Messages in this thread | | | Date | Fri, 20 Apr 2018 09:31:08 +0800 | From | yuankuiz@codeauro ... | Subject | Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions |
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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 >>> >>>> >>>>
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