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

[Rasmus Villemoes]

On Tue, Nov 04 2014, Maxime COQUELIN <maxime.coquelin@xxxxxx> 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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