From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 84F32C2D0DB for ; Mon, 20 Jan 2020 17:28:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6296821835 for ; Mon, 20 Jan 2020 17:28:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726942AbgATR2T (ORCPT ); Mon, 20 Jan 2020 12:28:19 -0500 Received: from gate.crashing.org ([63.228.1.57]:54272 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726642AbgATR2T (ORCPT ); Mon, 20 Jan 2020 12:28:19 -0500 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 00KHRXcN008864; Mon, 20 Jan 2020 11:27:33 -0600 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 00KHRW40008862; Mon, 20 Jan 2020 11:27:32 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 20 Jan 2020 11:27:32 -0600 From: Segher Boessenkool To: Christophe Leroy Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , nathanl@linux.ibm.com, arnd@arndb.de, tglx@linutronix.de, vincenzo.frascino@arm.com, luto@kernel.org, x86@kernel.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org Subject: Re: [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation. Message-ID: <20200120172732.GC3191@gate.crashing.org> References: <20200117085851.GS3191@gate.crashing.org> <3027b6d2-47a9-a871-7c52-050a5f9c6ab7@c-s.fr> <20200120151936.GB3191@gate.crashing.org> <4b0e5941-c37e-3c85-3809-45f33ce35657@c-s.fr> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4b0e5941-c37e-3c85-3809-45f33ce35657@c-s.fr> User-Agent: Mutt/1.4.2.3i Sender: linux-mips-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org On Mon, Jan 20, 2020 at 06:08:23PM +0100, Christophe Leroy wrote: > Not easy I think. > > First we have the unavoidable ASM entry function that can't be dropped > because of the CR[SO] bit the set on error or clear on no error and that > can't be done in C. Yup. > In our ASM VDSO, fixed shifts are used, while in generic C VDSO, shifts > are generic and read from the VDSO data. Does that cost more than just a few cycles? > And there is still some funny code generated by GCC (8.1), like: > > 620: 7d 29 3c 30 srw r9,r9,r7 > 624: 21 87 00 20 subfic r12,r7,32 > 628: 7d 07 3c 31 srw. r7,r8,r7 > 62c: 7d 08 60 30 slw r8,r8,r12 > 630: 7d 0b 4b 78 or r11,r8,r9 (This can be done cheaper for fixed shifts, you can use rlwimi then). > 634: 39 40 00 00 li r10,0 > 638: 40 82 00 84 bne 6bc <__c_kernel_clock_gettime+0x114> > 63c: 81 23 00 24 lwz r9,36(r3) > 640: 81 05 00 00 lwz r8,0(r5) > ... > 6bc: 7d 69 5b 78 mr r9,r11 > 6c0: 7c ea 3b 78 mr r10,r7 > 6c4: 7d 2b 4b 78 mr r11,r9 > 6c8: 4b ff ff 74 b 63c <__c_kernel_clock_gettime+0x94> > > This branch to 6bc is totally useless: > - copying r11 into r9 is pointless as r9 is overwritten in 63c > - copying back r9 into r11 is pointless as r11 has not been modified > inbetween. Yeah, huh, how did that happen. > - loading r10 with 0 then overwritting r10 with r7 when r7 is not 0 is > pointless as well, could have directly put the result of srw. in r10. This may be harder to make the compiler do. But the r9/r11 thing suggests you are preventing optimisation somewhere, maybe with some asm? Do you have some small testcase I can compile? Segher