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    SubjectRe: [PATCH] BUILD_BUG_ON: make it handle more cases
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    On 10/20/09, Américo Wang <xiyou.wangcong@gmail.com> wrote:
    > 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?

    Well, we haven't added a definition for it in this patch. I'm sure
    grep will tell you it wasn't defined before hand either. So any
    reference to it is an error - which will be reported at link time.

    >>+#define BUILD_BUG_ON(condition) \
    >>+ do { \
    >>+ ((void)sizeof(char[1 - 2*!!(condition)])); \
    >>+ if (condition) __build_bug_on_failed = 1; \

    If "condition" is known false at compile time, gcc -O will eliminate
    the code which refers to __build_bug_on_failed. If it's not proved to
    be false - it will break the build, which is exactly what we want
    BUILD_BUG_ON to do.

    >>+ } 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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