From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38889) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cUVhO-0008P0-OH for qemu-devel@nongnu.org; Fri, 20 Jan 2017 04:41:35 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cUVhJ-0000H7-T0 for qemu-devel@nongnu.org; Fri, 20 Jan 2017 04:41:34 -0500 Received: from mx1.redhat.com ([209.132.183.28]:48110) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cUVhJ-0000GD-NJ for qemu-devel@nongnu.org; Fri, 20 Jan 2017 04:41:29 -0500 Date: Fri, 20 Jan 2017 09:41:25 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20170120094124.GB2658@work-vm> References: <1484859998-25074-1-git-send-email-mst@redhat.com> <1484859998-25074-3-git-send-email-mst@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1484859998-25074-3-git-send-email-mst@redhat.com> Subject: Re: [Qemu-devel] [PATCH v3 2/4] compiler: rework BUG_ON using a struct List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org, Peter Maydell , Richard Henderson , Paolo Bonzini * Michael S. Tsirkin (mst@redhat.com) wrote: > There are theoretical concerns that some compilers might not trigger > build failures on attempts to define an array of size -1 and make it a > variable sized array instead. Let rewrite using a struct with a negative > bit field size instead as there are no dynamic bit field sizes. This is > similar to what Linux does. > > Signed-off-by: Michael S. Tsirkin > --- > include/qemu/compiler.h | 9 ++++++--- > 1 file changed, 6 insertions(+), 3 deletions(-) > > diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h > index 7512082..c6f673e 100644 > --- a/include/qemu/compiler.h > +++ b/include/qemu/compiler.h > @@ -85,9 +85,12 @@ > #define typeof_field(type, field) typeof(((type *)0)->field) > #define type_check(t1,t2) ((t1*)0 - (t2*)0) > > -#define QEMU_BUILD_BUG_ON(x) \ > - typedef char glue(qemu_build_bug_on__, __LINE__)[(x) ? -1 : 1] \ > - __attribute__((unused)) > +#define QEMU_BUILD_BUG_ON_STRUCT(x) \ > + struct { \ > + int qemu_build_bug_on : (x) ? -1 : 1; \ > + } The problem with this is it can't be used as an expression, where as your previous version could. I've got a similar case (see previous reply) that needed an expression bug-on that would evaluate to zero. Dave > +#define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \ > + glue(qemu_build_bug_on__, __LINE__) __attribute__((unused)) > > #if defined __GNUC__ > # if !QEMU_GNUC_PREREQ(4, 4) > -- > MST > > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK