On Mon, Oct 24, 2016 at 04:34:40PM +0200, Marek Polacek wrote:
> On Mon, Oct 24, 2016 at 08:18:25AM -0600, Martin Sebor wrote:
> > > The case above is just a case where with -O GCC can figure out what the
> > > value
> > > of a const-qualified variable is, see
> > > decl_constant_value_for_optimization.
> > > Since the warning is implemented even before gimplifying, optimizations
> > > like
> > > CP don't come into play yet.
> >
> > Ah, okay. That should limit the number of these false positives.
> > (I saw -O2 in dg-options and assumed it was important. It sounds
> > like the test case should pass even with -O1).
>
> Yep--even -O is enough.
>
> > But even without constant propagation there will be similar cases
> > (though probably less pervasive). For instance, if j were defined
> > to something like this:
> >
> > const int j = 4 == sizeof (size_t);
>
> Well, I think the warning would still be desirable:
>
> const int j = 4 == sizeof (long);
> if (j == 0)
> {
> if (i > 10) /* { dg-warning "this condition has identical branches" } */
> *p = j * 2 + 1;
> else
> *p = 1;
> }
>
> Given the j == 0 check, the branches really are duplicated. This is actually
> a distilled version of what I found in gcov-io.c.
Are you hashing before folding or after folding?
If before folding, you wouldn't complain about the gcov-io.c test.
With folding, one can imagine something like:
const int a = sizeof (long);
const int b = sizeof (long long);
int c;
if (cond)
c = a;
else
c = b;
or similar, where a and b can be the same on some targets and different on
others. I believe the warning is still useful, but it will probably be
never false positive free.
Jakub
