From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758843AbZBSMiT (ORCPT ); Thu, 19 Feb 2009 07:38:19 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754369AbZBSMiG (ORCPT ); Thu, 19 Feb 2009 07:38:06 -0500 Received: from cantor2.suse.de ([195.135.220.15]:46914 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753352AbZBSMiE (ORCPT ); Thu, 19 Feb 2009 07:38:04 -0500 Subject: Re: Definition of BUG on x86 From: Petr Tesarik To: Ingo Molnar Cc: "H. Peter Anvin" , Jeremy Fitzhardinge , LKML In-Reply-To: <20090219122211.GE1703@elte.hu> References: <1234975856.15053.16.camel@nathan.suse.cz> <499C4786.5010504@goop.org> <1235043648.15053.35.camel@nathan.suse.cz> <20090219121027.GB1703@elte.hu> <1235045971.15053.42.camel@nathan.suse.cz> <20090219122211.GE1703@elte.hu> Content-Type: text/plain; charset="UTF-8" Organization: SUSE LINUX Date: Thu, 19 Feb 2009 13:38:02 +0100 Message-Id: <1235047082.15053.49.camel@nathan.suse.cz> Mime-Version: 1.0 X-Mailer: Evolution 2.24.1.1 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Ingo Molnar píše v Čt 19. 02. 2009 v 13:22 +0100: > * Petr Tesarik wrote: > > > Ingo Molnar píše v Čt 19. 02. 2009 v 13:10 +0100: > > > * Petr Tesarik wrote: > > > > > > > So, the only method I could invent was using gas macros. It > > > > works but is quite ugly, because it relies on the actual > > > > assembler instruction which is generated by the compiler. Now, > > > > AFAIK gcc has always translated "for(;;)" into a jump to self, > > > > and that with any conceivable compiler options, but I don't > > > > know anything about Intel cc. > > > > > > > +static inline __noreturn void discarded_jmp(void) > > > > +{ > > > > + asm volatile(".macro jmp target\n" > > > > + "\t.purgem jmp\n" > > > > + ".endm\n"); > > > > + for (;;) ; > > > > +} > > > > > > hm, that's very fragile. > > > > > > Why not just: > > > > > > static inline __noreturn void x86_u2d(void) > > > { > > > asm volatile("u2d\n"); > > > } > > > > > > If GCC emits a bogus warning about _that_, then it's a bug in > > > the compiler that should be fixed. > > > > I wouldn't call it a bug. The compiler has no idea about what > > the inline assembly actualy does. So it cannot recognize that > > the ud2 instruction does not return (which BTW might not even > > be the case, depending on the implementation of the Invalid > > Opcode exception). > > No, i'm not talking about the inline assembly. > > I'm talking about the x86_u2d() _inline function_, which has > the __noreturn attribute. > > Shouldnt that be enough to tell the compiler that it ... wont > return? Nope, that's not how it works. You _may_ specify a noreturn attribute to any function (and GCC will honour it AFAICS), but if GCC _thinks_ that the function does return, it will issue the above-mentioned warning: /usr/src/linux-2.6/arch/x86/include/asm/bug.h:10: warning: 'noreturn' function does return And that's what your function will do. :-( Yes, I also thinks that this behaviour is counter-intuitive. Besides, I haven't found a gcc switch to turn this warning off, which would be my next recommendation, since the GCC heuristics is broken, of course. Petr Tesarik