linux-kernel - Re: [PATCH v6 1/3] lib: Add strongly typed 64bit int

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Date:   Tue, 19 Dec 2017 10:32:56 +0100
From:   Crt Mori <cmo@...exis.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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 <linux-iio@...r.kernel.org>
Subject: Re: [PATCH v6 1/3] lib: Add strongly typed 64bit int_sqrt

On 18 December 2017 at 17:24, Joe Perches <joe@...ches.com> wrote:
> On Mon, 2017-12-18 at 16:05 +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. Although int_sqrt() is a rough
>> approximation, the same algorithm is used in int_sqrt64() as good
>> enough on 32bit platform.
> []
>> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> []
>> @@ -459,6 +459,7 @@ extern int kernel_text_address(unsigned long addr);
>>  extern int func_ptr_is_kernel_text(void *ptr);
>>
>>  unsigned long int_sqrt(unsigned long);
>> +u64 int_sqrt64(u64 x);
>>
>>  extern void bust_spinlocks(int yes);
>>  extern int oops_in_progress;         /* If set, an oops, panic(), BUG() or die() is in progress */
>> diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
> []
>> @@ -36,3 +36,29 @@ unsigned long int_sqrt(unsigned long x)
>>       return y;
>>  }
>>  EXPORT_SYMBOL(int_sqrt);
>> +
>> +/**
>> + * int_sqrt64 - strongly typed int_sqrt function
>> + * @x: 64bit integer of which to calculate the sqrt
>> + */
>> +u64 int_sqrt64(u64 x)
>> +{
>> +     u64 b, m, y = 0;
>> +
>> +     if (x <= 1)
>> +             return x;
>> +
>> +     m = 1ULL << (64 - 2);
>> +     while (m != 0) {
>> +             b = y + m;
>> +             y >>= 1;
>> +
>> +             if (x >= b) {
>> +                     x -= b;
>> +                     y += m;
>> +             }
>> +             m >>= 2;
>> +     }
>> +     return y;
>> +}
>> +EXPORT_SYMBOL(int_sqrt64);
>
> int_sqrt is now optimized with __fls
>
> Because u64 is unsigned long on 64 bit arches, this
> could probably be optimized for 64 bit distributions
> with something like:
>
> #if BITS_PER_LONG != 64
> #define int_sqrt64 _int_sqrt64
> u64 _int_sqrt64(u64 x);
> #else
> #define int_sqrt64(x) int_sqrt
> #endif
>
> ...
>
> #if BITS_PER_LONG != 64
> u64 _int_sqrt64(u64 x)
> { etc.. }
> #endif
>

Thanks for the tip. I will respin with this change so that 64 bit
arches use the optimized int_sqrt from above.