Re: A case where Ada defaults to unsafe?

[Hyman Rosen <hyrosen@mail.com>]

Robert A Duff wrote:

> Matthew Woodcraft <mattheww@chiark.greenend.org.uk> writes:
>>Doesn't RM 11.6 allow the compiler to ignore this exception?
> 
> The compiler is always allowed to get the right answer instead of
> overflowing.  But I don't think that rule is stated in 11.6 -- somewhere
> in chap 3 or 4, I think.
> 
> But replace "x + y < 4" with something that fails in some other way (not
> overflow). For example, "Integer'(x+y) < 4" must fail if x+y is out of
> bounds.


Here's what 11.6 says:


	An implementation need not always raise an exception when
	a language-defined check fails. Instead, the operation that
	failed the check can simply yield an undefined result.
	The exception need be raised by the implementation only if,
	in the absence of raising it, the value of this undefined
	result would have some effect on the external interactions
	of the program.

So it seems to me that the case of
	if FALSE and Integer'(x+y) < 4
is also allowed not raise an overflow exception. I think that the
compiler is allowed to short-circuit boolean expressions as long as
doing so doesn't bypass any external interactions. I guess a similar
result applies to

	0 * (x + y)

which is optimizable to 0 without coding an overflow check.