Instruction-level parallelism

Instruction level parallelism (ILP) is a measure of how many of the operations in a program can be dealt with at once. Consider the following program:

1. e = a + b
2. f = c + d
3. g = e * f

Operation 3 depends on the results of operations 1 and 2, so it cannot be calculated until both of them are completed. However, operations 1 and 2 do not depend on any other operation, so they can be calculated simultaneously. Assuming that each operation can be completed in one unit of time, than these three instructions can be completed in a total of two units of time, giving an ILP of 3/2.

A goal of compiler and processor designers is to identify and take advantage of as much ILP as possible. Processors that can execute multiple instructions in parallel are refered to as superscalar processors.