[racket-dev] [plt] Push #28817: master branch updated

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Thu May 29 01:19:50 EDT 2014

Ok, I see. I'll revise my comment to "this would be better done with a
more general form of type inference", leaving out the claim of where
that inference should live.

I don't currently know how to do it other than building inference into
the complier. Matthias's plug-in rules sounds like a point that we hope
to eventually reach through macros as a compiler API.

On Sam's general question, I agree that there's no simple answer.

Some languages/libraries will provide particular optimizations that are
made possible by syntactic constraints. A type system is a particularly
fancy syntactic constraint, and it can offer particularly fancy
optimizations (such as splitting complex numbers).

Syntactic constraints are the reason to have multiple languages and a
choice, instead of just one language and compiler. I suppose a single
compiler could try several languages and find the one that a program
matches syntactically, but often the constraints are complex enough
that programs won't fit without careful attention. In that case, a
programmer knows (and can declare, and would really prefer to declare
and get feedback on) the restricted form that they intend to use for a

Meanwhile, we have a lot of code in plain Racket. Optimizing by hand is
so painful that even writing more C code (for the current optimizer)
seems like a better trade-off than hand-optimization. I imagine that
the PR was provoked by actual code somewhere. When the compiler is
finally itself implemented in Racket, the balance should shift even
further toward optimizations for plain Racket, whether or not we find
better a macro API for optimizations.

At Wed, 28 May 2014 19:50:50 -0700, Eric Dobson wrote:
> I don't think that TR should provide the majority of the optimizations
> in its current form because it has to run before inlining, and this
> limits what it can do.
> Here is an example program:
> #lang typed/racket
> (: my-sequence-map
>    (All (A B)
>      (case->
>        ((A -> B) (Vectorof A) -> (Vectorof B))
>        ((A -> B) (Listof A) -> (Listof B)))))
> (define (my-sequence-map f s)
>   (if (vector? s)
>       (vector-map f s)
>       (map f s)))
> (my-sequence-map add1 (vector 1 2 3))
> (my-sequence-map add1 (list 1 2 3))
> I would like this to be optimized to:
> (vector-map add1 (vector 1 2 3))
> (map add1 (list 1 2 3))
> I think this case of code will be very common if we move to a world
> where we work over generic sequences/datastructures, and specializing
> the call sites will be a big win.
> TR cannot do this optimization because it requires inlining. And the
> current version of racket cannot optimize this either because it
> becomes
> (let ((s (vector 1 2 3)))
>   (if (vector? s)
>       (vector-map add1 s)
>       (map add1 s)))
> Which isn't optimized because when we see (vector? s) we don't know
> that s is a vector as Mathew's change only works if the constructor is
> inline (i.e. of the form (vector? (vector 1 2 3))). Cases like this
> make me think that we need something stronger than context free
> rewrite rules over the ast/bytecode.
> On Wed, May 28, 2014 at 6:36 PM, Matthias Felleisen
> <matthias at ccs.neu.edu> wrote:
> >
> > Perhaps the right answer is to organize the optimizer
> > as a rewriting engine to which other devs can add rules
> > as they discover them (and their absence in the existing
> > rule set). -- Indeed, one could then even have programmers
> > extend the rule set for a specific program (though then
> > we have to worry about soundness). With syntax-* we should
> > have no problem formulating the mostly context-free rules
> > and we could figure out in addition how to keep track of
> > contexts. (This is the other half of what we used to call
> > the 'open compiler' idea at Rice.)
> >
> > -- Matthias
> >
> >
> >
> >
> > On May 28, 2014, at 9:25 PM, Sam Tobin-Hochstadt wrote:
> >
> >> On Thu, May 29, 2014 at 4:26 AM,  <mflatt at racket-lang.org> wrote:
> >>>
> >>> | optimizer: ad hoc optimization of predicates applied to constructions
> >>> |
> >>> | This is probably more of a job for Typed Racket, but maybe it's
> >>> | useful to detect some obviously unnecessary allocations of lists, etc.
> >>
> >> I think this is a useful discussion to have. I think there are two
> >> questions to answer:
> >>
> >> 1. Do we want people to need to use a particular language for greater
> >> optimization, whether that's Typed Racket or some other optimizer?
> >>
> >> 2. How should we optimize the code that Typed Racket depends on?
> >> Since this is a finite amount, we could manually do this, but we might
> >> not want to.
> >>
> >> Of course, in the absence of other constraints, it would be great to
> >> have infinite optimizations at every level. But in our actual setting,
> >> I don't know what I think the answer to either of these questions is.
> >>
> >> Sam
> >> _________________________
> >>  Racket Developers list:
> >>  http://lists.racket-lang.org/dev
> >

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