On Sun, 30 Jan 2022, 10:42 Jakub Jelinek via Gcc, <gcc@gcc.gnu.org> wrote:
> On Sun, Jan 30, 2022 at 10:47:41AM +0100, Theodore Papadopoulo wrote:
> > Before creating a bug report, I want to check with the GCC community (all
> > the more that checking that the problem has not yet been reported is
> > complicated at leat for me).
> >
> > The following (admitedly buggy) program generates a segmentation
> violation
> > on fedora 35 (this is with g++ 11.2.1 20211203 (Red Hat 11.2.1-7) (GCC))
> > when compiled with -O3 (other versions replacing unisgned by std::string
> may
> > trigger the exception instead of the segv)
> >
> > bool assert_sthg(const unsigned s) {
> > if (s==123)
> > throw 1;
> > }
> >
> > int main() {
> > assert_sthg(0);
> > return 0;
> > }
> >
> > When compiling, we indeed get a warning:
> >
> > test.C:4:1: warning: control reaches end of non-void function
> > [-Wreturn-type]
> >
> > I can well understand that the program being buggy that the optimizer is
> > allowed to do anything including the observed segmentation violation.
> > Yet the result is quite surprising....
>
> Undefined behavior can have any kind of surprising behavior.
>
> > The question is, in that case, wouldn't it be better to turn the warning
> > into an error at -O3 ?
>
> No, it can't be an error by default, it is undefined behavior only at
> runtime, if you never call the function or always call it with
> assert_sthg(123), then the program can be valid.
>
We could put a trap instruction at the end of the function though, which
would make the result a bit less arbitrary.
I've come around to thinking that's preferable for cases like this.
