Re: Checking if an Integer is an Exact Binary Power

[Andrei Alexandrescu via Digitalmars-d]

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