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From: bn@alsys.com (Brian Nettleton @pulsar)
Subject: Re: next "big" language?? (disagree)
Date: 1996/06/25
Message-ID: <DtKsIz.Dp6@thomsoft.com>#1/1
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sender: news@thomsoft.com (USENET News Admin @flash)
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references: <ag129.511.000A9CE9@ucs.cam.ac.uk> <dewar.834584250@schonberg>
 	<JSA.96Jun15175541@organon.com> <4q707h$1r2@krusty.irvine.com>
 <JSA.96Jun19115535@organon.com> <dewar.835306677@schonberg>
organization: Thomson Software Products, San Diego, CA, USA
newsgroups: 
 comp.lang.pascal,comp.lang.c,comp.lang.misc,comp.lang.pl1,comp.lang.ada
Date: 1996-06-25T00:00:00+00:00
List-Id: <comp.lang.ada>


In <dewar.835306677@schonberg> dewar@cs.nyu.edu (Robert Dewar) writes:

>In GNAT, 

>   pragma Assert (X);

>means EXACTLY

>   if not X then
>	raise Assert_Error;
>   end if;

>which is well defined, but not at all in the category of assertions that
>the compiler can take advantage of, which can behave in a completely
>differrent (and possibly suprising manner).

>For instance, another view of assertions is exemplified by the following:

>    pragma Assert (X /= 0);
>    ...
>    Q := 30 / X;

>where the compiler "knows" that X is non-zero, and therefore skips the
>test for X being zero in the division, and consequently generates some
>junk value in Q. In this model, assertions do not correspond to any
>executable code.

I'm not sure I see the complete distinction.  Wouldn't it be possible
for a compiler to eliminate any divide-by-zero check from the
following code sequence:

  if not (X /= 0) then
    raise Assert_Error;
  end if;
  Q := 30 / X;


The divide is unreachable when X = 0.  Now this admittedly does
cause a change in the generated code when an assertion is used.
I'm trying to understand the "assertions should not affect things"
camp.

-Brian Nettleton