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    SubjectRe: [PATCH] BUILD_BUG_ON: make it handle more cases
    On Tue, Oct 20, 2009 at 02:15:33PM +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.
    >
    >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;

    Hmm, what exactly is __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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