From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48486) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cUcUt-00064k-Pw for qemu-devel@nongnu.org; Fri, 20 Jan 2017 11:57:08 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cUcUq-0003R6-Pk for qemu-devel@nongnu.org; Fri, 20 Jan 2017 11:57:07 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57074) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cUcUq-0003Q3-Jn for qemu-devel@nongnu.org; Fri, 20 Jan 2017 11:57:04 -0500 Date: Fri, 20 Jan 2017 18:57:02 +0200 From: "Michael S. Tsirkin" Message-ID: <20170120185551-mutt-send-email-mst@kernel.org> References: <1484859998-25074-1-git-send-email-mst@redhat.com> <1484859998-25074-3-git-send-email-mst@redhat.com> <87wpdq2n4e.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87wpdq2n4e.fsf@dusky.pond.sub.org> 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: Markus Armbruster Cc: qemu-devel@nongnu.org, Peter Maydell , Richard Henderson , Paolo Bonzini On Fri, Jan 20, 2017 at 08:42:41AM +0100, Markus Armbruster wrote: > "Michael S. Tsirkin" writes: > > > 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 qemu_build_bug_on name space pollution is harmless, but quite > unnecessary: the name can be simply omitted (unnamed bit-field). I have concerns about it's portability though. I remember we had to get rid of unnamed fields in some structs at some point for the sake of some old compiler. > > +#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)