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

[Federico Serafini <federico.serafini@xxxxxxxxxxx>]

Introduce macro static_asser_unreachable() to check that a program
point is considered unreachable by the static analysis performed by the
compiler, even at optimization level -O0.

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@xxxxxxxxxxx>
---
 xen/include/xen/compiler.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index 16d554f2a5..ad0520f5d4 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -64,6 +64,14 @@
 # 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,
+ * even at optimization level -O0.
+ */
+#define static_assert_unreachable() \
+    asm(".error \"unreachable program point reached\"");
+
 #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