From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42941) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cUTVw-0004ne-Qt for qemu-devel@nongnu.org; Fri, 20 Jan 2017 02:21:37 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cUTVr-0007VU-T3 for qemu-devel@nongnu.org; Fri, 20 Jan 2017 02:21:36 -0500 Received: from mx1.redhat.com ([209.132.183.28]:47944) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cUTVr-0007V3-KI for qemu-devel@nongnu.org; Fri, 20 Jan 2017 02:21:31 -0500 From: Markus Armbruster 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> Date: Fri, 20 Jan 2017 08:21:28 +0100 In-Reply-To: <20170119212409-mutt-send-email-mst@kernel.org> (Michael S. Tsirkin's message of "Thu, 19 Jan 2017 21:25:49 +0200") Message-ID: <87d1fi42o7.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain 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: "Michael S. Tsirkin" Cc: Paolo Bonzini , Richard Henderson , qemu-devel@nongnu.org, Peter Maydell "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.