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

From: Jacob Keller <jacob.keller@gmail.com>
Date: Jan 23, 2018, 06:31 UTC
Message-ID: <CA+P7+xqfHnsoOq8EPb-4feu8Q6gdRTCeOGRVkSyjNJNtmtiw=Q@mail.gmail.com>
In-Reply-To: <20180123005950.GH26357@sigill.intra.peff.net>

On Mon, Jan 22, 2018 at 4:59 PM, Jeff King <peff@peff.net> wrote:

Show 50 quoted lines

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

I'd personally rather stick to the comment if we can, or use something like "fallthrough;" to make it appear like a keyword, instead of an all caps macro, since at least to my sensibility, the all caps is a bit too crazy.

Also, I would not put one inside an empty case statement that just falls through to the next branch and does nothing special itself.

Thanks, Jake