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Re: [libvirt] [PATCH 4/4] Add ATTRIBUTE_FALLTHROUGH for switch cases without break



On Thu, Feb 23, 2017 at 09:07:59PM +0400, Roman Bogorodskiy wrote:
>   Daniel P. Berrange wrote:
> 
> > On Wed, Feb 22, 2017 at 05:52:05PM +0000, Daniel P. Berrange wrote:
> > > In GCC 7 there is a new warning triggered when a switch
> > > case has a conditional statement (eg if ... else...) and
> > > some of the code paths fallthrough to the next switch
> > > statement. e.g.
> > > 
> > > conf/domain_conf.c: In function 'virDomainChrEquals':
> > > conf/domain_conf.c:14926:12: error: this statement may fall through [-Werror=implicit-fallthrough=]
> > >          if (src->targetTypeAttr != tgt->targetTypeAttr)
> > >             ^
> > > conf/domain_conf.c:14928:5: note: here
> > >      case VIR_DOMAIN_CHR_DEVICE_TYPE_CONSOLE:
> > >      ^~~~
> > > conf/domain_conf.c: In function 'virDomainChrDefFormat':
> > > conf/domain_conf.c:22143:12: error: this statement may fall through [-Werror=implicit-fallthrough=]
> > >          if (def->targetTypeAttr) {
> > >             ^
> > > conf/domain_conf.c:22151:5: note: here
> > >      default:
> > >      ^~~~~~~
> > > 
> > > GCC introduced a __attribute__((fallthrough)) to let you
> > > indicate that this is intentionale behaviour rather than
> > > a bug.
> > 
> > 
> > BTW, CLang has apparently had an -Wimplicit-fallthrough
> > warning flag for a while, and also seems to have the same
> > __attribute__((fallthrough)), but I've been unable to get
> > CLang to trigger such warnings, and hence did not enable
> > __attribute__((fallthrough)) on CLang. If someone can
> > figure out how to reproduce the warnings on clang we
> > could extend internal.h to surpress them for whichever
> > clang version introduced the warnings. As is, the
> > ATTRIBUTE_FALLTHROUGH turns into a no-op for CLang unless
> > it claims GCC 7.0 compatibility.
> 
> It *looks* like -Wimplicit-fallthrough in clang is only effective for
> C++11:
> 
> http://clang-developers.42468.n3.nabble.com/should-Wimplicit-fallthrough-require-C-11-td4028144.html
> 
> It mentions this commit:
> 
> http://llvm.org/viewvc/llvm-project?revision=167655&view=revision
> 
> And it seems it wasn't changed since than.

Ah I see. So they disabled it for C, because their [[clang::fallthrough]]
magic annotation would not work under C. Hopefully they'll enable it one
day with the more normal  __attribute__((fallthrough)) syntax for C.

Regards,
Daniel
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