[MLton] Vector.fromList[] performance
Matthew Fluet
fluet at cs.cornell.edu
Tue Oct 24 13:30:26 PDT 2006
> I have a largeish program that uses vectors a lot: it's a geometry
> code that is written in arbitrary-dimensional fashion. Some of the
> code is in special dimension, so I often find myself converting tuples
> (x,y) into vectors using Vector.fromList[x,y].
>
> Unfortunately, the performance of this utterly sucks: I don't know how
> to read the generated bytecode to see exactly what's happening, but I
> can't fathom how it could be so bad without paying for building the
> list, then converting it to a vector.
Well, building the list and then converting it to a vector is exactly what
it does, since that is what 'Vector.fromList [x,y]' does.
> How hard would it be to special-case Vector.fromList on a literal
> list?
It's not trivial. 'Vector.fromList' is implemented in SML, so the
compiler doesn't treat it specially. Trying to identify exactly this code
would be pretty tough.
On the other hand, a loop-unrolling optimization would likely help both
this idom and other code.
> SML/NJ allows this using the #[] construct, which helps SML/NJ
> on this test program knock the pants off mlton. But really the
> compiler should just handle it automatically so that I can write
> standard-compliant code.
Right. You could probably use MLton.Vector.unfoldi or the even more
powerful MLton.Vector.create (in SVN) to build the vector directly, but it
suffers from the same portability problem.
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