Re: [PATCH] Fixes compile warning with -Wimplicit-fallthrough CFLAGS
- From
Jeff King <peff@peff.net>
- Date
- Jan 23, 2018, 00:07 UTC
- Message-ID
- <20180123000727.GB26357@sigill.intra.peff.net>
- In-Reply-To
- <1516665078-107974-1-git-send-email-gitter.spiros@gmail.com>
On Mon, Jan 22, 2018 at 11:51:18PM +0000, Elia Pinto wrote:
Show 13 quoted lines
> 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.
Show 10 quoted lines
> 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