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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: Waiver question
Date: 1997/05/04
Message-ID: <dewar.862718682@merv>#1/1
X-Deja-AN: 239224450
References: <dewar.861594356@merv> <5kb8ol$18o@bcrkh13.bnr.ca>
 <E9J2rB.G04@world.std.com> <1997May2.063334.1@eisner>
 <E9KA78.DxH@world.std.com> <1997May2.164921.1@eisner>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-05-04T00:00:00+00:00
List-Id: <comp.lang.ada>


Larry said

<<A casual programmer would presume that if variable A is only modified
by task AA and variable B is only modified by task BB then they are
task-safe.  This is not true if the variables share a longword on
Alpha.  Having variables default to not sharing a longword goes a
long way toward eliminating this exposure.
>>

Well of course if you have two separate variables, you have to make sure
they can be indepednetly accessed by tasks, and if the compiler cannot
verify that this might not occur, then of cours they cannot occupy the
same longword.

The more uncomfortable case is one like this:

   type x is array (1 .. 64) of character;

now, it is really uncomfortable to by default allocate one longword
for each character for this declaration, and in practice it is
extremely unlikely that two tasks would access different characters
of this string.

Note that in Ada 83, such access would be erroneous in any case. In Ada 95,
this kind of access is valid, but the consequence of this change is rather
unpleasant for the (old style) Alpha.