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



  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.

Roman Bogorodskiy

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