[PATCH v4 08/11] xen/compiler: import 'fallthrough' keyword from linux

[Rahul Singh <rahul.singh@xxxxxxx>]

-Wimplicit-fallthrough warns when a switch case falls through. Warning
can be suppress by either adding a /* fallthrough */ comment, or by
using a null statement: __attribute__ ((fallthrough))

Define the pseudo keyword 'fallthrough' for the ability to convert the
various case block /* fallthrough */ style comments to null statement
"__attribute__((__fallthrough__))"

In C mode, GCC supports the __fallthrough__ attribute since 7.1,
the same time the warning and the comment parsing were introduced.

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: Rahul Singh <rahul.singh@xxxxxxx>
---
Changes in V4:
 - This patch is introduce in this verison.
---
 xen/include/xen/compiler.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index e643e69128..0ec0b4698e 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -33,6 +33,22 @@
 #define unreachable() __builtin_unreachable()
 #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 (!defined(__clang__) && (__GNUC__ >= 7))
+# define fallthrough        __attribute__((__fallthrough__))
+#else
+# define fallthrough        do {} while (0)  /* fallthrough */
+#endif
+
 #ifdef __clang__
 /* Clang can replace some vars with new automatic ones that go in .data;
  * mark all explicit-segment vars 'used' to prevent that. */
-- 
2.17.1