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

[yuankuiz]

On 2018-04-19 06:42 PM, yuankuiz@xxxxxxxxxxxxxx wrote:
> On 2018-04-19 02:48 PM, yuankuiz@xxxxxxxxxxxxxx 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@xxxxxxxxxxxxxx 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
> > >
> > > >