Return-Path: Received: from mail-wr1-f65.google.com ([209.85.221.65]:34187 "EHLO mail-wr1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727688AbeJVB3i (ORCPT ); Sun, 21 Oct 2018 21:29:38 -0400 From: Miguel Ojeda To: Greg Kroah-Hartman Cc: linux-kernel@vger.kernel.org, Miguel Ojeda , Dan Carpenter , Andreas Dilger , Masahiro Yamada , Michal Marek , Steven Rostedt , Mauro Carvalho Chehab , Olof Johansson , Konstantin Ryabitsev , "David S . Miller" , Andrey Ryabinin , Kees Cook , Thomas Gleixner , Ingo Molnar , Paul Lawrence , Sandipan Das , Andrey Konovalov , David Woodhouse , Will Deacon , Philippe Ombredanne , Paul Burton , David Rientjes , Willy Tarreau , Martin Sebor , Christopher Li , Jonathan Corbet , Theodore Ts'o , Geert Uytterhoeven , Rasmus Villemoes , Joe Perches , Arnd Bergmann , Dominique Martinet , Stefan Agner , Luc Van Oostenryck , Nick Desaulniers , Andrew Morton , Linus Torvalds , linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org, linux-sparse@vger.kernel.org, linux-kbuild@vger.kernel.org Subject: [PATCH 1/2] Compiler Attributes: add support for __fallthrough (gcc >= 7.1) Date: Sun, 21 Oct 2018 19:14:13 +0200 Message-Id: <20181021171414.22674-2-miguel.ojeda.sandonis@gmail.com> In-Reply-To: <20181021171414.22674-1-miguel.ojeda.sandonis@gmail.com> References: <20181021171414.22674-1-miguel.ojeda.sandonis@gmail.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: >From the GCC manual: fallthrough The fallthrough attribute with a null statement serves as a fallthrough statement. It hints to the compiler that a statement that falls through to another case label, or user-defined label in a switch statement is intentional and thus the -Wimplicit-fallthrough warning must not trigger. The fallthrough attribute may appear at most once in each attribute list, and may not be mixed with other attributes. It can only be used in a switch statement (the compiler will issue an error otherwise), after a preceding statement and before a logically succeeding case label, or user-defined label. https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html Currently, most of the kernel uses fallthrough comments to silence the -Wimplicit-fallthrough warnings (enabled by -Wextra, in W=1). However, C++17 standarized an "statement attribute" (the first of its kind) to deal with this: [[fallthrough]] is meant to be a new control keyword in the form of an extension. In C mode, GCC supports the __fallthrough__ attribute since 7.1, the same time the warning and the comment parsing were introduced. While comment parsing is a good idea to deal with old codebases that used such a comment as documentation for humans, the best solution is to use the attribute: * It is a "real" part of the AST (=> better for tooling). * It does not follow arbitrary rules for parsing (e.g. regexps for the comment parsing). * It may even become standarized in C as well: there are ongoing proposals to import some C++ standard attributes into the C standard, e.g. for fallthrough: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2268.pdf On top of that, it is also a better solution for the kernel, because: * We can actually use a #define for it like for the rest of attributes/extensions, which is not possible with a comment, so that its naming/usage is consistent across the entire kernel. * Whenever the migration from the comments to the attribute is complete, we may increase the level of the GCC warning up to 5, i.e. comments will not longer be considered for warning surpression: only the attribute must be used. This would enforce consistency by leveraging the compiler directly (instead of enforcing it with other tools). * Further into the future, we can consider moving the warning up to W=0 or even making it an error. It is worth noting that clang >= 3.2 supports the warning and the attribute, but only in C++ mode (and it is not enabled by -Wall/-Wextra/-Wpedantic like in gcc). Hopefully, they will also support it for C as well. Further, icc >= 18 does not seem to know anything about the warning; except that it accepts (i.e. ignores) [[fallthrough]] in C++17 mode (to be conformant, probably). Link: https://lore.kernel.org/lkml/20181017062255.oiu44y4zuuwilan3@mwanda/ Suggested-by: Dan Carpenter Signed-off-by: Miguel Ojeda --- include/linux/compiler_attributes.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h index 6b28c1b7310c..9e2153f85462 100644 --- a/include/linux/compiler_attributes.h +++ b/include/linux/compiler_attributes.h @@ -32,6 +32,7 @@ # define __GCC4_has_attribute___assume_aligned__ (__GNUC_MINOR__ >= 9) # define __GCC4_has_attribute___designated_init__ 0 # define __GCC4_has_attribute___externally_visible__ 1 +# define __GCC4_has_attribute___fallthrough__ 0 # define __GCC4_has_attribute___noclone__ 1 # define __GCC4_has_attribute___optimize__ 1 # define __GCC4_has_attribute___nonstring__ 0 @@ -133,6 +134,23 @@ # define __visible #endif +/* + * Currently, most of the kernel uses fallthrough comments to silence + * the -Wimplicit-fallthrough warnings (enabled by -Wextra, in W=1). + * For new instances, please use this attribute instead. + * + * Optional: only supported since gcc >= 7.1 + * Optional: not supported by clang + * Optional: not supported by icc + * + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#index-fallthrough-statement-attribute + */ +#if __has_attribute(__fallthrough__) +# define __fallthrough __attribute__((__fallthrough__)) +#else +# define __fallthrough +#endif + /* * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-format-function-attribute * clang: https://clang.llvm.org/docs/AttributeReference.html#format -- 2.17.1