linux-kernel - Re: [patch 3.5-rc3] mm, mempolicy: fix mbind() to do synchronous migration

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Date:	Thu, 21 Jun 2012 19:22:30 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Rientjes <rientjes@...gle.com>, Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 3.5-rc3] mm, mempolicy: fix mbind() to do synchronous migration

On Thu, Jun 21, 2012 at 6:45 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
>
> And casts to/from bool, perhaps.  To squish the warning we'd do things
> like a_bool = !!a_int.  That generates extra code, but gcc internally
> generates extra code for a_bool = a_int anyway, and a quick test here
> indicates that the generated code is identical (testl/setne).

It *has* to generate extra code. A cast to Bool is very much not at
all like a normal cast. All the traditional C casts just do a pure bit
truncate (or zero/sign extension) keeping the same value.

A cast to bool is totally different. It is exactly the same as "test
against zero" - so it in no way acts like a traditional integer cast
to a one-bit integer.

I'm not 100% sure the use of "bool" is a great idea, and people who
use pointers to bools are crazy mf's (you can break the fundamental
property of bools by assigning random values through the pointer), but
_Bool certainly ahs the _potential_ to be a good thing. The reason I'm
nervous about it is exactly that people get it wrong so easily because
they do *not* act like any other C type (the whole pointer-to-bool
thing being one example of people doing bad things - I personally
would be much happier if _Bool acted more like a one-bit bitfield and
could not have its address taken).

            Linus
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