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

[Joe Perches <joe@perches.com>]

On Thu, 2013-10-17 at 09:10 -0700, Tony Luck wrote:
> On Thu, Oct 17, 2013 at 1:55 AM, Joe Perches <joe@perches.com> wrote:
> > It's cost free to add the BUILD_BUG_ON
> > and perhaps you underestimate the runtime
> > bug checking effort,
> 
> This looks OK to me.  Gong: This doesn't stop people from using variables
> as arguments ... they just won't get a check for (h) < (l).

Another possibility is to swap high and low if necessary and
maybe warn when either is negative or too large for the bit width.

#define GENMASK(h, l)							\
({									\
	size_t high = h;						\
	size_t low = l;							\
	BUILD_BUG_ON(__builtin_constant_p(l) &&				\
		     __builtin_constant_p(h) &&				\
		     (l) > (h));					\
	BUILD_BUG_ON(__builtin_constant_p(l) &&				\
		     __builtin_constant_p(h) &&				\
		     ((l) >= BITS_PER_LONG ||				\
		      (h) >= BITS_PER_LONG));				\
	WARN_ONCE((!__builtin_constant_p(l) &&				\
		   ((l) < 0 || (l) > BITS_PER_LONG)) ||			\
		  (!__builtin_constant_p(h) &&				\
		   ((h) < 0 || (h) > BITS_PER_LONG)) ||			\
		  (l) > (h),						\
		  "GENMASK: invalid mask values: l: %u, h: %d\n",	\
		  (l), (h));						\
	if (low > high)							\
		swap(low, high);					\
	(((U32_C(1) << (high - low + 1)) - 1) << low);			\
})

And maybe this should be renamed something like
#define BIT_MASK_RANGE(h, l)
or
#define BIT_MASK_SHIFTED(h, l)