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| From: | George Neuner <gneuner2@comcast.net> |
| Newsgroups: | comp.compilers |
| Date: | Mon, 05 Mar 2018 15:51:44 -0500 |
| Organization: | A noiseless patient Spider |
| References: | 18-02-009 18-02-012 18-02-016 18-02-018 18-02-023 18-02-029 18-02-032 18-02-034 18-03-002 18-03-012 18-03-018 |
| Injection-Info: | gal.iecc.com; posting-host="news.iecc.com:2001:470:1f07:1126:0:676f:7373:6970"; logging-data="96049"; mail-complaints-to="abuse@iecc.com" |
| Keywords: | code, Pascal |
| Posted-Date: | 05 Mar 2018 16:57:26 EST |
On Mon, 05 Mar 2018 15:01:33 GMT, anton@mips.complang.tuwien.ac.at
(Anton Ertl) wrote:
>I also looked if I could find Wirth's response to Cohen's paper, and I
>found it:
>
><https://dl.acm.org/citation.cfm?id=214521> (probably behind a paywall
>for many).
>
>And there he gave the reason for not using a display for accessing
>non-local variables:
>
> |Upon closer analysis, we decided to discard the use of a display
> |that was already in our first Pascal compiler (1969). The reason
> |was that (1) the display requires updating for every procedure call
> |and return (!) that causes a change of context, and (2) variables at
> |intermediate levels (neither local nor global) are referenced quite
> |rarely. As a result, the maintenance of a display turned out to be
> |more costly than the inefficient access of nonlocal variables via a
> |static link.
>
>So apparently there was no influential paper in the 60s or 70s that
>damned the display. Wirth and his collaborators discovered the
>performance disadvantages of displays by original research, but
>apparently did not publish this piece of knowledge before 1991.
>
>- anton
I may be mistaken on the timeline, but it seems to me that in 1990
Wirth already was working on Oberon, which, like Modula-2, had
visibility rules that were problematic for a display implementation.
I think he also had seen how people had used Modula-2, and before that
the "unit" [modular] extended Pascals. Programmers in these languages
tended to prefer the modules to control visibility and to write
flatter code that made less use of nested functions.
OTOOH, there were the Lisp programmers.
Even in Lisp, deep scope accesses are not that common. But they
aren't exactly rare either ... sometimes you just have to step-wise
build up the context for a function.
I think the real issue was that, by 1990, everybody in the language
research community knew that closure conversion was a better choice
than either static chains or displays: not just for performance, but
because it enables more freedom and functionality in the language.
YMMV,
George
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