Re: [PATCH] lib/int_sqrt.c: Optimize square root function

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 2015-02-02 at 11:00 -0800, Linus Torvalds wrote:
> Hmm. I don't disagree, but would like some more feedback.
> 
> Davidlohr - you were the person to touch this function last (commit
> 30493cc9dddb: "lib/int_sqrt.c: optimize square root algorithm"), and
> you did so for performance reasons. And in fact, when you did that,
> you removed that initial loop:
> 
> -       one = 1UL << (BITS_PER_LONG - 2);
> -       while (one > op)
> -               one >>= 2;
> 
> but I'm not sure that was actually all that conscious, I think the
> real optimization was the changes inside the loop to make the final
> real loop faster and simpler.

I missed that. And yes, the real optimization should be in the loop.

> 
> Also, you had performance numbers, so presumably a test harness for it
> all. It probably depends a lot on the actual distribution of argument
> values, of course, but it would be good to accompany the patch with
> actual real numbers like lasty time.

Aha. In my case I recall I ran a usersapce program using each function
from 1 to a million, and throwing perf at it for 10 times.

> (I'm also not entirely sure what uses int_sqrt() that ends up being so
> performance-critical, so it would be good to document that too, since
> that probably also matters for the "what's the normal argument range"
> question..)

It's not a big deal afaik.

Thanks,
Davidlohr