[Haskell-cafe] Re: What I learned from my first serious attempt
low-level Haskell programming
lennart at augustsson.net
Thu Apr 5 15:10:11 EDT 2007
It's not that hard to figure out an order to permute the arguments on
the stack before a tail call that minimizes that number of moves and
temporary locations. Lmlc did this 20 years ago. :)
On Apr 5, 2007, at 19:17 , Claus Reinke wrote:
>> Stefan O'Rear wrote:
>> > 2. Parameters are very expensive. Our type of functions that build
>> > (ignoring CPS for the time being) was MBA# -> Int# ->
>> > where the Int# is the current write pointer. Adding an extra
>> > 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.
> just out of curiosity: what does GHC do with stack parameters in
> loops/tail calls?
> there tends to be a conflict as the old set of parameters is needed
> to build the new
> one for the recursive call/next loop iteration, while one wants to
> get rid of the old set before doing the call. unless one keeps the
> parameter frames away from the stack,
> or can figure out a safe order in which to overwrite the parameters
> in the frame,
> that seems to imply some copying, even though that may be cheap for
> parameters per frame.
> many years ago, i saw an abstract machine and RISC processor design
> aiming for good fp support that used two parameter stacks instead
> of one for just this reason.
> instead of moving stack frames around on a single stack, parameters
> were read
> from one stack, and built up on the other, followed by a cheap
> stack switch before
> the call. perhaps something like this might be of use here, too?
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