From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760795AbYEFCk3 (ORCPT ); Mon, 5 May 2008 22:40:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751956AbYEFCkN (ORCPT ); Mon, 5 May 2008 22:40:13 -0400 Received: from mx1.redhat.com ([66.187.233.31]:49815 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751517AbYEFCkL (ORCPT ); Mon, 5 May 2008 22:40:11 -0400 Message-ID: <481FC4E6.5060206@redhat.com> Date: Mon, 05 May 2008 19:39:34 -0700 From: Ulrich Drepper Organization: Red Hat, Inc. User-Agent: Thunderbird 2.0.0.12 (X11/20080226) MIME-Version: 1.0 To: Andrew Morton CC: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, davidel@xmailserver.org, mtk.manpages@gmail.com, torvalds@linux-foundation.org Subject: Re: [PATCH 01/18] flag parameters: helper function References: <200805050342.m453gkY4029814@devserv.devel.redhat.com> <20080505185103.b9299541.akpm@linux-foundation.org> In-Reply-To: <20080505185103.b9299541.akpm@linux-foundation.org> X-Enigmail-Version: 0.95.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Andrew Morton wrote: > hm, that looks expensive. The compiler will need to generate a deref of m > and rf multiple times around the loop. Copying them into locals does > improve that a lot. There really is no problem. The value is in L1d when it is reused. This is the generated code (%rdi is m): f: 85 17 test %edx,(%rdi) 11: 74 0b je 1e 13: 8b 47 04 mov 0x4(%rdi),%eax 16: 09 01 or %eax,(%rcx) 18: 8b 07 mov (%rdi),%eax 1a: f7 d0 not %eax 1c: 21 c2 and %eax,%edx At address 18 the load will be satisfied from L1d. If you would want to cache the value at address f you'd have to create one more instruction. This really is the best code sequence. The compiler could have chosen to move the value into a register because the array is const. But it didn't. > Also: sorry, but ugh-at-the-naming. We don't *gain* anything from having > idenitifers called f, of, m, n and rf. And we lose quite a lot in > readability and understandability. It would be much nicer to invest a > little bit more typing-time here, IMO. That's Davide's code and I didn't change it because it doesn't really matter. This is a trivial function which doesn't need more than 10 seconds to be understood. If you insist I'll rename the variables and elements but I consider this just busy work. - -- ➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (GNU/Linux) iD8DBQFIH8Tl2ijCOnn/RHQRAm+PAKDEArdgKWXLAMzzfYQ3Q8XzbJdWmgCfavq+ +THu/JEISm+IrX7oyhITYVU= =F2Co -----END PGP SIGNATURE-----