[PATCH 1/2] kernel.h: Add BUILD_BUG_ON_USED() macro.

[David Daney <ddaney.cavm@gmail.com>]

From: David Daney <david.daney@cavium.com>

We can place this in definitions that we expect the compiler to remove
by dead code elimination.  If this assertion fails, we get a nice
error message at build time.

The GCC function attribute error("message") was added in version 4.3,
so we define a new macro __linktime_error(message) to expand to this
for GCC-4.3 and later.  This will give us an error diagnostic from the
compiler on the line that fails.  For other compilers
__linktime_error(message) expands to nothing, and we have to be
content with a link time error, but at least we will still get a build
error.

BUILD_BUG_ON_USED() expands to the undefined function
__build_bug_on_used_failed() and will fail at link time if the
compiler ever emits code for it.  On GCC-4.3 and later,
attribute((error())) is used so that the failure will be noted at compile
time instead.

Signed-off-by: David Daney <david.daney@cavium.com>
---
 include/linux/compiler-gcc4.h |    1 +
 include/linux/compiler.h      |    4 +++-
 include/linux/kernel.h        |   16 ++++++++++++++++
 3 files changed, 20 insertions(+), 1 deletions(-)

diff --git a/include/linux/compiler-gcc4.h b/include/linux/compiler-gcc4.h
index dfadc96..2f40791 100644
--- a/include/linux/compiler-gcc4.h
+++ b/include/linux/compiler-gcc4.h
@@ -29,6 +29,7 @@
    the kernel context */
 #define __cold			__attribute__((__cold__))
 
+#define __linktime_error(message) __attribute__((__error__(message)))
 
 #if __GNUC_MINOR__ >= 5
 /*
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 320d6c9..4a24354 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -293,7 +293,9 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 #ifndef __compiletime_error
 # define __compiletime_error(message)
 #endif
-
+#ifndef __linktime_error
+# define __linktime_error(message)
+#endif
 /*
  * Prevent the compiler from merging or refetching accesses.  The compiler
  * is also forbidden from reordering successive instances of ACCESS_ONCE(),
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index b93c66e..ebe54515 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -666,6 +666,7 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
 #define BUILD_BUG_ON_ZERO(e) (0)
 #define BUILD_BUG_ON_NULL(e) ((void*)0)
 #define BUILD_BUG_ON(condition)
+#define BUILD_BUG_ON_USED() (0)
 #else /* __CHECKER__ */
 
 /* Force a compilation error if a constant expression is not a power of 2 */
@@ -704,6 +705,21 @@ extern int __build_bug_on_failed;
 		if (condition) __build_bug_on_failed = 1;	\
 	} while(0)
 #endif
+
+/**
+ * BUILD_BUG_ON_USED - break compile if used.
+ *
+ * If you have some code that you expect the compiler to eliminate at
+ * build time, you should use BUILD_BUG_ON_USED to detect if it is
+ * unexpectedly used.
+ */
+#define BUILD_BUG_ON_USED()						\
+	do {								\
+		extern void __build_bug_on_used_failed(void)		\
+			__linktime_error("BUILD_BUG_ON_USED failed");	\
+		__build_bug_on_used_failed();				\
+	} while (0)
+
 #endif	/* __CHECKER__ */
 
 /* Trap pasters of __FUNCTION__ at compile-time */
-- 
1.7.2.3