linux-kernel - Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions

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Date:   Wed, 18 Apr 2018 21:51:26 -0700
From:   Joe Perches <joe@...ches.com>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     yuankuiz@...eaurora.org, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andy Whitcroft <apw@...onical.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pm-owner@...r.kernel.org
Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool
 member definitions

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@...eaurora.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;
};