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

c++ c performance cpu low-level
Armen Tsirunyan picture Armen Tsirunyan · Nov 20, 2010 · Viewed 7.1k times · Source

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. shl, shr), which is a single command. But does it matter (nanosecond-wise, or CPU-tact-wise) how many bits you shift. In other words, is either of the following faster on any CPU?

x << 1;

and

x << 10;

And please don't hate me for this question. :)

Answer

nimrodm picture nimrodm · Nov 20, 2010

Potentially depends on the CPU.

However, all modern CPUs (x86, ARM) use a "barrel shifter" -- a hardware module specifically designed to perform arbitrary shifts in constant time.

So the bottom line is... no. No difference.

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