MIME-Version: 1.0
In-Reply-To: <CAKv63usChMeCNYJ6qshFcBCOLgEX9Q-mSv6miLG7YJkUKJ1gEA@mail.gmail.com>
References: <20180109151847.30258-1-cmo@melexis.com> <1515525824.9619.103.camel@perches.com>
 <CAKv63usChMeCNYJ6qshFcBCOLgEX9Q-mSv6miLG7YJkUKJ1gEA@mail.gmail.com>
From: Crt Mori <cmo@melexis.com>
Date: Wed, 10 Jan 2018 09:33:14 +0100
Message-ID: <CAKv63uuDya8LU-WHtKiqnEJCtg855aYduhowu0dv5qBd1r284Q@mail.gmail.com>
Subject: Re: [PATCH v12 1/3] lib: Add strongly typed 64bit int_sqrt
To: Joe Perches <joe@perches.com>
Cc: Jonathan Cameron <jic23@kernel.org>,
        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 <linux-iio@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        David Laight <David.Laight@aculab.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org

On 10 January 2018 at 09:15, Crt Mori <cmo@melexis.com> wrote:
> On 9 January 2018 at 20:23, Joe Perches <joe@perches.com> wrote:
>> 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.
>>
>
> In existing int_sqrt its only 1 and I assume that is more to protect
> from loop execution with 0 or 1. Since there is no difference (except
> fls64) with int_sqrt I assume there is no need to call it to avoid
> loop?
>

Nevermind, I see what you mean (should have thought longer before I
written). The cost of below loop is because of 64bit calculation is
not native on 32bit and we could just use 32bit calculation in that
loop. Will send v13 with a fix for this.

>>> +
>>> +     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