Checking if an Integer is an Exact Binary Power

[Xinok via Digitalmars-d]

On Monday, 25 April 2016 at 12:56:27 UTC, Andrei Alexandrescu 
wrote:
> On 4/25/16 6:42 AM, Solomon E wrote:
>> On Monday, 25 April 2016 at 05:35:12 UTC, Andrei Alexandrescu 
>> wrote:
>>>
>>> With gdc https://godbolt.org/g/jcU4np isPow2B is the winner 
>>> (shortest
>>> code, simplest instructions). -- Andrei
>>
>> I generalized function isPow2B to work for negative numbers in 
>> case of a
>> signed type, while keeping the assembly the same or better. 
>> (That also
>> incidentally made it correctly return false for zero.)
>>
>>> bool isPow2B(T)(T x)
>>> {
>>>    return (x & -x) > (x - 1);
>>> }
>>
>> bool isPow2F(T)(T x)
>> {
>>      return (x & -x) > (x >>> 1);
>> }
>>
>> assembly diff:
>> --        subl    $1, %edi
>> ++        shrl    %edi
>>
>
> That's smaller and faster, thanks. But how do you show it is 
> still correct? -- Andrei

Brute force.

http://dpaste.dzfl.pl/882d7cdc5f74