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, yuank...@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 > >
