From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ulrich Drepper Subject: Re: [PATCH 01/18] flag parameters: helper function Date: Mon, 05 May 2008 19:39:34 -0700 Message-ID: <481FC4E6.5060206@redhat.com> References: <200805050342.m453gkY4029814@devserv.devel.redhat.com> <20080505185103.b9299541.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, davidel@xmailserver.org, mtk.manpages@gmail.com, torvalds@linux-foundation.org To: Andrew Morton Return-path: 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 In-Reply-To: <20080505185103.b9299541.akpm@linux-foundation.org> Sender: netdev-owner@vger.kernel.org List-ID: -----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 t= o 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 ha= ving > 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. - -- =E2=9E=A7 Ulrich Drepper =E2=9E=A7 Red Hat, Inc. =E2=9E=A7 444 Castro S= t =E2=9E=A7 Mountain View, CA =E2=9D=96 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (GNU/Linux) iD8DBQFIH8Tl2ijCOnn/RHQRAm+PAKDEArdgKWXLAMzzfYQ3Q8XzbJdWmgCfavq+ +THu/JEISm+IrX7oyhITYVU=3D =3DF2Co -----END PGP SIGNATURE-----