Most efficient way to modify a stream of data

Question

I have a stream of 16 bit values, and I need to adjust the 4 least significant bits of each sample. The new values are different for each short, but repeat every X shorts - essentially tagging each short with an ID.

Are there any bit twiddling tricks to do this faster than just a for-loop?

More details I'm converting a file from one format to another. Currently implemented with FILE* but I could use Windows specific APIs if helpful.

[while data remaining]
{
   read X shorts from input
   tag 4 LSB's
   write modified data to output
}

---

In addition to bulk operations, I guess I was looking for opinions on the best way to stomp those last 4 bits.

1. Shift right 4, shift left 4, | in the new values
2. & in my zero bits, then | in the 1 bits
3. modulus 16, add new value

We're only supporting win7 (32 or 64) right now, so hardware would be whatever people choose for that.

Answer 1

If you're working on e.g. a 32-bit platform, you can do them 2 at a time. Or on a modern x86 equivalent, you could use SIMD instructions to operate on 128 bits at a time.

Other than that, there are no bit-twiddling methods to avoid looping through your entire data set, given that it sounds like you must modify every element!

Answer 2

Best way to stomp those last 4 bits is your option 2:

int i;
i &= 0xFFF0;
i |= tag;

Doing this on a long would be faster if you know tag values in advance. You can memcpy 4 shorts in one long and then do the same operations as above on 4 shorts at a time:

long l;
l &= 0xFFF0FFF0FFF0FFF0;
l |= tags;

where tags = (long) tag1 << 48 + (long) tag2 << 32 + (long) tag3 << 16 + (long) tag4; This has sense if you are reusing this value tags often, not if you have to build it differently for each set of 4 shorts.