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

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Date:	Mon, 02 Feb 2015 11:10:56 -0800
From:	Joe Perches <joe@...ches.com>
To:	Anshul Garg <aksgarg1989@...il.com>
Cc:	linux-kernel@...r.kernel.org, anshul.g@...sung.com,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH] lib/int_sqrt.c: Optimize square root function

On Mon, 2015-02-02 at 09:12 -0800, Anshul Garg wrote:
> From: Anshul Garg <aksgarg1989@...il.com>
> 
> Unnecessary instructions are executing even though m is
> greater than x so added logic to make m less than equal to
> x before performing these operations.
> 
> Signed-off-by: Anshul Garg <aksgarg1989@...il.com>
> ---
>  lib/int_sqrt.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
[]
> @@ -22,6 +22,9 @@ unsigned long int_sqrt(unsigned long x)
>  		return x;
>  
>  	m = 1UL << (BITS_PER_LONG - 2);
> +
> +	while (m > x)
> +		m >>= 2;

Perhaps removing the while and using fls(x) would be better.

Perhaps something like:

	m = 1UL << min(sizeof(unsigned long) == sizeof(u64) ?
		       fls64(x) : fls(x),
		       BITS_PER_LONG - 2);

>  	while (m != 0) {
>  		b = y + m;
>  		y >>= 1;



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