Path: utzoo!mnetor!uunet!husc6!bloom-beacon!mit-eddie!uw-beaver!cornell!rochester!PT.CS.CMU.EDU!sei!sei.cmu.edu!firth
From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.lang.pascal
Subject: Re:  Global variables
Message-ID: <4770@aw.sei.cmu.edu>
Date: 29 Mar 88 13:47:10 GMT
References: <12654@brl-adm.ARPA> <529@taux01.UUCP>
Sender: netnews@sei.cmu.edu
Reply-To: firth@bd.sei.cmu.edu.UUCP (Robert Firth)
Organization: Carnegie-Mellon University, SEI, Pgh, Pa
Lines: 18
Keywords: gloabl optimizing compilers

In article <529@taux01.UUCP> orr@taux01.UUCP (Orr Michael ) writes:
>As one of a team of compiler-writers, including a pascal compiler,
>let me note that global variables reduce the compiler's room for
>optimization. this is because any time you erite through a pointer,
>potentially you may be changing the value of any global var...

I don't think that's right.  For example, in Pascal, an explicit
pointer (^Thing) cannot point anywhere except into the heap, ie
at an object created by NEW.  It cannot alias any global variable.

That leaves only VAR parameters as potential aliases.  Even here,
a VAR parameter of one type (VAR p: ThingA) cannot alias a global
variable of any other type (VAR g: ThingB).

The extent to which this matters depends on people's coding style,
but with good Pascal programs (lots of different types) the amount
of possible aliasing is less than 5% (ie for any pointer you know
it cannot alias 95% or more of globals in scope).