Here's version 3 of the patch, incorporating comments from Andrew Morton.

Andrew Morton <akpm@osdl.org> wrote:
> We have an opportunity to make this loop less baroque.

That seems reasonable. gcc's optimiser should be able to render them the same.

> whereas in other places you _have_ made the definition of __init functions
> include the "__init".  I'm not sure which is best, really, but it seems
> better to include the __init in the declaration.

You're probably right. This should probably apply to vfs_caches_init() and
inode_init() too.

> What's the attribute((pure)) for?

It tells gcc that the function's result only depends on its arguments... it
makes no references at all to external data. It's like __attribute__((const))
but stronger. This means that gcc can generate code that caches the result.

See the gcc manual.

> It generates a warning on older gcc - please use __attribute_pure__.

We still support gcc's that old?

> The other four or five implementations of log2() use ffx(~n).

Yes... but ffs() and ffz() take int args, not long args. I suspect that
shouldn't matter (that would require the hash table to be calculated at 4Gig
buckets in size or greater to be a problem), but why take the chance when we
can avoid it easily?

David