I recently had a BUG_ON() go off spuriously on a gcc 4.0 compiled kernel.
It turns out gcc-4.0 was removing a sign extension while earlier gcc
versions would not.  Thinking this to be a compiler bug, I submitted a
report:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23422

It turns out we need to cast the input in order to tell gcc to sign extend
it.

Thanks to Andrew Pinski for his help on this bug.
Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>