Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757401AbZGCMl3 (ORCPT ); Fri, 3 Jul 2009 08:41:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756363AbZGCMlT (ORCPT ); Fri, 3 Jul 2009 08:41:19 -0400 Received: from gw1.cosmosbay.com ([212.99.114.194]:44328 "EHLO gw1.cosmosbay.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757159AbZGCMlS (ORCPT ); Fri, 3 Jul 2009 08:41:18 -0400 Message-ID: <4A4DFC17.4090809@cosmosbay.com> Date: Fri, 03 Jul 2009 14:39:51 +0200 From: Eric Dumazet User-Agent: Thunderbird 2.0.0.22 (Windows/20090605) MIME-Version: 1.0 To: Ingo Molnar CC: Eric Dumazet , Linus Torvalds , David Howells , akpm@linux-foundation.org, paulus@samba.org, arnd@arndb.de, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] FRV: Implement atomic64_t References: <20090701144913.GA28172@elte.hu> <20090701164700.29780.15103.stgit@warthog.procyon.org.uk> <4A4D2239.5000602@gmail.com> <4A4D9FC4.1070201@gmail.com> <20090703122710.GC7161@elte.hu> In-Reply-To: <20090703122710.GC7161@elte.hu> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-1.6 (gw1.cosmosbay.com [0.0.0.0]); Fri, 03 Jul 2009 14:39:55 +0200 (CEST) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1662 Lines: 55 Ingo Molnar a ?crit : > * Eric Dumazet wrote: > >> My last suggestion would be : >> >> static inline unsigned long long atomic64_read(const atomic64_t *ptr) >> { >> unsigned long long res; >> >> asm volatile( >> "mov %%ebx, %%eax\n\t" >> "mov %%ecx, %%edx\n\t" >> LOCK_PREFIX "cmpxchg8b %1\n" >> : "=A" (res) >> : "m" (*ptr) >> ); >> return res; >> } >> >> ebx/ecx being read only, and their value can be random, they are >> not even mentioned in asm constraints, so gcc is allowed to keep >> useful values in these registers. >> >> So the following (stupid) example >> >> for (i = 0; i < 10000000; i++) { >> res += atomic64_read(&myvar); >> } >> >> gives : >> xorl %esi, %esi >> .L2: >> mov %ebx, %eax >> mov %ecx, %edx >> lock;cmpxchg8b myvar >> addl %eax, %ecx >> adcl %edx, %ebx >> addl $1, %esi >> cmpl $10000000, %esi >> jne .L2 > > Ok, agreed. We dont want to inline it - cmpxchg8b is way too fat - > but your code above is a valid optimization for the out-of-line > variant as well. So i have applied it as such, will post the whole > atomic64_t series soon, after some testing. I just changed "=A" constraint to "+A" or gcc could use %edx/%eax in "ptr" address computation. I discovered this after crashing my box :) -- 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/