Which is faster: x<<1 or x<<10?

Question

I don\'t want to optimize anything, I swear, I just want to ask this question out of curiosity. I know that on most hardware there\'s an assembly command of bit-shift (e.g.

Answer 1

On some generations of Intel CPUs (P2 or P3? Not AMD though, if I remember right), the bitshift operations are ridiculously slow. Bitshift by 1 bit should always be fast though since it can just use addition. Another question to consider is whether bitshifts by a constant number of bits are faster than variable-length shifts. Even if the opcodes are the same speed, on x86 the nonconstant righthand operand of a bitshift must occupy the CL register, which imposes additional constrains on register allocation and may slow the program down that way too.