[plt-scheme] Statistics for Sequences

When you use `in-vector', the expansion of `for' uses `vector-ref' etc.
directly, so those operations can be inlined by the JIT.

When you use a generic sequence, the expansion of `for' extracts a set
of procedures from the sequence. The compiler can't tell that the
extracted procedures will sometimes be `vector-ref' etc.

At Thu, 10 Sep 2009 09:03:24 -0600, Doug Williams wrote:
> Actually, I'd like to dig into the sequence code (specifically,
> sequence-generate) that should give you the sequencing function dynamically.
> I'm not sure why it isn't as efficient as anything the for macro can
> generate for vectors.
> 
> On Thu, Sep 10, 2009 at 8:47 AM, Jakub Piotr Cłapa <jpc-ml at zenburn.net>wrote:
> 
> > On 09-09-10 16:30, Doug Williams wrote:
> >
> >> It's interesting that if I use (in-vector ...) in the for/fold
> >> statements, the times for the for/fold version are about the same as for
> >> the (uglier) do version (with vector-refs). [This one probably would
> >> benefit from Matthew's performance improvements.] Actually using it
> >> would mean giving up the flexibility in going to sequences in the first
> >> place, but it means there is some hope of eventually getting the same
> >> performance for the sequence versions (at least for vectors).
> >>
> >> using in-vector in the for
> >> cpu time: 266 real time: 265 gc time: 0
> >> cpu time: 250 real time: 250 gc time: 47
> >>
> >> current science collection routines
> >> cpu time: 250 real time: 249 gc time: 0
> >> cpu time: 218 real time: 218 gc time: 16
> >>
> >> It would be nice if (for ((x some-vector)) ...) and (for ((x (in-vector
> >> some-vector))) ...) had similar performance. I realize that at expansion
> >> time the latter knows to expect a vector while the former does not and
> >> can generate code accordingly. But, I can dream.
> >>
> >
> > AFAIU you could special case vectors (duplicating the code) if you expect
> > them to be used frequently. Probably a for-like macro expanding into
> > shortcuts for specified fast iterators would be nice to have. Something like
> >
> > (for ([x (in (list vector string) lst)])
> >  x)
> >
> > would expand to
> >
> > (cond
> >  [(list? x) (for ([x (in-list lst)]) x)]
> >  ...
> >  [else (for ([x lst]) x)])
> >
> > PS. And what about generating such special cases by evaling a dynamically
> > generated lambda at runtime? I guess it would make really long iterations
> > faster but the eval overhead would kill the performance for short ones?
> >
> > --
> > regards,
> > Jakub Piotr Cłapa
> >
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