Re: [PATCH] Fixes compile warning with -Wimplicit-fallthrough CFLAGS

From: Jeff King <peff@peff.net>
Date: Jan 23, 2018, 00:59 UTC
Message-ID: <20180123005950.GH26357@sigill.intra.peff.net>
In-Reply-To: <CAPig+cSuBRcM7BBw1MKOOVSZv+CVVrY-dWfSGj+31DA4QMrruA@mail.gmail.com>

On Mon, Jan 22, 2018 at 07:54:18PM -0500, Eric Sunshine wrote:

Show 22 quoted lines

> On Mon, Jan 22, 2018 at 6:51 PM, Elia Pinto <gitter.spiros@gmail.com> wrote:
> > This patch add explicit fallthrough compiler attribute
> > when needed on switch case statement eliminating
> > the compile warning [-Werror=implicit-fallthrough=].
> > It does this by means of a macro that takes into account
> > the versions of the compilers that include that attribute.
> > [...]
> > Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
> > ---
> > diff --git a/convert.c b/convert.c
> > @@ -1554,7 +1554,7 @@ static int ident_filter_fn(struct stream_filter *filter,
> >                 switch (ident->state) {
> >                 default:
> >                         strbuf_add(&ident->left, head, ident->state);
> > -                       /* fallthrough */
> > +                       GIT_FALLTHROUGH;
> >                 case IDENT_SKIPPING:
> >                         /* fallthrough */
> 
> Why doesn't this /* fallthrough */ deserve the same treatment?
> 
> >                 case IDENT_DRAINING:

I can't answer that philosophically, but I can tell you why the compiler does not complain. :)

Usually case arms with no statements between them are exempt from fallthrough warnings. So:

  switch (foo)
  case 1:
  case 2:
  case 3:
     /* do one thing */
     break;
  case 4:
     /* do another thing */
     break;
  }

does not need any annotations for cases 1 and 2 to fallthrough. Which means that the original comment was not actually necessary for the compiler, though the original author considered it a useful comment.

So there you get into philosophy. Should it be converted to a compiler-visible annotation, or is it better left as a comment?

-Peff