Path: utzoo!mnetor!uunet!husc6!uwvax!oddjob!hao!ames!pacbell!att-ih!ihnp4!inuxc!iuvax!pur-ee!uiucdcs!uiucdcsb!robison
From: robison@uiucdcsb.cs.uiuc.edu
Newsgroups: comp.lang.misc
Subject: Re: Var scoping in Wirth-type languages
Message-ID: <170500016@uiucdcsb>
Date: 12 Mar 88 07:35:00 GMT
References: <3949@ihlpf.ATT.COM>
Lines: 32
Nf-ID: #R:ihlpf.ATT.COM:3949:uiucdcsb:170500016:000:989
Nf-From: uiucdcsb.cs.uiuc.edu!robison    Mar 12 01:35:00 1988


For languages without "upward funargs" (e.g. Pascal), a global display
is sufficient, and furthermore requires only one extra push/pop per
procedure call. 

Let D[0..n] be the display registers.  Then the code for entering a procedure 
with lexical level n is:

	push D[n];
	D[n] := local_frame_pointer;

And the code for procedure exit is:

	pop D[n];
	return

One need not copy the entire display for each procedure call.
This was the Burroughs 6700's way of handling the display. I also notice 
that it was rediscovered in SIPLAN notices last year.  I suppose the
``expensive display'' myth is related to the ``expensive procedure call'' 
myth.  

I've heard that the "static link" approach is not inefficient with a
global optimizer.  The "chase" becomes a common subexpression and is
computed just once.

Arch D. Robison
University of Illinois at Urbana-Champaign
	
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