I think Racket handles this pretty well. The rule it defines is that a module cannot directly mutate another module's top-level symbols. This means that if a module itself does not contain any mutations of a top-level function, that definition cannot be changed at runtime. Therefore, everyone can rely on a simpler calling convention that doesn't require the extra level of indirection needed to handle changeable top-level symbols.
However, if the module contains a 'set!' or equivalent to a top-level symbol,
then references to that symbol will have to be loaded each time, since it is
possible for the value to change.
e.g.
#lang racket
(define (f1)
(println "f1"))
(define (f2)
(println "f2"))
;; The existence of the "set!" in this function means that 'f1' cannot be
directly called, within or without this module.
(define (alter)
(set! f1 f2))
(module+ main
(f1)
; (set! f1 f2) ;; Not allowed
(alter) ;; Allowed
(f2))
On 8/14/18, 7:51 AM, "Arthur Nunes-Harwitt" <racket-users@googlegroups.com on
behalf of a...@cs.rit.edu> wrote:
Hi,
Racket seems to depricate redefinition of functions at run-time. Since
Racket isn't Scheme, why not prevent that and get back that performance?
-Arthur
==============================================================
Arthur Nunes-Harwitt
Computer Science Department, Rochester Institute of Technology
Room 70-3509
585-475-4916
==============================================================
"I don't know what the language of the future will be
called, but it will look like LISP."
This email is confidential and intended for the named recipient(s). In
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please notify the sender at a...@cs.rit.edu.
On Mon, 13 Aug 2018, George Neuner wrote:
>
> On 8/13/2018 10:02 AM, 'Paulo Matos' via Racket Users wrote
>> On 11/08/18 19:41, Sam Tobin-Hochstadt wrote:
>> > There are basically two differences between the `unsafe-lsb` function
>> > in Racket and the C one:
>> > - the Racket calling convention vs the C calling convention
>> > - the instruction used to perform the LSB calculation
>> > > For a variety of reasons Racket's function calling convention is more
>> > heavyweight than C's, but if that code is inlined into some larger
>> > function that will go away (as it would in C). The simpler instruction
>> > used by the C compiler is because the C compiler recognizes that
>> > pattern of bitwise and, whereas the Racket JIT does the obvious `and`.
>> > That could of course be fixed in the JIT, although it's not clear how
>> > much of a win it is.
>> Is this something that will change with Racket-on-Chez? i.e. Racket will
>> then use the Chez optimizer to generate machine code?
>
> Racket's function call convention is heavier than C's because it has to
deal
> with the possibility that functions may be redefined at runtime, or may
be
> overloaded ... things C does not have to deal with. The same function
may
> have multiple entry points depending on number of arguments, or presence
of
> keywords, etc. And functions that may be redefined must be called
indirectly
> to avoid (re)patching every call site when/if the definition changes.
These
> things make (average) function calling in Racket slower than in C.
>
> It is expected that the Chez compiler will do a better job at optimizing
> code, but it can't overcome differences in call semantics. If you do
> something "schemely" with your functions, then you WILL pay the price for
it.
>
> George
>
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