[plt-scheme] getting better speed with JIT

On Dec  2, Jens Axel Soegaard wrote:
> Prabhakar Ragde skrev:
> > Will Clinger recently posted in comp.lang.scheme on the speed of
> > various Scheme implementations, and mentioned in passing that one
> > has to (require mzscheme) to get the JIT to work.

To clarify -- the JIT is always working; it's just that it can do a
better job as Jens described below.  Also, even on platforms where the
JIT is unavailable (sparc-solaris), the require line will speed things
up.


> [...]
> However, since it is illegal to mutate a module-required
> variable, the compiler can safely inline the + in:
> 
>    (require mzscheme)
>    (define (foo a b) (+ a b))
> 
> There are no problems with module based programs.

And this is related to what Matthew said -- in the above, `+' cannot
change so the compiler can optimize it, but `foo' is not immutable.
This can make a significant difference too.  For example (on my
machine),

  (define (loop n) (when (> n 0) (loop (- n 1))))
  (time (loop 1000000000))
  ;; -> cpu time: 41489 real time: 41496 gc time: 0

And when you (require mzscheme):

  (require mzscheme)
  (define (loop n) (when (> n 0) (loop (- n 1))))
  (time (loop 1000000000))
  ;; -> cpu time: 7743 real time: 7745 gc time: 0

Dropping the whole thing into a module makes it even better:

  (module foo mzscheme
    (define (loop n) (when (> n 0) (loop (- n 1))))
    (time (loop 1000000000)))
  (require foo)
  ;; -> cpu time: 3446 real time: 3447 gc time: 0

because it's similar to doing this:

  (require mzscheme)
  (let ()
    (define (loop n) (when (> n 0) (loop (- n 1))))
    (time (loop 1000000000)))
  ;; -> cpu time: 2958 real time: 2960 gc time: 0

IIUC, that trick is in common use in Larceny.  I don't really know why
it's faster than the module code -- probably because `foo' can be
folded to a simple loop instead of having a function entry point.

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!