Re: Rules for Representation of Subtypes

[kilgallen@eisner.decus.org (Larry Kilgallen)]

In article <DyGwtp.Bs1@world.std.com>, bobduff@world.std.com (Robert A Duff) writes:
> In article <1996Sep28.155354.1@eisner>,
> Larry Kilgallen <kilgallen@eisner.decus.org> wrote:
>>If the assignment of the output causes erroneousness,
>>then why isn't the name of the operation Checked Conversion ?
> 
> Heh?  If it were checked, it would do something sensible, like raise an
> exception, or return a well-defined result, or give a compile-time
> error.  The UNchecked means, it's *not* checked -- if you do something
> wrong, you get unpredictable behavior, i.e. erroneous execution.

But my X'valid should still work.  And compilers are not allowed to
change the flow of a program.

	X := my_unchecked_conversion(Y);

	if X'valid
	then
		Z(247) := X;
	else
		raise my_exception;
	end if;

No optimizer should stash my X into Z until it determines the
result of my conditional.  Likewise for:

	X:= my_unchecked_conversion(Y);

	if not X'valid then raise my_exception; end if;

	Z(247) := X;

no optimizer should stash my X into Z until it knows the result
of the possible flow control change represented by raise my_exception.

Larry Kilgallen