From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47910) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cU9Wz-00076L-5T for qemu-devel@nongnu.org; Thu, 19 Jan 2017 05:01:25 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cU9Wt-00055p-5Y for qemu-devel@nongnu.org; Thu, 19 Jan 2017 05:01:21 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56674) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cU9Wt-000559-0D for qemu-devel@nongnu.org; Thu, 19 Jan 2017 05:01:15 -0500 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> From: Paolo Bonzini Message-ID: <8721518c-5063-bd5b-60a9-3a3752894e74@redhat.com> Date: Thu, 19 Jan 2017 11:01:11 +0100 MIME-Version: 1.0 In-Reply-To: <87r33zii2m.fsf@dusky.pond.sub.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable 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 , "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org, Richard Henderson , Peter Maydell On 19/01/2017 09:12, Markus Armbruster wrote: > "Michael S. Tsirkin" writes: >=20 >> 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. >=20 > 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. 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. >> 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)) >> =20 >> +#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). Paolo >> #if defined __GNUC__ >> # if !QEMU_GNUC_PREREQ(4, 4) >> /* gcc versions before 4.4.x don't support gnu_printf, so use prin= tf. */ >=20 > More so since your QEMU_BUILD_BUG_ON_ZERO seems easier to understand: n= o > token pasting. >=20 > Anyway, > Reviewed-by: Markus Armbruster >=20