From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44252) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cUILN-0006El-Un for qemu-devel@nongnu.org; Thu, 19 Jan 2017 14:25:58 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cUILI-0001ed-Sz for qemu-devel@nongnu.org; Thu, 19 Jan 2017 14:25:57 -0500 Received: from mx1.redhat.com ([209.132.183.28]:58768) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cUILI-0001cl-N0 for qemu-devel@nongnu.org; Thu, 19 Jan 2017 14:25:52 -0500 Date: Thu, 19 Jan 2017 21:25:49 +0200 From: "Michael S. Tsirkin" Message-ID: <20170119212409-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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8737gfb2dn.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 , Peter Maydell , qemu-devel@nongnu.org, Richard Henderson 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. > >>> 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. -- MST