从字节中提取位的效率

Question

I'm extracting the 8th bit from a byte in C. Here is my example.

register unsigned char byte;

int pos = 7;

int x =(byte >> pos) & 1;   //Method I

int y =(byte & 0x80) >> pos;    //Method II

Both techniques will result in the same output, but is one of the methods more efficient than the other?

Answer 1

Both will be the same. Both AND and SHR instructions are 1-clock instructions on intel CPUs.

Answer 2

Bitwise operations a basically one of the fastest things you'll find on a computer. I'd imagine that any difference would be incredibly minor, such that it doesn't really matter.

Answer 3

If you know the bit you are extracting at compile-time, then either method should have approximately the same speed:

unsigned char val;
const int pos = 4;
...
int x = (val >> pos) & 1;
int y = (val & 0x10) >> pos;

However if you are calculating the position at runtime and not as a constant, doing the AND last should be faster:

unsigned char val;
int pos;
...
/* requires only a shift and AND */
int x = (val >> pos) & 1;
/* requires two shifts and AND */
int y = (val & (1 << pos)) >> pos;