Branch Predictor

A branch predictor is a digital circuit designed to enhance the performance of a central processing unit (CPU) by breaking the instruction pipeline into stages while trying to guess the outcome of conditional statements in the code. The most straightforward way to execute code is to read each line, execute the entirety of it, move on to the next line, and repeat. However, this can be slow or inefficient. Often, different parts of the code could be executed in parallel, as long as they are independent of each other, using a technique called pipelining, which can save CPU time. A branch predictor is a vital piece of hardware used in pipelining.

Working Logic of a Branch Predictor

Branches appear in the code when a statement is used, as the code ‘branches out’, according to the outcome of a condition. If the condition is true, a given set of instructions are followed, but if the condition is false, a different set of instructions are applied. Furthermore, the code may have multiple, nested statements or branches. This makes the code highly serialized, with every group of instructions depending on previous ones. A branch predictor guesses the outcome of a condition before it is evaluated to pipeline the instructions ahead. The prediction may be made based on previous data/behavior, such as dynamic branch prediction, or other algorithms (static or random branch prediction, among others). There are multiple ways to ensure that predictions are mostly correct, and the overall performance benefits of pipelining generally outweigh the cost of a wrong prediction.