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

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Message-ID: <545A4682.2080302@st.com>
Date:	Wed, 5 Nov 2014 16:47:14 +0100
From:	Maxime Coquelin <maxime.coquelin@...com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
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 11/05/2014 12:10 PM, Rasmus Villemoes wrote:
> 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).
That's a good point.

Peter, what is your view?

Kind regards,
Maxime
>
> Rasmus

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