From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754390AbYIEPF0 (ORCPT ); Fri, 5 Sep 2008 11:05:26 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753391AbYIEPFR (ORCPT ); Fri, 5 Sep 2008 11:05:17 -0400 Received: from one.firstfloor.org ([213.235.205.2]:42978 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753357AbYIEPFQ (ORCPT ); Fri, 5 Sep 2008 11:05:16 -0400 Date: Fri, 5 Sep 2008 17:08:33 +0200 From: Andi Kleen To: Arjan van de Ven Cc: Bernd Schubert , linux-kernel@vger.kernel.org, Andi Kleen Subject: Re: frame unwinder patches Message-ID: <20080905150833.GU18288@one.firstfloor.org> References: <200809051552.47929.bs@q-leap.de> <20080905071337.3373ab86@infradead.org> <200809051648.52767.bs@q-leap.de> <20080905075703.6041650e@infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080905075703.6041650e@infradead.org> User-Agent: Mutt/1.4.2.1i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > to be honest, on 64 bit the overhead is quite small (the extra > instructions it adds are optimized for by the modern cpus that you use The pipeline dependency stalls yes, the icache/decode overhead no. Also CONFIG_FRAME_POINTER currently enables -fno-sibling-calls which generates significantly worse code for a lot of common kernel constructs. > in the systems you're selling); on 32 bit the overhead is.. well a > little bigger but not THAT much. yes it loses a register for the > compiler to use, but no it's not a general purpose register, and with %rbp is a general purpose register. In fact it's even better than a general purpose register because it has often shorter encoding than the other registers, but is as versatile. > the register renaming that today's cpus do, I'd be surprised if you > could see anything significant. Even on modern CPUs you can measure it in macro benchmarks, at least that was the state last time that was investigated. On older CPUs without the magic hardware it was even more significant. Also there are even new CPUs like Atom which don't have the magic hardware. -Andi