Re: [PATCH][4.3] Deprecate -ftrapv

[Robert Dewar <dewar at adacore dot com>]

Richard Kenner wrote:
> > int f(int a, int b) { return 0 * (a + b); }
> >
> > get the potentially trapping arithmetic folded away.  If -ftrapv is to 
> > have properly defined semantics, those must include trapping if (a + b) 
> > overflows in the above code.
>
> Why?  Is there any language that requires such semantics?

For C, the above fragment of code is undefined if a+b overflows. Since
the purpose of -ftrapv for C, it is desirable that it catch this case
as well, since gcc might otherwise do peculiar things, or on some other
compiler you might get other peculiar things or a trap etc.

Obviously the C language does not require this, since it does not
require -ftrapv in the first place, that's just a handy addition
that we make to gcc, and we get to decide what's most useful.

For Ada, as I discussed in my last note, max performance says
you can ignore the check here, but generally for max performance
you turn off checks anyway. It is certainly allowed to optimize
this to zero ..

in fact there are two rules that let you optimize the above to
zero in Ada.

1. The rule about correct results for intermediate computations
being OK even if they are out of range.

2. The rule about not doing operations whose only purpose is
to see if they raise a predfined exception.

Certainly it is acceptable to do the check in the Ada case,
and if you are concerned about writing maximally portable
Ada code, desirable.