Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761429AbXHQGmq (ORCPT ); Fri, 17 Aug 2007 02:42:46 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752870AbXHQGme (ORCPT ); Fri, 17 Aug 2007 02:42:34 -0400 Received: from hoboe1bl1.telenet-ops.be ([195.130.137.72]:55825 "EHLO hoboe1bl1.telenet-ops.be" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750814AbXHQGmb (ORCPT ); Fri, 17 Aug 2007 02:42:31 -0400 X-Greylist: delayed 168262 seconds by postgrey-1.27 at vger.kernel.org; Fri, 17 Aug 2007 02:42:31 EDT Date: Fri, 17 Aug 2007 08:42:22 +0200 (CEST) From: Geert Uytterhoeven To: Linus Torvalds Cc: Paul Mackerras , Nick Piggin , Segher Boessenkool , heiko.carstens@de.ibm.com, horms@verge.net.au, linux-kernel@vger.kernel.org, rpjday@mindspring.com, ak@suse.de, netdev@vger.kernel.org, cfriesen@nortel.com, akpm@linux-foundation.org, jesper.juhl@gmail.com, linux-arch@vger.kernel.org, zlynx@acm.org, satyam@infradead.org, clameter@sgi.com, schwidefsky@de.ibm.com, Chris Snook , Herbert Xu , davem@davemloft.net, wensong@linux-vs.org, wjiang@resilience.com Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures In-Reply-To: Message-ID: References: <46C32618.2080108@redhat.com> <20070815234021.GA28775@gondor.apana.org.au> <3694fb2e4ed1e4d9bf873c0d050c911e@kernel.crashing.org> <46C3B50E.7010702@yahoo.com.au> <194369f4c96ea0e24decf8f9197d5bad@kernel.crashing.org> <46C505B2.6030704@yahoo.com.au> <18117.4848.695269.72976@cargo.ozlabs.ibm.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1846 Lines: 49 On Thu, 16 Aug 2007, Linus Torvalds wrote: > On Fri, 17 Aug 2007, Paul Mackerras wrote: > > I'm really surprised it's as much as a few K. I tried it on powerpc > > and it only saved 40 bytes (10 instructions) for a G5 config. > > One of the things that "volatile" generally screws up is a simple > > volatile int i; > > i++; > > which a compiler will generally get horribly, horribly wrong. > > In a reasonable world, gcc should just make that be (on x86) > > addl $1,i(%rip) > > on x86-64, which is indeed what it does without the volatile. But with the > volatile, the compiler gets really nervous, and doesn't dare do it in one > instruction, and thus generates crap like > > movl i(%rip), %eax > addl $1, %eax > movl %eax, i(%rip) > > instead. For no good reason, except that "volatile" just doesn't have any > good/clear semantics for the compiler, so most compilers will just make it > be "I will not touch this access in any way, shape, or form". Including > even trivially correct instruction optimization/combination. Apart from having to fetch more bytes for the instructions (which does matter), execution time is probably the same on modern processors, as they convert the single instruction to RISC-style load, modify, store anyway. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds - 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/