Rx3i Count bits set in word

In a Rx3i what would be the most efficient way to count the number of bits set in a word variable?

The naive approach is to simply shift the bits out of the word and count the ones that are set. But is this the smartest way of doing it?

One smarter example is Brian Kernighan’s Algorithm which has the advantage that it only loops for the number of bits that are set in the number. And while I have no problem implementing it in ladder code I have no idea if this is the best way in an Rx3i.

So am I missing any tricks?

There is no single instruction in the Rx3i function set that will do it.
It will still be an iterative process, but there is a "bit position' instruction which returns the location of the first set bit in a word or double word. Combine that with a counter and a "bit clear" instruction and you can count them in the least number of iterations.

Locate the first true bit
Increment counter
Clear that bit
Repeat until all bits cleared (value = zero).

Steve Bailey said:

There is no single instruction in the Rx3i function set that will do it.
It will still be an iterative process, but there is a "bit position' instruction which returns the location of the first set bit in a word or double word. Combine that with a counter and a "bit clear" instruction and you can count them in the least number of iterations.

Locate the first true bit
Increment counter
Clear that bit
Repeat until all bits cleared (value = zero).

Click to expand...

That's effectively the Kernighan algorithm. But implemented in a smart manner.

Thanks!