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

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Date:	Mon, 2 Feb 2015 11:13:44 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Anshul Garg <aksgarg1989@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	anshul.g@...sung.com
Subject: Re: [PATCH] lib/int_sqrt.c: Optimize square root function

On Mon, Feb 2, 2015 at 11:00 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> (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 also not entirely clear that we need a whole new loop. We
might just instead start off with a better guess for 'm' using some
calculation that might be doable with a single conditional move
instruction instead of a loop. Because I suspect that the inevitable
branch misprediction of a new loop is likely as expensive as a few
iterations through the core one.

IOW, instead of

  m = 1UL << (BITS_PER_LONG - 2);

perhaps something like

   m = 1UL << (BITS_PER_LONG/2- 2);
   if (m < x)
       m <<= BITS_PER_LONG/2;

(assuming gcc can change that code into a "cmov") might cut down the
"lots of empty loops" case in half for small values of 'x'.

There's probably some other better cheap initial guess value estimator.

                         Linus
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