail at changes 
that you thought gave counter-intuitive results.

Simon Peyton-Jones wrote:
> | 5. State# threads clog the optimizer quite effectively.  Replacing
> |    st(n-1)# with realWorld# everywhere I could count on data
> |    dependencies to do the same job doubled performance.
> 
> The idea is that the optimiser should allow you to write at a high level, and do the book keeping for you.  When it doesn't, I like to know, and preferably fix.
> 
> If you had a moment to boil out a small, reproducible example of this kind of optimisation failure (with as few dependencies as poss), then I'll look to see if the optimiser can be cleverer.

Yes, and *please* add some of this folklore to the performance wiki at 
http://haskell.org/haskellwiki/Performance, if you have the time.

Cheers,
	Simon