From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:51617) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cUchM-00007E-M3 for qemu-devel@nongnu.org; Fri, 20 Jan 2017 12:10:01 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cUchH-0002NU-N0 for qemu-devel@nongnu.org; Fri, 20 Jan 2017 12:10:00 -0500 Received: from mx1.redhat.com ([209.132.183.28]:58850) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cUchH-0002NC-Gx for qemu-devel@nongnu.org; Fri, 20 Jan 2017 12:09:55 -0500 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> <20170120185551-mutt-send-email-mst@kernel.org> From: Paolo Bonzini Message-ID: <024df752-27f3-88f3-99a3-a7f5e18069b4@redhat.com> Date: Fri, 20 Jan 2017 18:09:52 +0100 MIME-Version: 1.0 In-Reply-To: <20170120185551-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit 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" , Markus Armbruster Cc: qemu-devel@nongnu.org, Peter Maydell , Richard Henderson On 20/01/2017 17:57, Michael S. Tsirkin wrote: > 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. Unnamed bitfields are in C89 and we definitely use unnamed unions. Maybe that was an unnamed struct or scalar. Paolo >>> +#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)