lkml.org 
[lkml]   [2018]   [Apr]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions
On 2018-04-19 06:42 PM, yuankuiz@codeaurora.org wrote:
> On 2018-04-19 02:48 PM, yuankuiz@codeaurora.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@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;
>>>> };
> [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
>>>
>>>>
>>>>

\
 
 \ /
  Last update: 2018-04-20 03:31    [W:2.618 / U:0.536 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site