Date:   Fri, 20 Apr 2018 09:31:08 +0800
From:   yuankuiz@...eaurora.org
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Joe Perches <joe@...ches.com>,
        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 2018-04-19 06:42 PM, yuankuiz@...eaurora.org wrote:
> On 2018-04-19 02:48 PM, yuankuiz@...eaurora.org wrote:
>> On 2018-04-19 01:16 PM, Julia Lawall wrote:
>>> 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;
>>>> };
> [ZJ] Even though, two bool:1 are grouped in the #3, finally 4 bytes are 
> padded
>      due for int is the most significant in the type size.
>      At least, they are all the same per x86 and arm based on gcc.(12 
> bytes).
[ZJ] However, #3 could be difference to #4 if compiling it if the size 
of (_Bool)
      is a bigger value(4 bytes maybe available in Alpha EV45 for ex.).
>>> 
>>> 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.
>> [ZJ] Typically, addition saving is due for difference padding.
>>> 
>>> 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.
> [ZJ] As my example, int (*)() requested 8 bytes in x86 arch, then 8
> bytes is similiar to that.
>      While it request 4 bytes in arm arch. Typically, my previous
> example struct can
>      reach to 32 bytes in x86 arch(compared to 40 bytes for original 
> version).
>      Similarly, 20 bytes in arm arch(compared to 24 bytes per original 
> version).
>>> 
>>> julia
>>> 
>>>> 
>>>> 

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