On Thu, 5 Nov 2009 10:50:20 am Stephen Rothwell wrote:

Hi Rusty,

On Tue, 20 Oct 2009 14:15:33 +1030 Rusty Russell [off-list ref] wrote:

quoted

+#ifndef __OPTIMIZE__
+#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
+#else
+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
+#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
+

I decided to try this in linux-next, but an x86_64 allmodconfig build
gave this (gcc 4.4.0):

ERROR: "__build_bug_on_failed" [drivers/net/virtio_net.ko] undefined!
ERROR: "__build_bug_on_failed" [drivers/block/virtio_blk.ko] undefined!

I assume that this is caused by the "MAYBE_BUILD_BUG_ON(fbit >= 32)" in
virtio_has_feature() (in include/linux/virtio_config.h) which is called
all over the place.  Unfortunately, virtio_has_feature() gets uninlined
in those two files ...

Huh?  virtio_has_feature does:

	if (__builtin_constant_p(fbit))
		BUILD_BUG_ON(fbit >= 32);
	else
		BUG_ON(fbit >= 32);

So, if it's not a constant, gcc should throw away that first branch.  If it
is, it should optimize it away (and no, these are not real bugs AFAICT).

I did a 4.3.3 allmodconfig with this patch on 64-bit a few days back and all
was fine.  Will try 4.4.0 now.

Thanks,
Rusty.