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Commit 1399ff86 authored by David Daney's avatar David Daney Committed by Linus Torvalds
Browse files

kernel.h: add BUILD_BUG() macro 



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: default avatarDavid Daney <david.daney@cavium.com>
Acked-by: default avatarDavid Rientjes <rientjes@google.com>
Cc: DM <dm.n9107@gmail.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Acked-by: default avatarDavid Howells <dhowells@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1e16a539

include/linux/compiler-gcc4.h

+1 −0
Original line number Diff line number Diff line
@@ -29,6 +29,7 @@ 
   the kernel context */

#define __cold			__attribute__((__cold__))



#define __linktime_error(message) __attribute__((__error__(message)))



#if __GNUC_MINOR__ >= 5

/*

include/linux/compiler.h

+3 −1
Original line number Diff line number Diff line
@@ -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(),

include/linux/kernel.h

+16 −0
Original line number Diff line number Diff line
@@ -665,6 +665,7 @@ 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() (0)

#else /* __CHECKER__ */



/* Force a compilation error if a constant expression is not a power of 2 */
@@ -703,6 +704,21 @@ 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 */