[PATCH 3.2 008/221] kernel.h: add BUILD_BUG() macro

Ben Hutchings
2015-05-05T02:29:36Z

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

commit 1399ff86f2a2bbacbbe68fa00c5f8c752b344723 upstream.

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() expands to the undefined function __build_bug_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>
Acked-by: David Rientjes <rientjes@google.com>
Cc: DM <dm.n9107@gmail.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/include/linux/compiler-gcc4.h
+++ b/include/linux/compiler-gcc4.h
@@ -29,6 +29,8 @@
the kernel context */
#define __cold __attribute__((__cold__))

+#define __linktime_error(message) __attribute__((__error__(message)))
+
/*
* GCC 'asm goto' miscompiles certain code sequences:
*
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -297,7 +297,9 @@ void ftrace_likely_update(struct ftrace_
#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(),
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -679,6 +679,7 @@ static inline void ftrace_dump(enum ftra
#define BUILD_BUG_ON_ZERO(e) (0)
#define BUILD_BUG_ON_NULL(e) ((void*)0)
#define BUILD_BUG_ON(condition)
+#define BUILD_BUG() (0)
#else /* __CHECKER__ */

/* Force a compilation error if a constant expression is not a power of 2 */
@@ -717,6 +718,21 @@ extern int __build_bug_on_failed;
if (condition) __build_bug_on_failed = 1; \
} while(0)
#endif
+
+/**
+ * BUILD_BUG - break compile if used.
+ *
+ * If you have some code that you expect the compiler to eliminate at
+ * build time, you should use BUILD_BUG to detect if it is
+ * unexpectedly used.
+ */
+#define BUILD_BUG() \
+ do { \
+ extern void __build_bug_failed(void) \
+ __linktime_error("BUILD_BUG failed"); \
+ __build_bug_failed(); \
+ } while (0)
+
#endif /* __CHECKER__ */

/* Trap pasters of __FUNCTION__ at compile-time */