Re: [PATCH 1/4] LOG2: Implement a general integer log2 facility in the kernel [try #4]

Christoph Lameter
2006-10-09T11:07:12Z

On Mon, 9 Oct 2006, David Howells wrote:

> There are a number of reasons:
>
> (1) There are a bunch of independent log2 implementations lying around in the
> code. It'd be nice to just have one set that anyone can use.

True. So wrap around what is there but do not add gazillion of definitions
to all arches.

> (2) Not everyone realises that fls() can be used to do log2().

So this is a case for a wrapper.

> (3) ilog2(n) != fls(n)
>
> This means that the asm-optimised version for one might be less optimal
> for the other (for example, ilog2() produces an undefined result if n <=
> 1, fls() must return 0).

Ok these are boundary checks that are easily coded around. Some
variations on fls even exist that also do various flavors of end case
handling.

> (4) There are occasions when you might want to take a log2 of a constant.
> With the totally inline asm approach, it would always execute some code,
> though it should be unnecessary. What I've done permits you to avoid that
> as the answer is always going to be the same.

Good stuff. I have always wanted that. The wrapper could check for a
constant.

> (5) fls() and fls64() can't be used to initialise a variable at compile time,
> ilog2() can.

Well that is the same issue as (4).