linux-kernel - Re: [PATCH v2 3/9] bitops: Introduce a more generic BITMASK macro

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Date:	Wed, 16 Oct 2013 23:58:56 -0700
From:	Joe Perches <joe@...ches.com>
To:	Chen Gong <gong.chen@...ux.intel.com>
Cc:	tony.luck@...el.com, bp@...en8.de, naveen.n.rao@...ux.vnet.ibm.com,
	m.chehab@...sung.com, arozansk@...hat.com,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Thomas Winischhofer <thomas@...ischhofer.net>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>
Subject: Re: [PATCH v2 3/9] bitops: Introduce a more generic BITMASK macro

On Thu, 2013-10-17 at 02:30 -0400, Chen Gong wrote:
> On Wed, Oct 16, 2013 at 07:59:09PM -0700, Joe Perches wrote:
[]
> > Maybe add a
> > 
> > BUILD_BUG_ON(__builtin_constant_p(l) && __builtin_constant_p(h) && \
> > 	     (h) < (l))
> > 
> No, if so, users can't use variables for this macro.

__builtin_constant_p checks for constants

Built-in Function: int __builtin_constant_p (exp)
        You can use the built-in function __builtin_constant_p to
        determine if a value is known to be constant at compile-time and
        hence that GCC can perform constant-folding on expressions
        involving that value. The argument of the function is the value
        to test. The function returns the integer 1 if the argument is
        known to be a compile-time constant and 0 if it is not known to
        be a compile-time constant. A return of 0 does not indicate that
        the value is not a constant, but merely that GCC cannot prove it
        is a constant with the specified value of the -O option. 
        


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