On Mon, Oct 24, 2016 at 04:39:20PM +0200, Jakub Jelinek wrote:
> 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?
After, e.g. I think build_modify_expr c_fully_fold operands.
> 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.
Unfortunately :(. Maybe tolerable for -Wextra, though.
Marek
