Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754292AbbGAPbr (ORCPT ); Wed, 1 Jul 2015 11:31:47 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35879 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752995AbbGAPbj (ORCPT ); Wed, 1 Jul 2015 11:31:39 -0400 Date: Wed, 1 Jul 2015 17:31:33 +0200 From: Jakub Jelinek To: Vladimir Makarov Cc: Andy Lutomirski , gcc@gcc.gnu.org, "linux-kernel@vger.kernel.org" , Linus Torvalds , "H. Peter Anvin" , Ingo Molnar , Thomas Gleixner Subject: Re: gcc feature request / RFC: extra clobbered regs Message-ID: <20150701153133.GY10247@tucnak.redhat.com> Reply-To: Jakub Jelinek References: <20150630213736.GQ10247@tucnak.redhat.com> <559405E5.7000405@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <559405E5.7000405@redhat.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2213 Lines: 39 On Wed, Jul 01, 2015 at 11:23:17AM -0400, Vladimir Makarov wrote: > >>(I'm not necessarily suggesting that we do this for the syscall bodies > >>themselves. I want to do it for the entry and exit helpers, so we'd > >>still lose the five cycles in the full fast-path case, but we'd do > >>better in the slower paths, and the slower paths are becoming > >>increasingly important in real workloads.) > >GCC already supports -ffixed-REG, -fcall-used-REG and -fcall-saved-REG > >options, which allow to tweak the calling conventions; but it is per > >translation unit right now. It isn't clear which of these options > >you mean with the extra_clobber. > >I assume you are looking for a possibility to change this to be > >per-function, with caller with a different calling convention having to > >adjust for different ABI callee. To some extent, recent GCC versions > >do that automatically with -fipa-ra already - if some call used registers > >are not clobbered by some call and the caller can analyze that callee, > >it can stick values in such registers across the call. > >I'd say the most natural API for this would be to allow > >f{fixed,call-{used,saved}}-REG in target attribute. > > > > > One consequence of frequent changing calling convention per function or > register usage could be GCC slowdown. RA calculates too many data and it > requires a lot of time to recalculate them after something in the register > usage convention is changed. That is true. i?86/x86_64 is a switchable target, so at least for the case of info computed for the callee with non-standard calling convention such info can be computed just once when the function with such a target attribute would be seen first. But for the caller side, I agree not everything can be precomputed, if we can't use e.g. regsets saved in the callee; as a single function can call different functions with different ABIs. But to some extent we have that already with -fipa-ra, don't we? Jakub -- 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/