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Re: [C/C++ PATCH] Improve -Wlogical-op (PR c/63357)


On Mon, Apr 27, 2015 at 04:47:12PM +0200, Marek Polacek wrote:
> On Sat, Apr 25, 2015 at 10:18:59PM +0200, Gerald Pfeifer wrote:
> > In case this example feels too contrived (even though it is an
> > excerpt of Wine code), we now also warn about the following where
> > the two types and variables are defined in different places and
> > the size of one is set implicitly:
> > 
> >   typedef int r_fun_t (int);
> > 
> >   r_fun_t * text_funcs[] = {0,0,0};
> > 
> >   int report (unsigned t)
> >   {
> >     typedef int s_fun_t (long, char);
> > 
> >     static s_fun_t * GUI_funcs[3];
> > 
> >     return (t < sizeof text_funcs / sizeof text_funcs[0] &&
> >             t < sizeof GUI_funcs / sizeof GUI_funcs[0]);
> >   }
> > 
> > (I also filed this as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65891
> > so that we keep track.)
> 
> I'm afraid there isn't an easy solution to this; the problem is that we fold
> sizeof early, so the warning sees 
> 
>   t < sizeof 4 && t < 4
> 
> and warns.  Maybe using SIZEOF_EXPR would help...

That said, this isn't something that regressed with my patch.  Consider e.g.:

struct S { int i; };
struct T { int i; };
int
foo (unsigned int t)
{
  return t < sizeof (struct S) && t > sizeof (struct T);
}

with -Wlogical-op:
w2.c: In function âfooâ:
w2.c:6:32: warning: logical âandâ of mutually exclusive tests is always false [-Wlogical-op]
   return t < sizeof (struct S) && t > sizeof (struct T);
                                ^


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