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

[Jeff King <peff@peff.net>]

On Mon, Jan 22, 2018 at 11:51:18PM +0000, Elia Pinto 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.
> 
> The fallthrough (or clang::fallthrough) attribute is used to annotate
> intentional fall-through between switch labels.
> Traditionally these are marked with a specific comment, but
> this attribute is meant to replace comments with a more strict
> annotation, which can be checked by the compiler (gcc-7 or clang).
> The flags in question were introduced in gcc 7 and are also enabled
> with -Wextra.

Hrm. Your subject says "fixes compile warnings", but don't we already
compile cleanly with -Wimplicit-fallthrough after my 1cf01a34ea
(consistently use "fallthrough" comments in switches, 2017-09-21)?

Certainly the tip of "master" seems to pass for me on both gcc 7 and
clang 4. You can pump the warning up to level 5 on gcc to insist on the
attribute, but I think the comments are more readable (and it is not
like we have a problem with false positive comments).

> It would also have been possible to introduce a specific comment
> accepted by gcc 7 instead of the fallthrough attribute for this warning,
> but clang does not have a similar implementation. The macro replaces
> the previous, not uniform, comments and can acts as a comment itself.

Interestingly clang seems to accept -Wimplicit-fallthrough, but I could
not get it to actually trigger a warning, even after removing some of
the existing comments.

What version of clang are you using? I'm certainly puzzled by the
behavior I'm seeing.

> diff --git a/apply.c b/apply.c
> index 321a9fa68..a22fb2881 100644
> --- a/apply.c
> +++ b/apply.c
> @@ -1450,7 +1450,7 @@ static void recount_diff(const char *line, int size, struct fragment *fragment)
>  		switch (*line) {
>  		case ' ': case '\n':
>  			newlines++;
> -			/* fall through */
> +			GIT_FALLTHROUGH;

Ugh, the semi-colon there makes it look like it's actual code. If we go
this route, I wonder if it's worth hiding it inside the macro.

-Peff