[PATCH] compiler/compiler-gcc: Add __fallthrough

[PATCH] compiler/compiler-gcc: Add __fallthrough

[Joe Perches]

gcc v7.0 can warn on missing break statements from case labels
using a special __attribute__((fallthrough))__ marker.

Add a __fallthrough convenience macro for gcc versions >= 7 and
make the generic use of __fallthrough a no-op.

Signed-off-by: Joe Perches <joe@perches.com>
---
 include/linux/compiler-gcc.h | 7 +++++++
 include/linux/compiler.h     | 6 ++++++
 2 files changed, 13 insertions(+)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index d5e1fedbad24..6af8d6448f10 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -304,6 +304,13 @@
 #define __no_sanitize_address __attribute__((no_sanitize_address))
 #endif
 
+#if GCC_VERSION >= 70000
+/*
+ * Tell the compiler not to warn when a switch/case fallthrough marker exists
+ */
+#define __fallthrough __attribute__ ((fallthrough))
+#endif
+
 #endif	/* gcc version >= 40000 specific checks */
 
 #if !defined(__noclone)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 6e8e160b1e4b..16b6efc877f4 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -477,6 +477,12 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 #define __assume_aligned(a, ...)
 #endif
 
+/*
+ * switch/case fallthrough checking
+ */
+#ifndef __fallthrough
+#define __fallthrough
+#endif
 
 /* Are two types/vars the same type (ignoring qualifiers)? */
 #ifndef __same_type
-- 
2.10.0.rc2.1.g053435c

Re: [PATCH] compiler/compiler-gcc: Add __fallthrough

[Arnaldo Carvalho de Melo]

Em Fri, Feb 10, 2017 at 09:08:35AM -0800, Joe Perches escreveu:
> gcc v7.0 can warn on missing break statements from case labels
> using a special __attribute__((fallthrough))__ marker.
> 
> Add a __fallthrough convenience macro for gcc versions >= 7 and
> make the generic use of __fallthrough a no-op.

Can you state in the log message were this idea came from? Say,
something like:

"This was introduced in the tools/include/linux/compiler.h, where it was
first noticed while buildint tools/perf/ in a Fedora Rawhide
environment."

BTW, I added this to my tree:

https://git.kernel.org/cgit/linux/kernel/git/acme/linux.git/commit/?h=perf/core&id=e058554b1418e00864f1702c84bfa1d7cf314627

tools include: Introduce linux/compiler-gcc.h

To match the kernel headers structure, setting up things that are
specific to gcc or to some specific version of gcc.

It gets included by linux/compiler.h when gcc is the compiler being
used.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Joe Perches <joe@perches.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-pi5mxx176lotfce90rn17j88@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Thanks,

- Arnaldo
 
> Signed-off-by: Joe Perches <joe@perches.com>
> ---
>  include/linux/compiler-gcc.h | 7 +++++++
>  include/linux/compiler.h     | 6 ++++++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index d5e1fedbad24..6af8d6448f10 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -304,6 +304,13 @@
>  #define __no_sanitize_address __attribute__((no_sanitize_address))
>  #endif
>  
> +#if GCC_VERSION >= 70000
> +/*
> + * Tell the compiler not to warn when a switch/case fallthrough marker exists
> + */
> +#define __fallthrough __attribute__ ((fallthrough))
> +#endif
> +
>  #endif	/* gcc version >= 40000 specific checks */
>  
>  #if !defined(__noclone)
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 6e8e160b1e4b..16b6efc877f4 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -477,6 +477,12 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>  #define __assume_aligned(a, ...)
>  #endif
>  
> +/*
> + * switch/case fallthrough checking
> + */
> +#ifndef __fallthrough
> +#define __fallthrough
> +#endif
>  
>  /* Are two types/vars the same type (ignoring qualifiers)? */
>  #ifndef __same_type
> -- 
> 2.10.0.rc2.1.g053435c

Re: [PATCH] compiler/compiler-gcc: Add __fallthrough

[Joe Perches]

On Fri, 2017-02-10 at 14:18 -0300, Arnaldo Carvalho de Melo wrote:
> Em Fri, Feb 10, 2017 at 09:08:35AM -0800, Joe Perches escreveu:
> > gcc v7.0 can warn on missing break statements from case labels
> > using a special __attribute__((fallthrough))__ marker.
> > 
> > Add a __fallthrough convenience macro for gcc versions >= 7 and
> > make the generic use of __fallthrough a no-op.
> 
> Can you state in the log message were this idea came from? Say,
> something like:
> 
> "This was introduced in the tools/include/linux/compiler.h, where it was
> first noticed while buildint tools/perf/ in a Fedora Rawhide
> environment."

The request is from at least 2002.

Maybe Andrew could add a link like:

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=7652

if he applies it.