merged thanks!
Mathieu
----- On Jun 7, 2018, at 12:24 PM, Michael Jeanson mjean...@efficios.com wrote:
> See upstream commits :
>
> commit 8c87df457cb58fe75b9b893007917cf8095660a0
> Author: Jan Beulich <jbeul...@novell.com>
> Date: Tue Sep 22 16:43:52 2009 -0700
>
> BUILD_BUG_ON(): fix it and a couple of bogus uses of it
>
> gcc permitting variable length arrays makes the current construct used for
> BUILD_BUG_ON() useless, as that doesn't produce any diagnostic if the
> controlling expression isn't really constant. Instead, this patch makes
> it so that a bit field gets used here. Consequently, those uses where the
> condition isn't really constant now also need fixing.
>
> Note that in the gfp.h, kmemcheck.h, and virtio_config.h cases
> MAYBE_BUILD_BUG_ON() really just serves documentation purposes - even if
> the expression is compile time constant (__builtin_constant_p() yields
> true), the array is still deemed of variable length by gcc, and hence the
> whole expression doesn't have the intended effect.
>
> commit 7ef88ad561457c0346355dfd1f53e503ddfde719
> Author: Rusty Russell <ru...@rustcorp.com.au>
> Date: Mon Jan 24 14:45:10 2011 -0600
>
> BUILD_BUG_ON: make it handle more cases
>
> 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.
>
> This forced us to change some non-constant cases to MAYBE_BUILD_BUG_ON();
> as Jan points out in that commit, it didn't work as intended anyway.
>
> 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.
>
> We also document it thoroughly.
>
> Signed-off-by: Michael Jeanson <mjean...@efficios.com>
> ---
> lib/bug.h | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/lib/bug.h b/lib/bug.h
> index ecd4928..3de40ca 100644
> --- a/lib/bug.h
> +++ b/lib/bug.h
> @@ -21,6 +21,14 @@
> * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301
> USA
> */
>
> +#include <linux/version.h>
> +
> +#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,38))
> +#define LTTNG_BUILD_BUG_ON(cond) BUILD_BUG_ON(cond)
> +#else
> +#define LTTNG_BUILD_BUG_ON(cond) MAYBE_BUILD_BUG_ON(cond)
> +#endif
> +
> /**
> * BUILD_RUNTIME_BUG_ON - check condition at build (if constant) or runtime
> * @condition: the condition which should be false.
> @@ -33,7 +41,7 @@
> #define BUILD_RUNTIME_BUG_ON(condition) \
> do { \
> if (__builtin_constant_p(condition)) \
> - BUILD_BUG_ON(condition); \
> + LTTNG_BUILD_BUG_ON(condition); \
> else \
> BUG_ON(condition); \
> } while (0)
> --
> 2.17.1
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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