Messages in this thread | | | Subject | Re: [PATCH v8 net-next 1/3] filter: add Extended BPF interpreter and converter | From | Joe Perches <> | Date | Mon, 10 Mar 2014 20:02:16 -0700 |
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On Mon, 2014-03-10 at 19:02 -0700, Linus Torvalds wrote: > On Mon, Mar 10, 2014 at 6:51 PM, David Miller <davem@davemloft.net> wrote: > > From: Alexei Starovoitov <ast@plumgrid.com> > > Date: Sun, 9 Mar 2014 23:04:02 -0700 > > > >> + unsigned int jited:1; > > > > The C language has a proper type for boolean states, please therefore > > use 'bool', true, and false. > > No, the C standard actually has no such thing. > > In a structure, a bitfield is actually better than bool, because it > takes only one bit. A "bool" takes at least a byte.
Bitfields can also be _Bool and at least for gcc _Bool bitfields are required to be :1.
> Now, in this case it may not be an issue (looks like there are no > other uses that can use the better packing, so bit/byte/word is all > the same), but I really really want to make it clear that it is not at > all true that "bool" is somehow better than a single-bit bitfield. The > bitfield can pack *much* better, and I would actually say that it's > generally a *better* idea to use a bitfield, because you can much more > easily expand on it later by adding other bitfields.
bitfields generate relatively poor code and are frequently disadvantageous due to read-modify-write requirements.
> There are very few actual real advantages to "bool". The magic casting > behavior is arguably an advantage (the implicit cast in assigning to a > bitfield truncates to the low bits, the implicit cast on assignment to > "bool" does a test against zero), but is also quite arguably a > possible source of confusion
Umm. Types are good.
> and can cause problems down the line when > converting from bool to a bitfield (for the afore-mentioned packing > reasons).
I don't see how.
> I would generally suggest that people only use "bool" for function > return types, and absolutely nothing else. Seriously.
I think using bool for function arguments, structure members and variables is good and frequently to mostly is an overall improvement.
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