svn commit: r358248 - head/sys/vm

[Kyle Evans]

On Sat, Feb 22, 2020 at 10:25 AM Ian Lepore <ian at freebsd.org> wrote:
>
> On Sat, 2020-02-22 at 16:20 +0000, Kyle Evans wrote:
> > Author: kevans
> > Date: Sat Feb 22 16:20:04 2020
> > New Revision: 358248
> > URL: https://svnweb.freebsd.org/changeset/base/358248
> >
> > Log:
> >   vm_radix: prefer __builtin_unreachable() to an unreachable panic()
> >
> >   This provides the needed hint to GCC and offers an annotation for readers to
> >   observe that it's in-fact impossible to hit this point. We'll get hit with a
> >   a -Wswitch error if the enum applicable to the switch above were to get
> >   expanded without the new value(s) being handled.
> >
> > Modified:
> >   head/sys/vm/vm_radix.c
> >
> > Modified: head/sys/vm/vm_radix.c
> > ==============================================================================
> > --- head/sys/vm/vm_radix.c    Sat Feb 22 13:23:27 2020        (r358247)
> > +++ head/sys/vm/vm_radix.c    Sat Feb 22 16:20:04 2020        (r358248)
> > @@ -208,8 +208,7 @@ vm_radix_node_load(smrnode_t *p, enum vm_radix_access
> >       case SMR:
> >               return (smr_entered_load(p, vm_radix_smr));
> >       }
> > -     /* This is unreachable, silence gcc. */
> > -     panic("vm_radix_node_get: Unknown access type");
> > +     __unreachable();
> >  }
> >
> >  static __inline void
>
> What does __unreachable() do if the code ever becomes reachable?  Like
> if a new enum value is added and this switch doesn't get updated?
>

__unreachable doesn't help here, but the compiler will error out on
the switch() if all enum values aren't addressed and there's no
default: case.

IMO, compilers could/should become smart enough to error if there's an
explicit __builtin_unreachable() and they can trivially determine that
all paths will terminate before this, independent of -Werror=switch*.