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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: Ada 83 - avoiding unchecked conversions.
Date: 1996/12/18
Message-ID: <dewar.850929268@merv>#1/1
X-Deja-AN: 204771814
references: <md5:8B831999BCF200C6E70994BDF6CC529F> <32AED68A.48BE@aisf.com>
 <32b03008.564464@netnews.worldnet.att.net> <58np72$9qp@news.nyu.edu>
 <32B6905F.FC4@tiac.net>
organization: New York University
newsgroups: comp.lang.ada
Date: 1996-12-18T00:00:00+00:00
List-Id: <comp.lang.ada>


iEric said


"using overlays.  Apparently on that compiler (VAX Ada 1.something)
unchecked conversion was doing some checks.
"


Note that the unchecked in unchecked conversion is talking about omitting
normal compile time checks for type correctness. It is NOT talking about
runtime checking (whatever that might mean). In general you cannot assume
that unchecked conversion takes no code. Some of the time it may take
no code, but there is no reason to think, or even considerable desirable,
that a compiler would never generate code for an unchecked conversion. 
Sometimes you would get the wrong result in the absence of generating
code. For example, suppose that packed 5 bit arrays were stored left
justified in a register, and 5 bit integers were stored right justified.
Well the compiler had beetter emit a shift for the unchecked conversion
or it will get the wrong result (remembr that the semantics of
unchecked conversion is quite well defined, it is not some kind of
implementation dependent unknown op0eration).