linux-kernel - Re: [PATCH v2] kbuild: Change fallthrough comments to attributes

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Date:   Mon, 12 Aug 2019 15:40:45 -0700
From:   Joe Perches <joe@...ches.com>
To:     Nathan Huckleberry <nhuck@...gle.com>,
        yamada.masahiro@...ionext.com, michal.lkml@...kovi.net,
        Nathan Chancellor <natechancellor@...il.com>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH v2] kbuild: Change fallthrough comments to attributes

On Mon, 2019-08-12 at 15:14 -0700, Nathan Huckleberry wrote:
> Clang does not support the use of comments to label
> intentional fallthrough. This patch replaces some uses
> of comments to attributesto cut down a significant number
> of warnings on clang (from ~50000 to ~200). Only comments
> in commonly used header files have been replaced.
> 
> Since there is still quite a bit of noise, this
> patch moves -Wimplicit-fallthrough to
> Makefile.extrawarn if you are compiling with
> clang.

Unmodified clang does not emit this warning without a patch.

> diff --git a/Makefile b/Makefile
[]
> @@ -846,7 +846,11 @@ NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
>  KBUILD_CFLAGS += -Wdeclaration-after-statement
>  
>  # Warn about unmarked fall-throughs in switch statement.
> +# If the compiler is clang, this warning is only enabled if W=1 in
> +# Makefile.extrawarn
> +ifndef CONFIG_CC_IS_CLANG
>  KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,)
> +endif

It'd be better to remove CONFIG_CC_IS_CLANG everywhere
eventually as it adds complexity and makes .config files
not portable to multiple systems.

> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
[]
> @@ -253,4 +253,8 @@
>   */
>  #define __weak                          __attribute__((__weak__))
>  
> +#if __has_attribute(fallthrough)
> +#define __fallthrough                   __attribute__((fallthrough))

This should be __attribute__((__fallthrough__))

And there is still no agreement about whether this should
be #define fallthrough or #define __fallthrough

https://lore.kernel.org/patchwork/patch/1108577/

> diff --git a/include/linux/jhash.h b/include/linux/jhash.h
[]
> @@ -86,19 +86,43 @@ static inline u32 jhash(const void *key, u32 length, u32 initval)
[]
> +	case 12:
> +		c += (u32)k[11]<<24;
> +		__fallthrough;

You might consider trying out the scripted conversion tool
attached to this email:

https://lore.kernel.org/lkml/61ddbb86d5e68a15e24ccb06d9b399bbf5ce2da7.camel@perches.com/