linux-kernel - Re: [PATCH v3] bitops: Fix shift overflow in GENMASK macros

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Date:	Wed, 05 Nov 2014 12:10:45 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Maxime COQUELIN <maxime.coquelin@...com>
Cc:	gong.chen@...ux.intel.com, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	" Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, tytso@....edu,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel@...inux.com, eric.paire@...com
Subject: Re: [PATCH v3] bitops: Fix shift overflow in GENMASK macros

On Tue, Nov 04 2014, Maxime COQUELIN <maxime.coquelin@...com> wrote:

> -#define GENMASK(h, l)		(((U32_C(1) << ((h) - (l) + 1)) - 1) << (l))
> -#define GENMASK_ULL(h, l)	(((U64_C(1) << ((h) - (l) + 1)) - 1) << (l))
> +#define GENMASK(h, l) \
> +	((~0UL >> (BITS_PER_LONG - ((h) - (l) + 1))) << (l))
> +
> +#define GENMASK_ULL(h, l) \
> +	((~0ULL >> (BITS_PER_LONG_LONG - ((h) - (l) + 1))) << (l))

Slightly bikeshedding here, but may I suggest spelling it

  (((~0UL) << (l)) & (~0UL >> (BITS_PER_LONG - 1 - (h))))

? This will ensure the arguments are expanded once each, and will, IMHO,
DTRT if/when l > h (namely, yield 0).

Rasmus
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