Re: What is the warning about builtin-function on gcc-4.6.0 ?

[Adam Beneschan <adam@irvine.com>]

On Mar 30, 5:57 am, Robert A Duff <bobd...@shell01.TheWorld.com>
wrote:

> >...If the
> > compiler's behavior is incorrect and doesn't raise C_E for
> > abs(Integer'First) when it's supposed to, then presumably
> > abs(Integer'First) will still be Integer'First (on a normal 2's
> > complement machine), and EQUAL will return TRUE.
>
> Right, that would be a compiler bug.
>
> I was imagining a compiler that returns the right answer for
> abs(Integer'First).  It's not entirely unreasonable.  Perhaps
> Integer is 32 bits, and the arithmetic is done in a 64-bit
> register, which can obviously represent 2**31 just fine.

Offhand, I don't think that makes sense, though.  The "abs" predefined
function is predefined to return Integer'Base; if the mathematical
answer doesn't fit in Integer'Base, I don't see how the right answer
can be returned.  Actually, 3.5.4(11) says that the predefined subtype
Integer is constrained to the base range of its type, and 3.5.4(9)
says that the base ranges of integer types are symmetric about zero
except possibly for an extra negative number.  So I don't think the
scenario you envision can legally exist.  If arithmetic is done in a
64-bit register, the program may temporarily have the correct
mathematical answer for abs(Integer'First); but if that answer doesn't
fit in 32 bits, nothing can be done with that value---the compiler
would have to raise C_E right away.  (*) So I don't know what you mean
by "returns the right answer", but I don't think it's possible in any
meaningful sense.  Maybe I've lost track of what you were trying to
say.

(*) If the value were used as part of a larger expression involving
predefined operators, there might be a Chapter 11 Implementation
Permission that would allow computation to continue using the 64-bit
value.  Here, though, it's just passed as a parameter of type Integer
to a user-defined function, so the constraint check would have to take
place.  Bottom line: there's really no way for Equal(abs(X),X) to
return False if X=Integer'First.

                             -- Adam