From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753325AbeDSFQQ (ORCPT ); Thu, 19 Apr 2018 01:16:16 -0400 Received: from mail2-relais-roc.national.inria.fr ([192.134.164.83]:22867 "EHLO mail2-relais-roc.national.inria.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752792AbeDSFQO (ORCPT ); Thu, 19 Apr 2018 01:16:14 -0400 X-IronPort-AV: E=Sophos;i="5.48,467,1517871600"; d="scan'208";a="323533673" Date: Thu, 19 Apr 2018 07:16:11 +0200 (CEST) From: Julia Lawall X-X-Sender: jll@hadrien To: Joe Perches cc: Julia Lawall , yuankuiz@codeaurora.org, Andrew Morton , Peter Zijlstra , "Rafael J. Wysocki" , Andy Whitcroft , Linux PM , "Rafael J. Wysocki" , Frederic Weisbecker , Thomas Gleixner , paulmck@linux.vnet.ibm.com, Ingo Molnar , Len Brown , Linux Kernel Mailing List , linux-pm-owner@vger.kernel.org Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions In-Reply-To: Message-ID: References: <891d4f632fbff5052e11f2d0b6fac35d@codeaurora.org> <20180410123305.GF4082@hirez.programming.kicks-ass.net> <95477c93db187bab6da8a8ba7c57836868446179.camel@perches.com> <20180410143950.4b8526073b4e3e34689f68cb@linux-foundation.org> <20180410150011.df9e036f57b5bcac7ac19686@linux-foundation.org> <20180411081502.GJ4082@hirez.programming.kicks-ass.net> <20180411092959.e666ec443e4d3bb6f43901d7@linux-foundation.org> <1c9f185f6086e9d89659f93720a27b660ee17c13.camel@perches.com> <5341b3b1945c4c1dbf7b356b1a7a4bd6ce304287.camel@perches.com> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 18 Apr 2018, Joe Perches wrote: > On Thu, 2018-04-19 at 06:40 +0200, Julia Lawall wrote: > > > > On Wed, 18 Apr 2018, Joe Perches wrote: > > > > > On Tue, 2018-04-17 at 17:07 +0800, yuankuiz@codeaurora.org wrote: > > > > Hi julia, > > > > > > > > On 2018-04-15 05:19 AM, Julia Lawall wrote: > > > > > On Wed, 11 Apr 2018, Joe Perches wrote: > > > > > > > > > > > On Thu, 2018-04-12 at 08:22 +0200, Julia Lawall wrote: > > > > > > > On Wed, 11 Apr 2018, Joe Perches wrote: > > > > > > > > On Wed, 2018-04-11 at 09:29 -0700, Andrew Morton wrote: > > > > > > > > > We already have some 500 bools-in-structs > > > > > > > > > > > > > > > > I got at least triple that only in include/ > > > > > > > > so I expect there are at probably an order > > > > > > > > of magnitude more than 500 in the kernel. > > > > > > > > > > > > > > > > I suppose some cocci script could count the > > > > > > > > actual number of instances. A regex can not. > > > > > > > > > > > > > > I got 12667. > > > > > > > > > > > > Could you please post the cocci script? > > > > > > > > > > > > > I'm not sure to understand the issue. Will using a bitfield help if there > > > > > > > are no other bitfields in the structure? > > > > > > > > > > > > IMO, not really. > > > > > > > > > > > > The primary issue is described by Linus here: > > > > > > https://lkml.org/lkml/2017/11/21/384 > > > > > > > > > > > > I personally do not find a significant issue with > > > > > > uncontrolled sizes of bool in kernel structs as > > > > > > all of the kernel structs are transitory and not > > > > > > written out to storage. > > > > > > > > > > > > I suppose bool bitfields are also OK, but for the > > > > > > RMW required. > > > > > > > > > > > > Using unsigned int :1 bitfield instead of bool :1 > > > > > > has the negative of truncation so that the uint > > > > > > has to be set with !! instead of a simple assign. > > > > > > > > > > At least with gcc 5.4.0, a number of structures become larger with > > > > > unsigned int :1. bool:1 seems to mostly solve this problem. The > > > > > structure > > > > > ichx_desc, defined in drivers/gpio/gpio-ich.c seems to become larger > > > > > with > > > > > both approaches. > > > > > > > > [ZJ] Hopefully, this could make it better in your environment. > > > > IMHO, this is just for double check. > > > > > > I doubt this is actually better or smaller code. > > > > > > Check the actual object code using objdump and the > > > struct alignment using pahole. > > > > I didn't have a chance to try it, but it looks quite likely to result in a > > smaller data structure based on the other examples that I looked at. > > I _really_ doubt there is any difference in size between the > below in any architecture > > struct foo { > int bar; > bool baz:1; > int qux; > }; > > and > > struct foo { > int bar; > bool baz; > int qux; > }; > > Where there would be a difference in size is > > struct foo { > int bar; > bool baz1:1; > bool baz2:1; > int qux; > }; > > and > > struct foo { > int bar; > bool baz1; > bool baz2; > > int qux; > }; In the situation of the example there are two bools together in the middle of the structure and one at the end. Somehow, even converting to bool:1 increases the size. But it seems plausible that putting all three bools together and converting them all to :1 would reduce the size. I don't know. The size increase (more than 8 bytes) seems out of proportion for 3 bools. I was able to check around 3000 structures that were not declared with any attributes, that don't declare named types internally, and that are compiled for x86. Around 10% become smaller whn using bool:1, typically by at most 8 bytes. julia > >