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=-5.3 required=3.0 tests=BAYES_00, 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 8E9A7C433DB for ; Tue, 16 Mar 2021 00:05:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4B45E64F69 for ; Tue, 16 Mar 2021 00:05:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233038AbhCPAEz (ORCPT ); Mon, 15 Mar 2021 20:04:55 -0400 Received: from gate.crashing.org ([63.228.1.57]:54045 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231950AbhCPAEX (ORCPT ); Mon, 15 Mar 2021 20:04:23 -0400 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 12FNxmMm004591; Mon, 15 Mar 2021 18:59:48 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 12FNxlbU004590; Mon, 15 Mar 2021 18:59:47 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 15 Mar 2021 18:59:47 -0500 From: Segher Boessenkool To: David Laight Cc: "'Rasmus Villemoes'" , Christophe Leroy , "linuxppc-dev@lists.ozlabs.org" , Paul Mackerras , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] powerpc/vdso32: Add missing _restgpr_31_x to fix build failure Message-ID: <20210315235947.GD16691@gate.crashing.org> References: <20210312022940.GO29191@gate.crashing.org> <023afd0c-dc61-5891-5145-5bcdce8227be@prevas.dk> <14e2cfb8c3f141aaba8fe0fb2d8f1885@AcuMS.aculab.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <14e2cfb8c3f141aaba8fe0fb2d8f1885@AcuMS.aculab.com> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 15, 2021 at 04:38:52PM +0000, David Laight wrote: > From: Rasmus Villemoes > > Sent: 15 March 2021 16:24 > > On 12/03/2021 03.29, Segher Boessenkool wrote: > > > On Tue, Mar 09, 2021 at 06:19:30AM +0000, Christophe Leroy wrote: > > >> With some defconfig including CONFIG_CC_OPTIMIZE_FOR_SIZE, > > >> (for instance mvme5100_defconfig and ps3_defconfig), gcc 5 > > >> generates a call to _restgpr_31_x. > > > > > >> I don't know if there is a way to tell GCC not to emit that call, because at the end we get more > > instructions than needed. > > > > > > The function is required by the ABI, you need to have it. > > > > > > You get *fewer* insns statically, and that is what -Os is about: reduce > > > the size of the binaries. > > > > Is there any reason to not just always build the vdso with -O2? It's one > > page/one VMA either way, and the vdso is about making certain system > > calls cheaper, so if unconditional -O2 could save a few cycles compared > > to -Os, why not? (And if, as it seems, there's only one user within the > > DSO of _restgpr_31_x, yes, the overall size of the .text segment > > probably increases slightly). > > Sometimes -Os generates such horrid code you really never want to use it. > A classic is on x86 where it replaces 'load register with byte constant' > with 'push byte' 'pop register'. > The code is actually smaller but the execution time is horrid. > > There are also cases where -O2 actually generates smaller code. Yes, as with all heuristics it doesn't always work out. But usually -Os is smaller. > Although you may need to disable loop unrolling (often dubious at best) > and either force or disable some function inlining. The cases where GCC does loop unrolling at -O2 always help quite a lot. Or, do you have a counter-example? We'd love to see one. And yup, inlining is hard. GCC's heuristics there are very good nowadays, but any single decision has big effects. Doing the important spots manually (always_inline or noinline) has good payoff. Segher