Re: Ada 83 - avoiding unchecked conversions.

[kst@aonix.com (Keith Thompson)]

In <dewar.850929268@merv> dewar@merv.cs.nyu.edu (Robert Dewar) writes:
[...]
> 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. 

Certainly unchecked conversion can require some code in some cases, but
one wouldn't normally expect that code to include constraint checks.

> 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).

I agree that the compiler needs to emit a shift for this case, but
it shouldn't generate a check that the result is within the declared
range of its subtype.  This makes it the programmer's responsibility to
guarantee that the result will be a valid value of the target subtype
(if possible, by using a target subtype that covers all possible bit
patterns), or to deal with it properly if it isn't.

On the other hand, if the result of a unchecked conversion is assigned
to an object, there may well be a constraint check on the assignment.

-- 
Keith Thompson (The_Other_Keith) kst@aonix.com <http://www.aonix.com> <*>
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