Implement -Wswitch-fallthrough

[Marek Polacek]

On Mon, Jul 11, 2016 at 04:28:32PM -0400, NightStrike wrote:
> On Mon, Jul 11, 2016 at 3:43 PM, Marek Polacek <polacek@redhat.com> wrote:
> > But then the [[fallthrough]] attribute was
> > approved for C++17 [1], and that's what has got me to do all this.
> >  ...
> > I added a new builtin,
> > __builtin_fallthrough, that prevents the warning from occurring.  It can only
> > be used in a switch; the compiler will issue an error otherwise.  The new C++
> > attribute can then be implemented in terms of this builtin.
> 
> This is a stupid question I'm sure, but if C++ can have the attribute
> version, why can't C have __attribute__((fallthrough)) ?

We could, but why?  I already added __builtin_fallthrough() and
__attribute__((fallthrough)) would just be turned into that.  C++17 adds the
fallthrough attribute for a null statement, so we need to implement that, but
again I expect that I just turn that to __builtin_fallthrough().  We plan to
add __Fallthrough to C, but that is a long way off.  Does
"__attribute__((fallthrough));" have any advantages over "__builtin_fallthrough()"?

	Marek