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 85A21C433E6 for ; Sun, 7 Mar 2021 19:15:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5B0C964F53 for ; Sun, 7 Mar 2021 19:15:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232234AbhCGTO1 (ORCPT ); Sun, 7 Mar 2021 14:14:27 -0500 Received: from mail3-relais-sop.national.inria.fr ([192.134.164.104]:19632 "EHLO mail3-relais-sop.national.inria.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232246AbhCGTOO (ORCPT ); Sun, 7 Mar 2021 14:14:14 -0500 X-IronPort-AV: E=Sophos;i="5.81,230,1610406000"; d="scan'208";a="375022443" Received: from 173.121.68.85.rev.sfr.net (HELO hadrien) ([85.68.121.173]) by mail3-relais-sop.national.inria.fr with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 07 Mar 2021 20:14:12 +0100 Date: Sun, 7 Mar 2021 20:14:11 +0100 (CET) From: Julia Lawall X-X-Sender: jll@hadrien To: Joe Perches cc: Rasmus Villemoes , kernelnewbies , kernel-janitors , cocci , LKML Subject: Re: linux-kernel janitorial RFP: Mark static arrays as const In-Reply-To: Message-ID: References: <053b06c47f08631675c295b5c893b90be4248347.camel@perches.com> User-Agent: Alpine 2.22 (DEB 394 2020-01-19) MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="8323329-1189131414-1615144452=:2930" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --8323329-1189131414-1615144452=:2930 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT On Wed, 3 Mar 2021, Joe Perches wrote: > On Wed, 2021-03-03 at 10:41 +0100, Rasmus Villemoes wrote: > > On 02/03/2021 18.42, Joe Perches wrote: > > > Here is a possible opportunity to reduce data usage in the kernel. > > > > > > $ git grep -P -n '^static\s+(?!const|struct)(?:\w+\s+){1,3}\w+\s*\[\s*\]' drivers/ | \ > > >   grep -v __initdata | \ > > >   wc -l > > > 3250 > > > > > > Meaning there are ~3000 declarations of arrays with what appears to be > > > file static const content that are not marked const. > > > > > > So there are many static arrays that could be marked const to move the > > > compiled object code from data to text minimizing the total amount of > > > exposed r/w data. > > > > You can add const if you like, but it will rarely change the generated > > code. gcc is already smart enough to take a static array whose contents > > are provably never modified within the TU and put it in .rodata: > > At least some or perhaps even most of the time, true, but the gcc compiler > from v5 through at least v10 seems inconsistent about when it does the > appropriate conversion. > > See the example I posted: > https://lore.kernel.org/lkml/6b8b250a06a98ce42120a14824531a8641f5e8aa.camel@perches.com/ > > It was a randomly chosen source file conversion btw, I had no prior > knowledge of whether the text/data use would change. > > I'm unsure about clang consistently moving static but provably const arrays > from data to text. I rarely use clang. At least for v11 it seems to be > better though. I didn't try 10.1. I tried the relevnt drivers in drivers/input/joystick. I got only one driver that changed with gcc 9.3, which was drivers/input/joystick/analog.c. It actually got larger: original: text data bss dec hex filename 22607 10560 320 33487 82cf drivers/input/joystick/analog.o after adding const: text data bss dec hex filename 22728 10816 320 33864 8448 drivers/input/joystick/analog.o This was the only case where bss was not 0, but I don't know if there is a connection. julia --8323329-1189131414-1615144452=:2930--