On Wed, Oct 30, 2013 at 10:00:07AM +0100, Kai Tietz wrote:
> 2013/10/30 Andrew Haley <a...@redhat.com>:
> > On 10/30/2013 08:34 AM, Ondřej Bílka wrote:
> >
> >>>
> >> The reasons of adding builtins is performance. Without that one can
> >> write a simple template to generically check overflows like
> >>
> >> template <class C> class overflow {
> >> public:
> >> C val;
> >> overflow <C> operator + (overflow <C> &y) {
> >> overflow <C> ret;
> >> if (val > 0 && y.val > 0 && val + y.val < val)
> >> throw std::overflow_error();
> >> /* ... */
> >> ret.val = val + y.val;
> >> return ret;
> >> }
> >> /* ... */
> >> };
> >
> > How is that going to work? The compiler can simply eliminate this line:
> >
> > if (val > 0 && y.val > 0 && val + y.val < val)
> > throw std::overflow_error();
> >
> > because it knows that the guard is always false. I suppose it could be
> > compiled with -fwrapv.
> >
> > Andrew.
>
> Right, as Andrew pointed out, this doesn't work. You are falling into
> the pit of optimization on assumptions of overflow.
>
> The way, which might work, is to do checks on unsigned types. As for
> unsigned-integer-scalar-types overflow is defined.
For an example of something that implements this see
http://mxr.mozilla.org/mozilla-central/source/mfbt/CheckedInt.h which is
used by WebKit / Blink as well aiui.
Trev
>
> Kai
