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    Subject[PATCH 3.2 008/221] kernel.h: add BUILD_BUG() macro
    3.2.69-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    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 */


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