linux-kernel - Re: [PATCH v12 1/3] lib: Add strongly typed 64bit int

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Date:   Tue, 09 Jan 2018 11:23:44 -0800
From:   Joe Perches <joe@...ches.com>
To:     Crt Mori <cmo@...exis.com>, Jonathan Cameron <jic23@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Ian Abbott <abbotti@....co.uk>,
        Larry Finger <Larry.Finger@...inger.net>,
        Niklas Soderlund <niklas.soderlund+renesas@...natech.se>,
        Thomas Gleixner <tglx@...utronix.de>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH v12 1/3] lib: Add strongly typed 64bit int_sqrt

On Tue, 2018-01-09 at 16:18 +0100, Crt Mori wrote:
> There is no option to perform 64bit integer sqrt on 32bit platform.
> Added stronger typed int_sqrt64 enables the 64bit calculations to
> be performed on 32bit platforms. Using same algorithm as int_sqrt()
> with strong typing provides enough precision also on 32bit platforms,
> but it sacrifices some performance.
[]
> diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
[]
> @@ -36,3 +37,34 @@ unsigned long int_sqrt(unsigned long x)
>  	return y;
>  }
>  EXPORT_SYMBOL(int_sqrt);
> +
> +#if BITS_PER_LONG < 64
> +/**
> + * int_sqrt64 - strongly typed int_sqrt function when minimum 64 bit input
> + * is expected.
> + * @x: 64bit integer of which to calculate the sqrt
> + */
> +u32 int_sqrt64(u64 x)
> +{
> +	u64 b, m;
> +	u32 y = 0;
> +
> +	if (x <= 1)
> +		return x;

I think this should instead be:

	if (x <= INT_MAX)
		return int_sqrt((int)x);

to reduce the loop cost below when the
value is small enough.

> +
> +	m = 1ULL << (fls64(x) & ~1ULL);
> +	while (m != 0) {
> +		b = y + m;
> +		y >>= 1;
> +
> +		if (x >= b) {
> +			x -= b;
> +			y += m;
> +		}
> +		m >>= 2;
> +	}
> +
> +	return y;
> +}
> +EXPORT_SYMBOL(int_sqrt64);
> +#endif