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From: jjacobs@well.UUCP
Newsgroups: comp.lang.lisp
Subject: Against the Tide of Common LISP
Message-ID: <2565@well.UUCP>
Date: Tue, 10-Feb-87 22:16:00 EST
Article-I.D.: well.2565
Posted: Tue Feb 10 22:16:00 1987
Date-Received: Thu, 12-Feb-87 00:09:28 EST
Reply-To: jjacobs@well.UUCP (Jeffrey Jacobs)
Organization: Whole Earth 'Lectronic Link, Sausalito, CA
Lines: 68


In <768@rpics.RPI.EDU>, Bill Yerazunis writes:

>Do I really need to keep that there?   :-)

 Only if you reproduce the whole thing somewhere else :-)

 >Why bother DEFVARing?  It says right in CLTL that it won't affect
>just efficiency.

It says right in CLTL "DEFVAR is the recommended way to declare the
use of a special variable".

I contend that this does affect correctness (and is also a good reason to
refer to the manual even when doing *simple* things).  :-)

>Common lisp does have an "interior logic" that you can get into.  It makes
>sense after a while.  Things like the scoping DO make a LOT of sense when
>you start seriously considering a compiler.

 Some of it makes sense, but not *good* sense :-) 

The issue I raise is not "lexical vs dynamic"; it's the godawful mess that CL
uses!

(As a general rule of language design I agree that lexical is better;
dynamic scoping for LISP is both a personal prejudice and a
performance issue).

>  I know my code runs faster
>with lexical than with dynamic.  Remember, when you scope lexically and
>compile, you have an absolute displacement onto the stack. This is
>a fast thing. 

>If you dynamically scope, you have to call a routine
>to go into a hash table and find out where whatever-it-is is kept.
>This is a not-so-fast thing for a compiled language.  On calling
>and return, you have to fix up the hash symbol table for each and
>every dynamically-scoped argument.  This is a very-not-fast thing

Say what???????????

The "value cell" is normally statically located at a known address!!!
No need to perform hash table lookup at all!!!  All references are by
address.

Access time may possibly be *faster*, i.e. MOV ADDR, dest instead of 
MOV INDEX(SP),dest.depending on CPU architecture!

Simplisticially, dynamic binding becomes:

PUSH	#SPEC_CELL_ADDR	; save address for later restoral 
PUSH	SPEC_CELL_ADDR	; save current value
MOV	new_value, SPEC_CELL_ADDR ; set value

restoring becomes simply

POP	R1	; get address of special cell, R1 assumed to be a reg.
POP	(R1)	; restore value.

 Jeffrey M. Jacobs
 CONSART Systems Inc.
 Technical and Managerial Consultants
 P.O. Box 3016, Manhattan Beach, CA 90266
 (213)376-3802
 CIS:75076,2603
 BIX:jeffjacobs
 USENET: well!jjacobs