What is the most efficient way to represent small values in a struct?

Question

Often I find myself having to represent a structure that consists of very small values. For example, Foo has 4 values, a, b, c, d that, range from

Answer 1

I did video decompression for a while. The fastest thing to do is something like this:

short ABCD; //use a 16 bit data type for your example

and set up some macros. Maybe:

#define GETA ((ABCD >> 12) & 0x000F)
#define GETB ((ABCD >> 8) & 0x000F)
#define GETC ((ABCD >> 4) & 0x000F)
#define GETD (ABCD  & 0x000F)  // no need to shift D

In practice you should try to be moving 32 bit longs or 64 bit long long because thats the native MOVE size on most modern processors.

Using a struct will always create the overhead in your compiled code of extra instructions from the base address of you struct to the field. So get away from that if you really want to tighten your loop.

Edit: Above example gives you 4 bit values. If you really just need values of 0..3 then you can do the same things to pull out your 2 bit numbers so,,,GETA might look like this:

GETA ((ABCD >> 14) & 0x0003)

And if you are really moving billions of things things, and I don't doubt it, just fill up a 32bit variable and shift and mask your way through it.

Hope this helps.