From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756624AbZETPqT (ORCPT ); Wed, 20 May 2009 11:46:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753027AbZETPqM (ORCPT ); Wed, 20 May 2009 11:46:12 -0400 Received: from mx2.redhat.com ([66.187.237.31]:59117 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752811AbZETPqL (ORCPT ); Wed, 20 May 2009 11:46:11 -0400 Date: Wed, 20 May 2009 11:42:41 -0400 From: Jason Baron To: Ingo Molnar Cc: linux-kernel@vger.kernel.org, fweisbec@gmail.com, laijs@cn.fujitsu.com, rostedt@goodmis.org, peterz@infradead.org, mathieu.desnoyers@polymtl.ca, jiayingz@google.com, mbligh@google.com, roland@redhat.com, fche@redhat.com Subject: Re: [PATCH 0/3] tracepoints: delay argument evaluation Message-ID: <20090520154241.GC3081@redhat.com> References: <20090520073348.GA12316@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090520073348.GA12316@elte.hu> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 20, 2009 at 09:33:48AM +0200, Ingo Molnar wrote: > hm, this is really a compiler bug in essence - the compiler should > delay the construction of arguments into unlikely branches - if the > arguments are only used there. > > We'd basically open-code a clear-cut: > > trace_block_bio_complete(md->queue, bio); > > into this form: > > trace(block_bio_complete, md->queue, bio); > > .. and this latter form could become moot (and a nuisance) if the > compiler is fixed. > > Have you tried very latest GCC, does it still have this optimization > problem? > > Note that the compiler getting this right would help a _lot_ of > other inline functions in the kernel as well. Arguments only used > within unlikely() branches are quite common. > > Ingo hi, I e-mailed the gcc list, where they suggested using a macro, as I've done. They also suggested filing an enhancement request for this, which I've done: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40207 It seems like they agree with the suggestion. It still might make sense to make this requirement explicit (by adding the extra macro), as the tracepoint off case should really be as optimized as possible. thanks, -Jason