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Subject: Re: [RFC][PATCH] A generic boolean (version 6)
From: Nicholas Miell <nmiell@comcast.net>
To: ricknu-0@student.ltu.se
Cc: Arnd Bergmann <arnd.bergmann@de.ibm.com>, linux-kernel@vger.kernel.org,
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Date: Thu, 27 Jul 2006 18:56:32 -0700
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On Fri, 2006-07-28 at 03:29 +0200, ricknu-0@student.ltu.se wrote:
> Citerar Nicholas Miell <nmiell@comcast.net>:
> 
> > On Thu, 2006-07-27 at 21:55 +0200, ricknu-0@student.ltu.se wrote:
> > > Citerar Nicholas Miell <nmiell@comcast.net>:
> > > 
> > > > If _Bool does end up in the user-kernel ABI, be advised that validating
> > > > them will be tricky ("b == true || b == false" or "!!b" won't work), and
> > > 
> > > Why would !!b not work?
> > > I don't think it should end up in the ABI (at least, not yet). Just asking
> > > because I'm curious. :)
> > > 
> > 
> > The compiler knows that "b = !!b;" is a no-op.
> 
> In what gcc version? Using 4.0.2 myself and got that if b equals 12 (using a
> pointer to add the value to the boolean) then !!b equals 1.

gcc version 4.1.1 20060525 (Red Hat 4.1.1-1) compiles:

#include <stdbool.h>
bool validBool(bool b) { return (b == true || b == false); }
bool normalizeBool(bool b) { return !!b; }

to:

validBool:
        movl    $1, %eax
        ret

normalizeBool:
        movzbl  %dil, %eax
        ret

-- 
Nicholas Miell <nmiell@comcast.net>