Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933229AbZKXPs6 (ORCPT ); Tue, 24 Nov 2009 10:48:58 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758100AbZKXPs6 (ORCPT ); Tue, 24 Nov 2009 10:48:58 -0500 Received: from mx1.redhat.com ([209.132.183.28]:54273 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758099AbZKXPs5 (ORCPT ); Tue, 24 Nov 2009 10:48:57 -0500 Message-ID: <4B0BFFD0.2080203@redhat.com> Date: Tue, 24 Nov 2009 15:46:24 +0000 From: Andrew Haley User-Agent: Thunderbird 2.0.0.23 (X11/20090825) MIME-Version: 1.0 To: Jakub Jelinek CC: Thomas Gleixner , "H.J. Lu" , rostedt@goodmis.org, Ingo Molnar , "H. Peter Anvin" , LKML , Andrew Morton , Heiko Carstens , feng.tang@intel.com, Peter Zijlstra , Frederic Weisbecker , David Daney , Richard Guenther , gcc , Linus Torvalds Subject: Re: [PATCH][GIT PULL][v2.6.32] tracing/x86: Add check to detect GCC messing with mcount prologue References: <1258694593.22249.1012.camel@gandalf.stny.rr.com> <1258736456.22249.1032.camel@gandalf.stny.rr.com> <4B06EF6F.2050507@redhat.com> <6dc9ffc80911220138y15bfa91agccf5c29f1c30e09a@mail.gmail.com> <4B0972C9.302@redhat.com> <6dc9ffc80911221530t38d83cf6je739743c8d756667@mail.gmail.com> <4B0BF119.4070704@redhat.com> <20091124150604.GJ22813@hs20-bc2-1.build.redhat.com> <4B0BFC84.7070806@redhat.com> <20091124153634.GK22813@hs20-bc2-1.build.redhat.com> In-Reply-To: <20091124153634.GK22813@hs20-bc2-1.build.redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1498 Lines: 30 Jakub Jelinek wrote: > On Tue, Nov 24, 2009 at 03:32:20PM +0000, Andrew Haley wrote: >> Jakub Jelinek wrote: >>> On Tue, Nov 24, 2009 at 03:55:49PM +0100, Thomas Gleixner wrote: >>>>> you should compile your code with -maccumulate-outgoing-args, and there's >>>>> no need to use -mtune=generic. Is that right? >>>> Seems to work. What other side effects has that ? >>> Faster code, significant increase in code size though. >> Does it affect code size when we don't have to realign the stack pointer? > > Yes, a lot. The difference is that -maccumulate-outgoing-args allocates > space for arguments of the callee with most arguments in the prologue, using > subtraction from sp, then to pass arguments uses movl XXX, 4(%esp) etc. > and the stack pointer doesn't usually change within the function (except for > alloca/VLAs). > With -mno-accumulate-outgoing-args args are pushed using push instructions > and stack pointer is constantly changing. Alright. So, it is possible in theory for gcc to generate code that only uses -maccumulate-outgoing-args when it needs to realign SP. And, therefore, we could have a nice option for the kernel: one with (mostly) good code density and never generates the bizarre code sequence in the prologue. Andrew. -- 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/