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    SubjectRe: [PATCH] BUILD_BUG_ON: make it handle more cases
    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.
    > 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 <>

    Thanks Rusty, this indeed fixes the problem.

    Acked-by: Hollis Blanchard <>

    Hollis Blanchard
    IBM Linux Technology Center

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