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

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Message-ID: <alpine.DEB.2.20.1804190705580.2830@hadrien>
Date:   Thu, 19 Apr 2018 07:16:11 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Joe Perches <joe@...ches.com>
cc:     Julia Lawall <julia.lawall@...6.fr>, 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 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@...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;
> };

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

>
>