Date: Sat, 22 Feb 2020 10:29:17 -0600 From: Kyle Evans <kevans@freebsd.org> To: Ian Lepore <ian@freebsd.org> Cc: src-committers <src-committers@freebsd.org>, svn-src-all <svn-src-all@freebsd.org>, svn-src-head <svn-src-head@freebsd.org> Subject: Re: svn commit: r358248 - head/sys/vm Message-ID: <CACNAnaHZnrqRv9J-B7XRCc7eN7Hkccf1R-7e36LiAXvZR4etVw@mail.gmail.com> In-Reply-To: <a3b2125de10d214d6e422d183f1fdc7e0e38e014.camel@freebsd.org> References: <202002221620.01MGK46E072303@repo.freebsd.org> <a3b2125de10d214d6e422d183f1fdc7e0e38e014.camel@freebsd.org>
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On Sat, Feb 22, 2020 at 10:25 AM Ian Lepore <ian@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*.
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