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, URIBL_BLOCKED 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 A6478C433DB for ; Mon, 8 Mar 2021 05:39:21 +0000 (UTC) Received: from shelob.surriel.com (shelob.surriel.com [96.67.55.147]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id DF428651EE for ; Mon, 8 Mar 2021 05:39:20 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DF428651EE Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=perches.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernelnewbies-bounces@kernelnewbies.org Received: from localhost ([::1] helo=shelob.surriel.com) by shelob.surriel.com with esmtp (Exim 4.94) (envelope-from ) id 1lJ8bZ-0006aL-Uz; Mon, 08 Mar 2021 00:38:57 -0500 Received: from smtprelay0023.hostedemail.com ([216.40.44.23] helo=smtprelay.hostedemail.com) by shelob.surriel.com with esmtps (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94) (envelope-from ) id 1lJ8bY-0006aG-44 for kernelnewbies@kernelnewbies.org; Mon, 08 Mar 2021 00:38:56 -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 Date: Sun, 07 Mar 2021 21:38:49 -0800 In-Reply-To: References: <053b06c47f08631675c295b5c893b90be4248347.camel@perches.com> User-Agent: Evolution 3.38.1-1 MIME-Version: 1.0 Cc: LKML , kernelnewbies , kernel-janitors , cocci , Rasmus Villemoes X-BeenThere: kernelnewbies@kernelnewbies.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Learn about the Linux kernel List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Errors-To: kernelnewbies-bounces@kernelnewbies.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/ | \ > > > > =A0=A0grep -v __initdata | \ > > > > =A0=A0wc -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 conten= ts > > > 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 compi= ler > > 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.c= amel@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 ar= rays > > 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: > = > =A0=A0=A0text data bss dec hex filename > =A0=A022607 10560 320 33487 82cf drivers/input/joystick/analog= .o > = > after adding const: > = > =A0=A0=A0text data bss dec hex filename > =A0=A022728 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 _______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies