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From: gjc@paradigm.com
Newsgroups: comp.lang.lisp
Subject: Re: FUNCALL question, fun facts
Message-ID: <26@paradigm.com>
Date: 29 Jan 90 19:11:35 GMT
References: <3277@accuvax.nwu.edu> <1990Jan28.175437.19293@hellgate.utah.edu>
Organization: Paradigm Associates Inc, Cambridge MA
Lines: 35

There used to be a saying, FUNCALL CONSIDERED HARMFULL..

In article <1990Jan28.175437.19293@hellgate.utah.edu>, sandra%defun.utah.edu@cs.utah.edu (Sandra J Loosemore) writes:
> In article <3277@accuvax.nwu.edu> krulwich@ils.nwu.edu (Bruce Krulwich) writes:
>>Rough experiments in Lucid CL (on a SPARCstation) indicate that there is
>>indeed twice the procedure-call overhead in calling a closure stored in a
>>lexically bound variable as in calling a global function (stored in a
>>symbol's function slot).
> 
> Many implementations do optimize FUNCALL heavily.  It is theoretically
> possible to compile FUNCALL to be just as efficient as an ordinary
> call to a named global function ...

Enough theory, here are some fun-facts-to-know and tell.

(1) In the MIT CADR or LMI LAMBDA lispmachine (FUNCALL F X) and (F X) compiled
    into *exactly* the same machine instructions.
(2) The case (FUNCALL F X) where F was a local variable was very slightly,
    but measurably FASTER than the case of (F X) where F was a "global"
(3) Symbolics and then later Texas Instruments improved on the instruction
    set such that (FUNCALL F X) and (F X) were indeed different, resulting
    in some extra overhead for the FUNCALL case, although it is very
    difficult to measure the difference.
(4) On a VAX it is quite easy to implement lexical closures in such a way
    (inline-code trampolines) such that the two cases produce the same
    instructions.
(5) On some RISC architectures, e.g. SPARC from SUN, there is different
    code produce for {This is C code now, not lisp:} (*F)(X); vs. F(X);
    and in-fact a little subroutine called POINTERCALL is called
    in the (*F)(X); case.


-GJC

(Also known as the John Silber of Lisp Implementors?)