From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34681) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cUdKQ-0006zZ-5m for qemu-devel@nongnu.org; Fri, 20 Jan 2017 12:50:23 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cUdKM-0006Aw-Uu for qemu-devel@nongnu.org; Fri, 20 Jan 2017 12:50:22 -0500 Received: from mx1.redhat.com ([209.132.183.28]:48668) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cUdKM-0006AC-NX for qemu-devel@nongnu.org; Fri, 20 Jan 2017 12:50:18 -0500 Date: Fri, 20 Jan 2017 19:50:16 +0200 From: "Michael S. Tsirkin" Message-ID: <20170120194950-mutt-send-email-mst@kernel.org> References: <1484772931-16272-1-git-send-email-mst@redhat.com> <1484772931-16272-4-git-send-email-mst@redhat.com> <87r33zii2m.fsf@dusky.pond.sub.org> <8721518c-5063-bd5b-60a9-3a3752894e74@redhat.com> <8737gfb2dn.fsf@dusky.pond.sub.org> <20170119212409-mutt-send-email-mst@kernel.org> <87d1fi42o7.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87d1fi42o7.fsf@dusky.pond.sub.org> Subject: Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: Paolo Bonzini , Richard Henderson , qemu-devel@nongnu.org, Peter Maydell On Fri, Jan 20, 2017 at 08:21:28AM +0100, Markus Armbruster wrote: > "Michael S. Tsirkin" writes: > > > On Thu, Jan 19, 2017 at 02:33:40PM +0100, Markus Armbruster wrote: > >> Paolo Bonzini writes: > >> > >> > On 19/01/2017 09:12, Markus Armbruster wrote: > >> >> "Michael S. Tsirkin" writes: > >> >> > >> >>> QEMU_BUILD_BUG_ON uses a typedef in order to be safe > >> >>> to use outside functions, but sometimes it's useful > >> >>> to have a version that can be used within an expression. > >> >>> Following what Linux does, introduce QEMU_BUILD_BUG_ON_ZERO > >> >>> that return zero after checking condition at build time. > >> >> > >> >> Following Linux's example makes sense, but I can't help but wonder > >> >> whether we need both QEMU_BUILD_BUG_ON_ZERO() and QEMU_BUILD_BUG_ON(). > >> > > >> > I think so, most notably QEMU_BUILD_BUG_ON was added to C11 as > >> > _Static_assert but QEMU_BUILD_BUG_ON_ZERO wasn't. > >> > >> Okay. > >> > >> > But we can indeed redefine QEMU_BUILD_BUG_ON to > >> > (void)QEMU_BUILD_BUG_ON_ZERO(x) like Linux does, until we add optional > >> > support for _Static_assert. > >> > >> Yes, please. > > > > > > I don't think we can because we use QEMU_BUILD_BUG_ON outside > > any functions. I don't think you can put 0 there. > > Point taken. Still, we should be able to factor out a common core of > the bug condition. > > >> >>> Signed-off-by: Michael S. Tsirkin > >> >>> --- > >> >>> include/qemu/compiler.h | 2 ++ > >> >>> 1 file changed, 2 insertions(+) > >> >>> > >> >>> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h > >> >>> index 2882470..f4cf13b 100644 > >> >>> --- a/include/qemu/compiler.h > >> >>> +++ b/include/qemu/compiler.h > >> >>> @@ -89,6 +89,8 @@ > >> >>> typedef char glue(qemu_build_bug_on__,__LINE__)[(x) ? -1 : 1] \ > >> >>> __attribute__((unused)) > >> >>> > >> >>> +#define QEMU_BUILD_BUG_ON_ZERO(x) (sizeof(int[(x) ? -1 : 1]) - sizeof(int)) > >> > > >> > Linux here uses: > >> > > >> > #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); })) > >> > > >> > and the issue is that sizeof(int[(x) ? -1 : 1]) could be > >> > runtime-evaluated (the type is a variable-length array). > >> > >> Let's copy both macros from Linux. > > > > I prefer our variant, I don't think it's portable to assume that > > sizeof(struct {int:0}) is 0. Besides, Linux code is GPLv2 only and this > > file is 2 or later. > > Use (sizeof(struct { int: X }) - sizeof(struct { int: 0 })) if you care > for portability to lesser compilers. I don't, because we use common > extensions supported by both GCC and Clang all over the place. > > The idea to use bitfield size is not copyrightable, only expressions of > the idea. Right I did that before reading this reply. -- MST