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    SubjectRe: [PATCH] BUILD_BUG_ON: make it handle more cases
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    On Tue, 2009-10-20 at 14:15 +1030, Rusty Russell wrote:
    > 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.

    What's the status with this patch ? The lack of it breaks my KVM stuff
    in powerpc...

    Cheers,
    Ben.

    > 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__)
    >
    > _______________________________________________
    > Linuxppc-dev mailing list
    > Linuxppc-dev@lists.ozlabs.org
    > https://lists.ozlabs.org/listinfo/linuxppc-dev




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