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    Subject[PATCH] BUILD_BUG_ON: make it handle more cases
    Date
    BUILD_BUG_ON used to use the optimizer to do code elimination or fail
    at link time; it was changed to first the size of a negative array (a
    nicer compile time error), then (in
    8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.

    bitfields: needs a literal constant at parse time, and can't be put under
    "if (__builtin_constant_p(x))" for example.
    negative array: can handle anything, but if the compiler can't tell it's
    a constant, silently has no effect.
    link time: breaks link if the compiler can't determine the value, but the
    linker output is not usually as informative as a compiler error.

    If we use the negative-array-size method *and* the link time trick,
    we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
    branches, and maximal ability for the compiler to detect errors at
    build time.

    Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

    diff --git a/include/linux/kernel.h b/include/linux/kernel.h
    --- a/include/linux/kernel.h
    +++ b/include/linux/kernel.h
    @@ -683,12 +683,6 @@ struct sysinfo {
    char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */
    };

    -/* Force a compilation error if condition is true */
    -#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
    -
    -/* Force a compilation error if condition is constant and true */
    -#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
    -
    /* Force a compilation error if condition is true, but also produce a
    result (of value 0 and type size_t), so the expression can be used
    e.g. in a structure initializer (or where-ever else comma expressions
    @@ -696,6 +690,33 @@ struct sysinfo {
    #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
    #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))

    +/**
    + * BUILD_BUG_ON - break compile if a condition is true.
    + * @cond: the condition which the compiler should know is false.
    + *
    + * If you have some code which relies on certain constants being equal, or
    + * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
    + * detect if someone changes it.
    + *
    + * The implementation uses gcc's reluctance to create a negative array, but
    + * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
    + * to inline functions). So as a fallback we use the optimizer; if it can't
    + * prove the condition is false, it will cause a link error on the undefined
    + * "__build_bug_on_failed". This error message can be harder to track down
    + * though, hence the two different methods.
    + */
    +#ifndef __OPTIMIZE__
    +#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
    +#else
    +extern int __build_bug_on_failed;
    +#define BUILD_BUG_ON(condition) \
    + do { \
    + ((void)sizeof(char[1 - 2*!!(condition)])); \
    + if (condition) __build_bug_on_failed = 1; \
    + } while(0)
    +#endif
    +#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
    +
    /* Trap pasters of __FUNCTION__ at compile-time */
    #define __FUNCTION__ (__func__)


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