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

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Message-ID: <1381998750.22110.101.camel@joe-AO722>
Date:	Thu, 17 Oct 2013 01:32:30 -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 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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