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From: Geert Bosch <geert@gonzo.sun3.iaf.nl>
Subject: Re: array conversion - how to do?
Date: 1997/07/04
Message-ID: <5pjkpi$mvk$1@gonzo.sun3.iaf.nl>#1/1
X-Deja-AN: 254656913
References: <5opej8$rv5$1@gonzo.sun3.iaf.nl>
 <33B3DE95.41EF@does.not.exist.com> <dewar.867457041@merv>
 <5p3qk2$8mk$1@gonzo.sun3.iaf.nl> <dewar.867957369@merv>
Organization: La Calandre Infortunee
Newsgroups: comp.lang.ada
Date: 1997-07-04T00:00:00+00:00
List-Id: <comp.lang.ada>


Geert says
 ``Now I think about it, it is possible to rather closely model
   typecasting for arrays by implementing array conversion as showed
   below. It is a pity that this generates lots of code with GNAT
   although all code could be eliminated. Are there any compilers
   that correctly compile this into a null function?''

Robert Dewar <dewar@merv.cs.nyu.edu> replies:
 ``If the body of Error is not available, this is not a valid
   optimization, and indeed is typical of the kind of invalid
   optimizations that optimizing compilers are sometimes known
   to do.''

I think you placed my remark about optimization in the wrong context. 
Your reply seems to target the loop approach which makes a copy that
cannot trivially be eliminated.

The code for Array_Conversion that I showed uses Unchecked_Conversion
from one constrained array to another. All checks can be eliminated,
so Unchecked_Conversion can just provide a different view of the 
array which is the goal I want to achieve. I'll quote some references
to support my claim that the code can be optimized to null.

Regards,
   Geert

RM95 13.9           /Implementation permissions/

 (12)   An implementation may return the result of an unchecked
        conversion by reference, if the Source type is not
	a by-copy type.  In this case, the result of the unchecked
	conversion represents simply a different (read-only) view
	of the operand of the conversion.
        
RM95 13.9             /Implementation Advice/

 (14)   The Size of an array object should not include its bounds;
	hence, the bounds should not be part of the converted data.

 (15)   The implementation should not generate unnecessary run-time
	checks to ensure that the representation of S is a
	representation of the target type. It should take advantage
	of the permission to return by reference when possible.
	Restrictions on unchecked conversions should be avoided
	unless required by the target environment.