Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions

From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Joe Perches <joe@perches.com>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Andy Whitcroft <apw@canonical.com>,
	yuankuiz@codeaurora.org, Linux PM <linux-pm@vger.kernel.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	paulmck@linux.vnet.ibm.com, Ingo Molnar <mingo@kernel.org>,
	Len Brown <len.brown@intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions
Date: Wed, 11 Apr 2018 10:15:02 +0200	[thread overview]
Message-ID: <20180411081502.GJ4082@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180410150011.df9e036f57b5bcac7ac19686@linux-foundation.org>

On Tue, Apr 10, 2018 at 03:00:11PM -0700, Andrew Morton wrote:
> On Tue, 10 Apr 2018 14:53:51 -0700 Joe Perches <joe@perches.com> wrote:
> 
> > On Tue, 2018-04-10 at 14:39 -0700, Andrew Morton wrote:
> > > On Tue, 10 Apr 2018 11:19:54 -0700 Joe Perches <joe@perches.com> wrote:
> > > 
> > > > A struct with a bool member can have different sizes on various
> > > > architectures because neither bool size nor alignment is standardized.
> > > 
> > > What's wrong with bools in structs?
> > 
> > See above.
> 
> Yeah, but so what?  `long' has different sizes on different
> architectures too.

Right, so we have ILP32/LP64 for all our 32/64 bit archs respectively.
So only 2 possible variations to consider, and if you know your bitness
you know your layout.

(+- some really unfortunate alignment exceptions, the worst of which
Arnd recently removed, hooray!)

But neither says anything about sizeof(_Bool), and the standard leaves
it undefined and only mandates it is large enough to store either 0 or
1 (and I suspect this vagueness is because there are architectures that
either have no byte addressibility or it's more expensive than word
addressibility).

Typically GCC chooses a single byte to represent _Bool, but there are no
guarantees. This means that when you care about structure layout (as we
all really should) things go wobbly when you use _Bool.

If GCC were to guarantee a 1 byte _Bool for all Linux ABIs we could
reconsider.