Path: utzoo!mnetor!uunet!husc6!bbn!mit-eddie!uw-beaver!cornell!rochester!ur-tut!sunybcs!boulder!michael@boulder.Colorado.EDU
From: michael@boulder.Colorado.EDU (Michael Schmidt)
Newsgroups: comp.lang.misc
Subject: Re: Displays are not expensive (Was:  Var scoping in Wirth-type languages)
Message-ID: <4818@sigi.Colorado.EDU>
Date: 13 Mar 88 18:43:05 GMT
References: <3821@ihlpf.ATT.COM> <2791@enea.se> <3949@ihlpf.ATT.COM> <4426@june.cs.washington.edu> <5673@watdragon.waterloo.edu>
Sender: news@sigi.Colorado.EDU
Reply-To: michael@boulder.Colorado.EDU (Michael Schmidt)
Organization: Uni-GH Paderborn, FB17/Informatik
Lines: 33
Keywords: static link, go!
In-reply-to: gvcormack@watdragon.waterloo.edu (Gordon V. Cormack)

Posting-Front-End: Gnews 1.1


In article <5673@watdragon.waterloo.edu> you write:
>This is just not true.  For ordinary procedure calls, a global
>display involves trivial entry and exit overhead.  The only time
>displays get expensive is if you do lots of "functional" style 
>programming - passing procedures as parameters and/or using
>procedure variables.

Carefull. If you have full procedure variables, you even cannot
use the usual stack of activation records, because the extent of
a procedure doesn't end necessarily, when you leave it.

>Even this is quite bearable - it uses time
>and space proportional to n to build a local copy of the display
>in order to pass a procedure (or procedures) as a parameter.

And you have to pass variable length, big objects around.

>Here is a comparison of the time to do static links vs. displays.
>(in terms of "n", the level of nesting).
>
>    STRATEGY:        static links          global disply
>
>    ENTRY CODE:        O(1)                    O(1)
>    EXIT CODE:         0                       O(1)
>    NONLOCAL REF:      O(n)                    O(1)
>    FUNC PARM:         O(1)                    O(n)*

Yes, and now? If you realize, that NONLOCAL REF account for less
than ONE PERCENT of all references (if we consider LOCAL-1 to be
nearly LOCAL, cf. <4808@sigi.Colorado.EDU>), why should one use
displays?? 

	Michael Schmidt