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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 AEF4BC433E6 for ; Mon, 8 Mar 2021 05:40:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7FC6964FAE for ; Mon, 8 Mar 2021 05:40:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234586AbhCHFj0 (ORCPT ); Mon, 8 Mar 2021 00:39:26 -0500 Received: from smtprelay0251.hostedemail.com ([216.40.44.251]:35518 "EHLO smtprelay.hostedemail.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S234459AbhCHFi5 (ORCPT ); Mon, 8 Mar 2021 00:38:57 -0500 Received: from filter.hostedemail.com (clb03-v110.bra.tucows.net [216.40.38.60]) by smtprelay04.hostedemail.com (Postfix) with ESMTP id 3521118027FAC; Mon, 8 Mar 2021 05:38:52 +0000 (UTC) X-Session-Marker: 6A6F6540706572636865732E636F6D X-HE-Tag: snake11_360cef1276ee X-Filterd-Recvd-Size: 3958 Received: from [192.168.1.159] (unknown [47.151.137.21]) (Authenticated sender: joe@perches.com) by omf07.hostedemail.com (Postfix) with ESMTPA; Mon, 8 Mar 2021 05:38:50 +0000 (UTC) Message-ID: Subject: Re: linux-kernel janitorial RFP: Mark static arrays as const From: Joe Perches To: Julia Lawall Cc: Rasmus Villemoes , kernelnewbies , kernel-janitors , cocci , LKML Date: Sun, 07 Mar 2021 21:38:49 -0800 In-Reply-To: References: <053b06c47f08631675c295b5c893b90be4248347.camel@perches.com> Content-Type: text/plain; charset="ISO-8859-1" User-Agent: Evolution 3.38.1-1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 2021-03-07 at 20:14 +0100, Julia Lawall wrote: > > 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. You really need consider using defconfig so whatever object code does not have tracing/debugging support. For instance, this code with defconfig and analog joystick: Original: $ size drivers/input/joystick/analog.o text data bss dec hex filename 8115 261 224 8600 2198 drivers/input/joystick/analog.o with const: $ size drivers/input/joystick/analog.o text data bss dec hex filename 8179 201 224 8604 219c drivers/input/joystick/analog.o