[Jtsummers]

The program bypasses the need to actually check the particular number against 3 and 5 using any kind of arithmetic instructions, instead it's keeping separate values for 3-count and 5-count which will be either 3 or 5 when the number is a multiple of either. It then performs a clear on it. So it has this state when it gets to the checks:

  n    3-count   5-count
  1    1         1
  2    2         2
  3    3         3
  4    1         4
  ...

So the check is just a cpi (compare immediate) instruction for whether the particular counter is 3 or 5. Which is probably faster than bit twiddling, especially since they don't have a pop count instruction available in the instruction set. On an instruction set with that, I imagine it would be a useful trick though.

[bjourne]

The point of bit-twiddling is to produce a number n that is 1 or -1 if counter is divisible by 3 or 5 and 0 otherwise, and then add either mul(n, counter) or and(n, counter) to the result. It avoids branches which could shave off a few bytes depending on how branch instructions are encoded. Since we're golfing, we're not interested in speed, only program size.

[Jtsummers]

Still not an option on the platform in the article, though. You'd have to make your own pop count and implement it likely with a loop and bit shifting. It's almost certainly going to take more instructions than this and end up with as many branches as a result. The instruction set is very limited. Though quoting myself:

>> On an instruction set with that [pop count], I imagine it would be a useful trick though.