Re: [PATCH v8 net-next 1/3] filter: add Extended BPF interpreter and converter

[H. Peter Anvin]

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 <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>On Mon, Mar 10, 2014 at 6:51 PM, David Miller <davem@xxxxxxxxxxxxx>
>wrote:
>> From: Alexei Starovoitov <ast@xxxxxxxxxxxx>
>> 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

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