From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthew Wilcox Subject: Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64() Date: Fri, 26 Mar 2010 12:16:41 -0600 Message-ID: <20100326181641.GP3875@parisc-linux.org> References: <20100326144241.8583.95617.stgit@warthog.procyon.org.uk> <28287.1269625325@redhat.com> <20100326175827.GD20055@linux-mips.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from palinux.external.hp.com ([192.25.206.14]:57756 "EHLO mail.parisc-linux.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753060Ab0CZSQm (ORCPT ); Fri, 26 Mar 2010 14:16:42 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Linus Torvalds Cc: Ralf Baechle , David Howells , mingo@elte.hu, tglx@linutronix.de, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org On Fri, Mar 26, 2010 at 11:03:09AM -0700, Linus Torvalds wrote: > On Fri, 26 Mar 2010, Ralf Baechle wrote: > > > > My trusty old 486 book [1] in the remarks about the BSF instruction: > > > > "The documentation on the 80386 and 80486 states that op1 is undefined if > > op2 is 0. In reality the 80386 will leave the value in op1 unchanged. > > The first versions of the 80486 will change op1 to an undefined value. > > Later version again will leave it unchanged." > > > > [1] Die Intel Familie in German language, by Robert Hummel, 1992 > > Ok, that explains my memory of us having tried this, at least. > > But I do wonder if any of the people working for Intel could ask the CPU > architects whether we could depend on the "don't write" for 64-bit mode. > If AMD already documents the don't-touch semantics, and if Intel were to > be ok with documenting it for their 64-bit capable CPU's, we wouldn't then > need to rely on undefined behavior. I'll drop one of them a note. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step."