C 中按位运算符的性能

Question

What is the fastest way to make the last 2 bits of a byte zero?

x = x >> 2 << 2;

OR

x &= 252;

Is there a better way?

Answer 1

Depends on many factors, including the compiler, the machine architecture (ie processor).

My experience is that

x &= 252; // or...
x &= ~3;  

are more efficient (and faster) than

x = x >> 2 << 2;

Answer 2

If your compiler is smart enough, it might replace

x = x >> 2 << 2;

by

x &= ~3;

The later is faster than the former, because the later is only one machine instruction, while the former is two. And all bit manipulation instructions can be expected to execute in precisely one cycle.

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Note:
The expression ~3 is the correct way to say: A bit mask with all bits set but the last two. For a one-byte type, this is equivalent to using 252 as you did, but ~3 will work for all types up to int . If you need to specify such a bitmask for a larger type like a long , add the appropriate suffix to the number, ~3l in the case of a long .