Question

Shifting bits left and right is apparently faster than multiplication and division operations on most, maybe even all, CPUs if you happen to be using a power of 2. However, it can reduce the clarity of code for some readers and some algorithms. Is bit-shifting really necessary for performance, or can I expect the compiler or VM to notice the case and optimize it (in particular, when the power-of-2 is a literal)? I am mainly interested in the Java and .NET behavior but welcome insights into other language implementations as well.

Answer 1

Most compilers today will do more than convert multiply or divide by a power-of-two to shift operations. When optimizing, many compilers can optimize a multiply or divide with a compile time constant even if it’s not a power of 2. Often a multiply or divide can be decomposed to a series of shifts and adds, and if that series of operations will be faster than the multiply or divide, the compiler will use it.

For division by a constant, the compiler can often convert the operation to a multiply by a ‘magic number’ followed by a shift. This can be a major clock-cycle saver since multiplication is often much faster than a division operation.

Henry Warren’s book, Hacker’s Delight, has a wealth of information on this topic, which is also covered quite well on the companion website:

• http://www.hackersdelight.org/

See also a discussion (with a link or two ) in:

• Reading assembly code

Anyway, all this boils down to allowing the compiler to take care of the tedious details of micro-optimizations. It’s been years since doing your own shifts outsmarted the compiler.