Bool vs bitfield are orthogonal, at least under gcc, unless I'm completely out to sea. It is probably not a good idea to create a bitfield when it doesn't buy you anything, lest you generate rmw instructions when byte stores would do.
On March 10, 2014 7:02:18 PM PDT, Linus Torvalds <torva...@linux-foundation.org> wrote: >On Mon, Mar 10, 2014 at 6:51 PM, David Miller <da...@davemloft.net> >wrote: >> From: Alexei Starovoitov <a...@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. > >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. > >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 and can cause problems down the line when >converting from bool to a bitfield (for the afore-mentioned packing >reasons). > >So please don't sell "bool" as some kind of panacea. It has at least >as many problems as it has advantages. > >I would generally suggest that people only use "bool" for function >return types, and absolutely nothing else. Seriously. > > Linus -- Sent from my mobile phone. Please pardon brevity and lack of formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
