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

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
......@@ -29,6 +29,7 @@
the kernel context */
#define __cold __attribute__((__cold__))
#define __linktime_error(message) __attribute__((__error__(message)))
#if __GNUC_MINOR__ >= 5
/*
......
......@@ -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(),
......
......@@ -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 */
......
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