[XEN PATCH v2 1/3] xen: introduce STATIC_ASSERT_UNREACHABLE()

[Federico Serafini <federico.serafini@bugseng.com>]

Introduce macro STATIC_ASSERT_UNREACHABLE() to check that a program
point is considered unreachable by the static analysis performed by the
compiler.

The use of such macro will lead to one of the following outcomes:
- the program point identified by the macro is considered unreachable,
  then the compiler removes the macro;
- the program point identified by the macro is not considered
  unreachable, then the compiler does not remove the macro, which will
  lead to a failure in the build process caused by an assembler error.

Signed-off-by: Federico Serafini <federico.serafini@bugseng.com>
---
Changes in v2:
- removed constraint about optimization level -O0;
- use capital letters for macro name;
- add missing blanks;
- remove stray semicolon;
- cite the assertion failure in the error message.
---
 xen/include/xen/compiler.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index 16d554f2a5..062f54449c 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -64,6 +64,13 @@
 # define fallthrough        do {} while (0)  /* fallthrough */
 #endif
 
+/*
+ * Add the following macro to check that a program point is considered
+ * unreachable by the static analysis performed by the compiler.
+ */
+#define STATIC_ASSERT_UNREACHABLE() \
+    asm ( ".error \"static assertion failed: unreachable\"" )
+
 #ifdef __clang__
 /* Clang can replace some vars with new automatic ones that go in .data;
  * mark all explicit-segment vars 'used' to prevent that. */
-- 
2.34.1