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

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Date:	Mon, 10 Mar 2014 20:02:16 -0700
From:	Joe Perches <joe@...ches.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	David Miller <davem@...emloft.net>, ast@...mgrid.com,
	dborkman@...hat.com, Ingo Molnar <mingo@...nel.org>,
	Will Drewry <wad@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Peter Anvin <hpa@...or.com>,
	Hagen Paul Pfeifer <hagen@...u.net>,
	Jesse Gross <jesse@...ira.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <edumazet@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	penberg@....fi, Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v8 net-next 1/3] filter: add Extended BPF interpreter
 and converter

On Mon, 2014-03-10 at 19:02 -0700, Linus Torvalds wrote:
> On Mon, Mar 10, 2014 at 6:51 PM, David Miller <davem@...emloft.net> wrote:
> > From: Alexei Starovoitov <ast@...mgrid.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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