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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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