Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Message-ID: <b17a1d0de54ca0db12bd38a0b7fb20a5befb6778.camel@perches.com>
Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool
 member definitions
From:   Joe Perches <joe@perches.com>
To:     Julia Lawall <julia.lawall@lip6.fr>
Cc:     yuankuiz@codeaurora.org, Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Andy Whitcroft <apw@canonical.com>,
        Linux PM <linux-pm@vger.kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        paulmck@linux.vnet.ibm.com, Ingo Molnar <mingo@kernel.org>,
        Len Brown <len.brown@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-pm-owner@vger.kernel.org
Date:   Wed, 18 Apr 2018 21:51:26 -0700
In-Reply-To: <alpine.DEB.2.20.1804190639410.2830@hadrien>
References: <891d4f632fbff5052e11f2d0b6fac35d@codeaurora.org>
          <CAJZ5v0h=Jeb5JjjE9vNQM29xYAy-Ogmx1mycRv-hPJ1+m=edzw@mail.gmail.com>
          <20180410123305.GF4082@hirez.programming.kicks-ass.net>
          <e22fb871b1b7f2fda4b22f3a24e0d7f092eb612c.camel@perches.com>
          <95477c93db187bab6da8a8ba7c57836868446179.camel@perches.com>
          <20180410143950.4b8526073b4e3e34689f68cb@linux-foundation.org>
          <b3d75281c3a7b4b217b612913894b346eaa3288b.camel@perches.com>
          <20180410150011.df9e036f57b5bcac7ac19686@linux-foundation.org>
          <20180411081502.GJ4082@hirez.programming.kicks-ass.net>
          <20180411092959.e666ec443e4d3bb6f43901d7@linux-foundation.org>
          <1c9f185f6086e9d89659f93720a27b660ee17c13.camel@perches.com>
          <alpine.DEB.2.20.1804120816130.3559@hadrien>
          <b2087cac13c3a1d01ebe1a17e66351f3777bc397.camel@perches.com>
          <alpine.DEB.2.20.1804142315300.2750@hadrien>
          <e7cbe275136b86083ff70d9af26f41bf@codeaurora.org>
         <5341b3b1945c4c1dbf7b356b1a7a4bd6ce304287.camel@perches.com>
         <alpine.DEB.2.20.1804190639410.2830@hadrien>
Content-Type: text/plain; charset="ISO-8859-1"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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;
};