Re: [PATCH v6 1/3] lib: Add strongly typed 64bit int_sqrt

[Crt Mori]

On 18 December 2017 at 17:44, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Mon, Dec 18, 2017 at 04:05:44PM +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.
>
> You clearly haven't read a recent version of the file you're patching.
> Please take a moment to do so now.
>

IIO kernel does not have the recent version in, so thanks for heads
up. It does not change much for my function.

>> +/**
>> + * int_sqrt64 - strongly typed int_sqrt function
>> + * @x: 64bit integer of which to calculate the sqrt
>> + */
>> +u64 int_sqrt64(u64 x)
>
> Please explain how the result of sqrt(u64) can be larger than u32.
>

My hand calculator tells me it could be.

ffffffffÃffffffff = FFFFFFFE00000001 which still has some margin which
will end up above the 32 bit number. Further more the __fls
optimization automatically casts the inputs to unsigned long (32 bit
on 32 bit machines), so that also makes it out of option.

> Also, I expect that this fact could be exploited to optimize this for
> 32bit archs if one were so inclined.
>
>> +{
>> +     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;
>> +}
>
> so yeah, no, please try again after reading the current file.