Re: Warning specifically for a returning noreturn

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Wed, 5 Jul 2023 at 12:01, Julian Waters via Gcc <gcc@gcc.gnu.org> wrote:
>
> I see, thanks Andrew.
>
> Anyone else have opinions on this besides Liu or Andrew? The responses have
> been surprisingly quiet thus far

IMHO all warnings should have an option controlling them, so that you
can disable them via pragmas.

But I agree that you shouldn't need to return from a noreturn
function, it can either throw or use __builtin_unreachable() on the
line where you currently return.


>
> best regards,
> Julian
>
> On Wed, 5 Jul 2023, 09:40 Andrew Pinski, <pinskia@gmail.com> wrote:
>
> > On Tue, Jul 4, 2023 at 6:32 PM Julian Waters <tanksherman27@gmail.com>
> > wrote:
> > >
> > > Hi Andrew, thanks for the quick response,
> > >
> > > What if the method has a return value? I know it sounds
> > counterintuitive, but in some places HotSpot relies on the noreturn
> > attribute being applied to methods that do return a value in an unreachable
> > code path. Does the unreachable builtin cover that case too?
> >
> > It is wrong to use noreturn on a function other than one which has a
> > return type of void as documented.
> >
> > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-noreturn-function-attribute
> > :
> > ```
> > It does not make sense for a noreturn function to have a return type
> > other than void.
> > ```
> >
> > Thanks,
> > Andrew Pinski
> >
> >
> > >
> > > best regards.
> > > Julian
> > >
> > > On Wed, Jul 5, 2023 at 9:07 AM Andrew Pinski <pinskia@gmail.com> wrote:
> > >>
> > >> On Tue, Jul 4, 2023 at 5:54 PM Julian Waters via Gcc <gcc@gcc.gnu.org>
> > wrote:
> > >> >
> > >> > Hi all,
> > >> >
> > >> > Currently to disable the warning that a noreturn method does return,
> > it's
> > >> > required to disable warnings entirely. This can be very inconvenient
> > when
> > >> > -Werror is enabled with a noreturn method that isn't specifically
> > calling
> > >> > something like std::abort() at the end, when one wants all other
> > -Wall and
> > >> > -Wextra warnings to be reported, for instance in the Java HotSpot VM
> > (which
> > >> > I'm currently adapting to compile with gcc on all supported
> > platforms). Is
> > >> > there a possibility we can add a disable warning option specifically
> > for
> > >> > this case? Something like -Wno-returning-noreturn. I'm interested in
> > adding
> > >> > this myself if it's not convenient for gcc's maintainers to do so at
> > the
> > >> > moment, but I'd need some guidance on where to look and what the
> > relevant
> > >> > code is
> > >>
> > >> You could just add
> > >> __builtin_unreachable(); (or std::unreachable(); if you are C++23 or
> > >> unreachable() if you are using C23).
> > >> Or even add while(true) ;
> > >>
> > >> I am pretty sure not having an option is on purpose and not really
> > >> interested in adding an option here because of the above workarounds.
> > >>
> > >> Thanks,
> > >> Andrew Pinski
> > >>
> > >> >
> > >> > best regards,
> > >> > Julian
> >