From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Mon, 10 Dec 2007 15:35:43 +0000 (GMT) Received: from localhost.localdomain ([127.0.0.1]:18831 "EHLO dl5rb.ham-radio-op.net") by ftp.linux-mips.org with ESMTP id S20024672AbXLJPfl (ORCPT ); Mon, 10 Dec 2007 15:35:41 +0000 Received: from denk.linux-mips.net (denk.linux-mips.net [127.0.0.1]) by dl5rb.ham-radio-op.net (8.14.1/8.13.8) with ESMTP id lBAFZNpI019476; Mon, 10 Dec 2007 15:35:24 GMT Received: (from ralf@localhost) by denk.linux-mips.net (8.14.1/8.14.1/Submit) id lBAFZNm5019475; Mon, 10 Dec 2007 15:35:23 GMT Date: Mon, 10 Dec 2007 15:35:23 +0000 From: Ralf Baechle To: "Maciej W. Rozycki" Cc: Kaz Kylheku , linux-mips@linux-mips.org Subject: Re: SiByte 1480 & Branch Likely instructions? Message-ID: <20071210153523.GA19384@linux-mips.org> References: <20071209051450.GA18181@linux-mips.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17 (2007-11-01) Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 17758 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: ralf@linux-mips.org Precedence: bulk X-list: linux-mips On Mon, Dec 10, 2007 at 03:28:52PM +0000, Maciej W. Rozycki wrote: > > > Not really a kernel-related question. I've discovered that GCC 4.1.1 > > > (which I'm not using for kernel compiling, but user space) generates > > > branch likely instructions by default, even though the documentation > > > says that their use is off by default for MIPS32 and MIPS64, because > > > they are considered deprecated. They are documented as obsolete for the > > > Broadcom chips I am working with. > > > > Microarchitecture guys love to hate branch likely. But the deprecation is > > a dream. Binary compatibility will always require those instructions to > > continue to exist so the genie is out of the bottle and so I feel very > > certain to predict that a future MIPS 3 specification will contain branch > > likely. > > We have been there before -- binary compatibility does not preclude > emulation. And I do not mean keeping the MIPS I toys (as they might be > seen these days) running, but serious products deployed commercially, like > newer VAX implementations that kept full binary compatibility with their > predecessors in the area of the some of the more arcane instructions only > by means of emulating them in the OS. It would devastate the performance of some binaries. As an intellectual challenge, how far can you strip down a MIPS implementation and emulate removed instructions in the kernel ;-) Ralf