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    SubjectRe: [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@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;
    > };

    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

    >
    >

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