Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752940AbbGAEUI (ORCPT ); Wed, 1 Jul 2015 00:20:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46222 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750715AbbGAEUA (ORCPT ); Wed, 1 Jul 2015 00:20:00 -0400 Message-ID: <55936A78.1060206@redhat.com> Date: Tue, 30 Jun 2015 22:20:08 -0600 From: Jeff Law User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: "H. Peter Anvin" , Andy Lutomirski CC: Jakub Jelinek , Andy Lutomirski , gcc@gcc.gnu.org, "linux-kernel@vger.kernel.org" , Linus Torvalds , Ingo Molnar , Thomas Gleixner , Vladimir Makarov Subject: Re: gcc feature request / RFC: extra clobbered regs References: <20150630213736.GQ10247@tucnak.redhat.com> <55930D24.4040105@zytor.com> <55930FB9.2070904@zytor.com> <55931203.7030102@zytor.com> In-Reply-To: <55931203.7030102@zytor.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1985 Lines: 45 On 06/30/2015 04:02 PM, H. Peter Anvin wrote: > On 06/30/2015 02:55 PM, Andy Lutomirski wrote: >> On Tue, Jun 30, 2015 at 2:52 PM, H. Peter Anvin wrote: >>> On 06/30/2015 02:48 PM, Andy Lutomirski wrote: >>>> On Tue, Jun 30, 2015 at 2:41 PM, H. Peter Anvin wrote: >>>>> On 06/30/2015 02:37 PM, Jakub Jelinek wrote: >>>>>> I'd say the most natural API for this would be to allow >>>>>> f{fixed,call-{used,saved}}-REG in target attribute. >>>>> >>>>> Either that or >>>>> >>>>> __attribute__((fixed(rbp,rcx),used(rax,rbx),saved(r11))) >>>>> >>>>> ... just to be shorter. Either way, I would consider this to be >>>>> desirable -- I have myself used this to good effect in a past life >>>>> (*cough* Transmeta *cough*) -- but not a high priority feature. >>>> >>>> I think I mean the per-function equivalent of -fcall-used-reg, so >>>> hpa's "used" suggestion would do the trick. >>>> >>>> I guess that clobbering the frame pointer is a non-starter, but five >>>> out of six isn't so bad. It would be nice to error out instead of >>>> producing "disastrous results", though, if another bad reg is chosen. >>>> (Presumably the PIC register on PIC builds would be an example of >>>> that.) >>>> >>> >>> Clobbering the frame pointer is perfectly fine, as is the PIC register. >>> However, gcc might need to handle them as "fixed" rather than "clobbered". >> >> Hmm. True, I guess, although I wouldn't necessarily expect gcc to be >> able to generate code to call a function like that. >> > > No, but you need to be able to call other functions, or you just push > the issue down one level. For ia32, the PIC register really isn't special anymore. I'd be surprised if you couldn't clobber it. jeff -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/