Checking if an Integer is an Exact Binary Power

[tsbockman via Digitalmars-d]

On Saturday, 23 April 2016 at 21:04:52 UTC, Nordlöw wrote:
> On Saturday, 23 April 2016 at 20:42:25 UTC, Lass Safin wrote:
>> CPUID: https://en.wikipedia.org/wiki/CPUID.
>> You can check for the presence of a lot of instructions with 
>> this instruction.
>> However this will only work on x86 and only run-time.
>
> Code you give a complete code example in D, please or point out 
> a suitable place in druntime/phobos?

core.bitop.popcnt() uses core.cpuid.hasPopcnt() to choose between 
the intrinsic _popcnt() and a software fallback algorithm at 
runtime:
     
https://github.com/dlang/druntime/blob/master/src/core/bitop.d#L343

The relative complexity of softPopcnt() also shows why you don't 
really want to use popcnt() for this task unless you know the 
intrinsic will be available.