From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hollis Blanchard Subject: Re: [PATCH] BUILD_BUG_ON: make it handle more cases Date: Thu, 22 Oct 2009 14:04:37 -0700 Message-ID: <1256245477.7495.30.camel@slab.austin.ibm.com> References: <4AC1E15502000078000516B5@vpn.id2.novell.com> <200910201142.34006.rusty@rustcorp.com.au> <1256002193.6546.2.camel@slab> <200910201415.34361.rusty@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <200910201415.34361.rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org> Sender: kvm-ppc-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Rusty Russell Cc: Jan Beulich , sfr-3FnU+UHB4dNDw9hX6IcOSA@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, linuxppc-dev-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org, kvm-ppc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-next-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-next.vger.kernel.org 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