On Thu, 25 Jan 2007, Andrew Morton wrote:
> On Thu, 25 Jan 2007 04:32:12 -0500 (EST)
> "Robert P. J. Day" <[email protected]> wrote:
> > +/*
> > + * round down to nearest power of two
> > + */
> > +static inline __attribute__((const))
> > +unsigned long __rounddown_pow_of_two(unsigned long n)
> > +{
> > + return 1UL << (fls_long(n) - 1);
> > +}
>
> So __rounddown_pow_of_two(16) returns 8?
it does? but if that was true, so would 17, and 18, and 19 ... i
didn't actually test this since it seemed so straightforward.
doesn't fls_long() return the most significant bit? oh, wait ...
reading further ...
>
> > /**
> > * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
> > * @n - parameter
> > @@ -154,4 +174,20 @@ unsigned long __roundup_pow_of_two(unsigned long n)
> > __roundup_pow_of_two(n) \
> > )
> >
> > +/**
> > + * rounddown_pow_of_two - round the given value down to nearest power of two
> > + * @n - parameter
> > + *
> > + * round the given value down to the nearest power of two
> > + * - the result is undefined when n == 0
> > + * - this can be used to initialise global variables from constant data
> > + */
> > +#define rounddown_pow_of_two(n) \
> > +( \
> > + __builtin_constant_p(n) ? ( \
> > + (n == 1) ? 0 : \
> > + (1UL << ilog2(n)) : \
> > + __rounddown_pow_of_two(n) \
> > + )
>
> But (1UL << ilog2(16)) returns 16?
>
>
> And, afiact, your __rounddown_pow_of_two() is basically equivalent to (1UL
> << ilog2(n)) anyway. So a suitable (and less buggy) implementation might be
>
> static inline unsigned long rounddown_pow_of_two(unsigned long n)
> {
> return (n == 1) ? 0 : (1UL << ilog2(n));
> }
i think you're right. it's been a long day so give me a few minutes
to convince myself.
rday
--
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA
http://www.fsdev.dreamhosters.com/wiki/index.php?title=Main_Page
========================================================================
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
