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Subject[RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use
Date
Reserve the pseudo keyword 'fallthrough' for the ability to convert the
various case block /* fallthrough */ style comments to appear to be an
actual reserved word with the same gcc case block missing fallthrough
warning capability.

All switch/case blocks now must end in one of:

break;
fallthrough;
goto <label>;
return [expression];

fallthough is gcc's __attribute__((__fallthrough__)) which was introduced
in gcc version 7..

fallthrough devolves to an empty "do {} while (0)" if the compiler version
(any version less than gcc 7) does not support the attribute.

Signed-off-by: Joe Perches <joe@perches.com>
---

For allow compilation of a kernel that includes net/sctp, this patch
also requires a rename of the use of fallthrough as a label in
net/sctp/sm_make_chunk.c that was submitted as:

https://lore.kernel.org/lkml/e0dd3af448e38e342c1ac6e7c0c802696eb77fd6.1564549413.git.joe@perches.com/

include/linux/compiler_attributes.h | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 6b318efd8a74..cdf016596659 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -40,6 +40,7 @@
# define __GCC4_has_attribute___noclone__ 1
# define __GCC4_has_attribute___nonstring__ 0
# define __GCC4_has_attribute___no_sanitize_address__ (__GNUC_MINOR__ >= 8)
+# define __GCC4_has_attribute___fallthrough__ 0
#endif

/*
@@ -185,6 +186,22 @@
# define __noclone
#endif

+/*
+ * Add the pseudo keyword 'fallthrough' so case statement blocks
+ * must end with any of these keywords:
+ * break;
+ * fallthrough;
+ * goto <label>;
+ * return [expression];
+ *
+ * gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
+ */
+#if __has_attribute(__fallthrough__)
+# define fallthrough __attribute__((__fallthrough__))
+#else
+# define fallthrough do {} while (0) /* fallthrough */
+#endif
+
/*
* Note the missing underscores.
*
--
2.15.0
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