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 54B03C43603 for ; Thu, 19 Dec 2019 16:09:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2A7092146E for ; Thu, 19 Dec 2019 16:09:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726909AbfLSQJA (ORCPT ); Thu, 19 Dec 2019 11:09:00 -0500 Received: from relay.sw.ru ([185.231.240.75]:57942 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726778AbfLSQJA (ORCPT ); Thu, 19 Dec 2019 11:09:00 -0500 Received: from dhcp-172-16-24-104.sw.ru ([172.16.24.104]) by relay.sw.ru with esmtp (Exim 4.92.3) (envelope-from ) id 1ihyM7-0007Tl-FY; Thu, 19 Dec 2019 19:08:51 +0300 Subject: Re: [Q] ld: Does LTO reorder ro variables in two files? To: Alexander Monakov Cc: Peter Zijlstra , gcc-help@gcc.gnu.org, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, linux-kernel@vger.kernel.org, Jan Hubicka References: <157675913272.349305.8936736338884044103.stgit@localhost.localdomain> <20191219131242.GK2827@hirez.programming.kicks-ass.net> <3db1b1c8-0228-56e4-a04f-e8d24cd1dd51@virtuozzo.com> From: Kirill Tkhai Message-ID: Date: Thu, 19 Dec 2019 19:08:50 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19.12.2019 18:45, Alexander Monakov wrote: > [adding Jan Hubicka, GCC LTO maintainer] > > On Thu, 19 Dec 2019, Kirill Tkhai wrote: > >> CC: gcc-help@gcc.gnu.org >> >> Hi, gcc guys, >> >> this thread starts here: https://lkml.org/lkml/2019/12/19/403 >> >> There are two const variables: >> >> struct sched_class idle_sched_class >> and >> struct sched_class fair_sched_class, >> >> which are declared in two files idle.c and fair.c. >> >> 1)In Makefile the order is: idle.o fair.o >> 2)the variables go to the same ro section >> 3)there is no SORT(.*) keyword in linker script. >> >> Is it always true, that after linkage &idle_sched_class < &fair_sched_class? > > No, with LTO you don't have that guarantee. For functions it's more obvious, > GCC wants to analyze functions in reverse topological order so callees are > generally optimized before callers, and it will emit assembly as it goes, so > function ordering with LTO does not give much care to translation unit > boundaries. For variables it's a bit more subtle, GCC partitions all variables > and functions so it can hand them off to multiple compiler processes while doing > LTO. There's no guarantees about order of variables that end up in different > partitions. > > There's __attribute__((no_reorder)) that is intended to enforce ordering even > with LTO (it's documented under "Common function attributes" but works for > global variables as well). Thanks, Alexander! Kirill