Message-ID: <1381998750.22110.101.camel@joe-AO722>
Subject: Re: [PATCH v2 3/9] bitops: Introduce a more generic BITMASK macro
From: Joe Perches <joe@perches.com>
To: Chen Gong <gong.chen@linux.intel.com>
Cc: tony.luck@intel.com, bp@alien8.de, naveen.n.rao@linux.vnet.ibm.com,
        m.chehab@samsung.com, arozansk@redhat.com, linux-acpi@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Thomas Winischhofer <thomas@winischhofer.net>,
        Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>,
        Tomi Valkeinen <tomi.valkeinen@ti.com>
Date: Thu, 17 Oct 2013 01:32:30 -0700
In-Reply-To: <20131017073824.GC14946@gchen.bj.intel.com>
References: <1381935366-11731-1-git-send-email-gong.chen@linux.intel.com>
	 <1381935366-11731-4-git-send-email-gong.chen@linux.intel.com>
	 <1381978749.22110.76.camel@joe-AO722>
	 <20131017063059.GB14946@gchen.bj.intel.com>
	 <1381993136.22110.95.camel@joe-AO722>
	 <20131017073824.GC14946@gchen.bj.intel.com>
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On Thu, 2013-10-17 at 03:38 -0400, Chen Gong wrote:
> the point is we can use GENMASK like GENMASK(end_bit, start_bit) but
> we don't know the value of end_bit/start_bit at compile-time.

True.

The BUILD_BUG_ON idea is just to avoid people using
	GENMASK(1, 2)
instead of
	GENMASK(2, 1)

#define GENMASK(h, l)						\
({								\
	BUILD_BUG_ON(__builtin_constant_p(l) &&			\
		     __builtin_constant_p(h) &&			\
		     (l) > (h));				\
	(((U32_C(1) << ((h) - (l) + 1)) - 1) << (l));		\
})


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