From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Darren Jenkins" Subject: Re: sparse using insane amounts of memory Date: Sat, 10 Mar 2007 16:05:08 +1100 Message-ID: <82faac5b0703092105x75149bcdx5813e18f96db8004@mail.gmail.com> References: <1173319356.3546.54.camel@johannes.berg> <1173375270.15842.24.camel@dv> <1173375791.3248.37.camel@johannes.berg> <200703081908.40997.IvDoorn@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Content-Disposition: inline Sender: linux-wireless-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Linus Torvalds Cc: Ivo van Doorn , Johannes Berg , Pavel Roskin , linux-wireless-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-sparse-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-sparse@vger.kernel.org On 3/9/07, Linus Torvalds wrote: > > > On Thu, 8 Mar 2007, Linus Torvalds wrote: > > > > To check a value for being a nice range of consecutive bits, you can > > simply do: > > > > #define is_power_of_two(x) (!((x) & ((x)-1))) There is already an inline for this in log2.h /* * Determine whether some value is a power of two, where zero is * *not* considered a power of two. */ static inline __attribute__((const)) bool is_power_of_2(unsigned long n) { return (n != 0 && ((n & (n - 1)) == 0)); } > - 0 is special, and is generally considered to be a power of two (and > this is more fundamental than you'd think: it's not just fall-out from > the particular expression chosen, it is fundamentally *required* to > handle overflow, and you can think of 0 as 2**x, x > wordsize if that > makes you more comfortable with the notion that zero is a power-of-two > in any finite representation of 2's complement) > > The "zero is special" thing means that if you don't want to accept zero as > a valid mask (it technically *is* a contiguous set of bits set - it's just > the empty set) you'd need to check for it specially. I guess the person who wrote it wasn't thinking discrete maths at the time. Darren J.