Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755969Ab3I3O4s (ORCPT ); Mon, 30 Sep 2013 10:56:48 -0400 Received: from eddie.linux-mips.org ([78.24.191.182]:60206 "EHLO cvs.linux-mips.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755819Ab3I3O4p (ORCPT ); Mon, 30 Sep 2013 10:56:45 -0400 Date: Mon, 30 Sep 2013 16:56:30 +0200 From: Ralf Baechle To: linux-kernel@vger.kernel.org, Arnd Bergmann , David Howells , Markos Chandras , linux-mips@linux-mips.org, David Daney , Andrew Pinski , John Crispin Subject: Issue with BUG() in asm-gemeric/bug.h if CONFIG_BUG=n Message-ID: <20130930145630.GA14672@linux-mips.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2126 Lines: 77 Lately I received several patches for build issues that only strike if CONFIG_BUG is disabled. Here's a test case extracted from one of them: /* * Definition of BUG taken from asm-generic/bug.h for the CONFIG_BUG=n case */ #define BUG() do {} while(0) int foo(int arg) { int res; if (arg == 1) res = 23; else if (arg == 2) res = 42; else BUG(); return res; } [ralf@h7 ~]$ gcc -O2 -Wall -c bug.c bug.c: In function ‘foo’: bug.c:17:2: warning: ‘res’ may be used uninitialized in this function [-Wmaybe-uninitialized] return res; ^ It's fairly obvious to see what's happening here - GCC doesn't know that the else case can not be reached, thus razorsharply concludes that res may be used uninitialized. There several locations where MIPS - possibly other architectures as well - is affected by this. I think the definition of BUG should be changed to something like #define BUG() unreachable() unreachable() will depending on the compiler being used, expand either into a call to __builtin_unreachable() or where that function is unavailable, into do {} while (1). __builtin_unreachable() was introduce for GCC 4.5.0. This means there'd be minor bloat for antique compilers - but probably even better code generation for compilers supporting __builtin_unreachable(). Ralf Signed-off-by: Ralf Baechle include/asm-generic/bug.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h index 7d10f96..6f78771 100644 --- a/include/asm-generic/bug.h +++ b/include/asm-generic/bug.h @@ -108,7 +108,7 @@ extern void warn_slowpath_null(const char *file, const int line); #else /* !CONFIG_BUG */ #ifndef HAVE_ARCH_BUG -#define BUG() do {} while(0) +#define BUG() unreachable() #endif #ifndef HAVE_ARCH_BUG_ON ----- End forwarded message ----- Ralf -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/