On 2016.12.16 at 18:27 +0100, Jakub Jelinek wrote:
> On Fri, Dec 16, 2016 at 10:10:00AM -0700, Martin Sebor wrote:
> > > No. The first call to sm_read_sector just doesn't exit. So it is warning
> > > about dead code.
> >
> > If the code is dead then GCC should eliminate it. With it eliminated
>
> There is (especially with jump threading, but not limited to that, other
> optimizations may result in that too), code that even very smart optimizing
> compiler isn't able to prove that is dead.
>
> > the warning would be gone. The warning isn't smart and it doesn't
> > try to be. It only works with what GCC gives it. In this case the
> > dump shows that GCC thinks the code is reachable. If it isn't that
> > would seem to highlight a missed optimization opportunity, not a need
> > to make the warning smarter than the optimizer.
>
> No, it highlights that the warning is done in a wrong place where it suffers
> from too many false positives.
Another issue with Martin's patch is that it adds many false positives
when one uses -fsanitize=undefined, e.g.:
% cat test.ii
struct A {
char *msg;
A(const char *);
};
A::A(const char *p1) {
msg = new char[__builtin_strlen(p1) + 1];
__builtin_strcpy(msg, p1);
}
% g++ -Wall -O2 -c test.ii
% g++ -Wall -fsanitize=undefined -O2 -c test.ii
test.ii: In constructor ‘A::A(const char*)’:
test.ii:6:34: warning: argument 1 null where non-null expected [-Wnonnull]
msg = new char[__builtin_strlen(p1) + 1];
~~~~~~~~~~~~~~~~^~~~
test.ii:6:34: note: in a call to built-in function ‘long unsigned int
__builtin_strlen(const char*)’
--
Markus
