Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755632Ab3I3Pxy (ORCPT ); Mon, 30 Sep 2013 11:53:54 -0400 Received: from mail-ie0-f177.google.com ([209.85.223.177]:49052 "EHLO mail-ie0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755209Ab3I3Pxv (ORCPT ); Mon, 30 Sep 2013 11:53:51 -0400 Message-ID: <52499E8B.6000702@gmail.com> Date: Mon, 30 Sep 2013 08:53:47 -0700 From: David Daney User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 MIME-Version: 1.0 To: Ralf Baechle CC: linux-kernel@vger.kernel.org, Arnd Bergmann , David Howells , Markos Chandras , linux-mips@linux-mips.org, Andrew Pinski , John Crispin Subject: Re: Issue with BUG() in asm-gemeric/bug.h if CONFIG_BUG=n References: <20130930145630.GA14672@linux-mips.org> In-Reply-To: <20130930145630.GA14672@linux-mips.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2653 Lines: 94 On 09/30/2013 07:56 AM, Ralf Baechle wrote: > 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() > 16304 > 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). The *only* reason we have CONFIG_BUG=n is to reduce code size. Sticking in that empty loop, negates the entire point. IMHO: We should do one of: o Make CONFIG_BUG=y mandatory o Ignore the warnings. o Fix the warning sites so they quit Warning. So I don't think the patch is really an improvement over the status quo. David Daney > > __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/