Re: linux-next: tree build failure

[Jan Beulich]

>>> Hollis Blanchard <hollisb@xxxxxxxxxx> 02.10.09 17:48 >>>
>On Wed, 2009-09-30 at 07:35 +0100, Jan Beulich wrote:
>> The one Rusty suggested the other day may help here. I don't like it
>> as a drop-in replacement for BUILD_BUG_ON() though (due to it
>> deferring the error generated to the linking stage), I'd rather view
>> this as an improvement to MAYBE_BUILD_BUG_ON() (which should
>> then be used here).
>
>Can you be more specific?
>
>I have no idea what Rusty suggested where. I can't even guess what

I'm attaching Rusty's response I was referring to.

>MAYBE_BUILD_BUG_ON() is supposed to do (sounds like a terrible name).

Agreed - but presumably better than just deleting the bogus instances
altogether...

Jan
> --- Begin Message ---
> On Wed, 19 Aug 2009 01:29:25 am Jan Beulich wrote:
> > 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.
> > 
> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
>
> We used to use an undefined symbol here; diagnostics are worse but it catches
> more stuff.
>
> Perhaps a hybrid is the way to go?
>
> #ifndef __OPTIMIZE__
> #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> #else
> /* If it's a constant, catch it at compile time, otherwise at link time. */
> extern int __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
>
> Thanks,
> Rusty.
>
> --- End Message ---