On Wed, Oct 30, 2013 at 10:49:09AM -0400, Trevor Saunders wrote:
> 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.
>
That also does not work in strict sense as it assumes two complement
arithmetic which standard does not guarantee and is one of reasons why
signed overflows has undefined behavior.
