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Subject: Re: [PATCH v12 1/3] lib: Add strongly typed 64bit int_sqrt
From: Joe Perches <joe@perches.com>
To: Crt Mori <cmo@melexis.com>, Jonathan Cameron <jic23@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>, Andrew Morton
 <akpm@linux-foundation.org>,  Kees Cook <keescook@chromium.org>, Rusty
 Russell <rusty@rustcorp.com.au>, Ian Abbott <abbotti@mev.co.uk>,  Larry
 Finger <Larry.Finger@lwfinger.net>, Niklas Soderlund
 <niklas.soderlund+renesas@ragnatech.se>,  Thomas Gleixner
 <tglx@linutronix.de>, Krzysztof Kozlowski <krzk@kernel.org>, Masahiro
 Yamada <yamada.masahiro@socionext.com>, linux-kernel@vger.kernel.org,
 linux-iio@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, David
 Laight <David.Laight@aculab.com>
Date: Tue, 09 Jan 2018 11:23:44 -0800
In-Reply-To: <20180109151847.30258-1-cmo@melexis.com>
References: <20180109151847.30258-1-cmo@melexis.com>
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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