On Sat, Feb 22, 2020 at 11:18 AM Mateusz Guzik <mjgu...@gmail.com> wrote: > > On 2/22/20, Kyle Evans <kev...@freebsd.org> wrote: > > On Sat, Feb 22, 2020 at 10:44 AM Mateusz Guzik <mjgu...@gmail.com> wrote: > >> > >> On 2/22/20, Kyle Evans <kev...@freebsd.org> wrote: > >> > On Sat, Feb 22, 2020 at 10:25 AM Ian Lepore <i...@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*. > >> > _______________________________________________ > >> > >> I think this is way too iffy, check this program: > >> > >> > >> #include <stdio.h> > >> > >> int > >> main(void) > >> { > >> > >> __builtin_unreachable(); > >> printf("test\n"); > >> } > >> > >> Neither clang nor gcc warn about this and both stop code generation > >> past the statement. Thus I think for production kernels __unreachable > >> can expand to to the builtin, but for debug it should be a panic with > >> func/file/line. This would work fine in terms of analysis since panic > >> is noreturn or so. > > > > I guess I'll repeat this again: our build will error out if this > > becomes reachable, because we compile with -Werror=switch. There's no > > point in having a panic that cannot physically be reached, you will > > never see the func/file/line. > > > > The keyword in the current lends itself towards misuse and according > to my grep this is precisely what happened in the tree: > > ddb/db_expr.c: > > switch(t) { > .. > default: > __unreachable(); > } > > similarly in dev/amdtemp/amdtemp.c >
The ddb one reads like crap at first, but the context does make it a little better- within that context, it's easily proven that all of the reachable cases are already handled. amdtemp looks more like an unexpected bug waiting to happen. > Another shady user is in dev/nvdimm/nvdimm.c -- read_label has few > loops and the function ends with __unreachable() > > Seems to be in all these cases the intent was to "warn at compilation > time if we ever get here and panic at runtime at least with debug" > > In contrast, the keyword is to explicitly tell the compiler that given > piece of code will never be executed no matter it thinks. Thus the > least which can be done is injecting a panic into it, according to the > original review which added it https://reviews.freebsd.org/D2536 llvm > developers do an equivalent (assert(0 && "blah")). > I don't see much use in adding a panic for this specific call site - it was deliberately structured for optimization, and the odds of it getting refactored in a way that anything after the switch becomes unreachable unintentionally are likely pretty slim. However, especially given the other cases you've pointed out, I do see the utility in general and I'm preparing a diff that will more generally turn __unreachable into a panic under INVARIANTS instead of dirtying up this particular call-site with the logic. Thanks, Kyle Evans _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
