Explain the concept of branch prediction and its role in code optimisation.

Branch prediction is a technique used in code optimization to improve the performance of a computer program by reducing the impact of conditional branches on the execution time.

In computer programs, conditional branches are instructions that determine the flow of execution based on a condition. For example, an if statement in a programming language. When a conditional branch is encountered, the processor needs to determine which path to take, either the true or false branch.

Branch prediction comes into play by attempting to predict the outcome of a conditional branch before it is actually evaluated. This prediction is based on historical information about the branch's behavior. The processor maintains a branch history table that records the outcomes of previous branches.

If the prediction is correct, the processor can continue executing instructions along the predicted path, resulting in a performance improvement. However, if the prediction is incorrect, a pipeline stall occurs, and the processor needs to discard the instructions it has already executed along the wrong path and start over. This can lead to a performance penalty.

The role of branch prediction in code optimization is to minimize the number of pipeline stalls caused by incorrect predictions. By accurately predicting the outcome of conditional branches, the processor can avoid unnecessary stalls and keep the pipeline filled with instructions, leading to improved performance.

There are different branch prediction techniques, such as static prediction, where the prediction is based on the branch's static behavior, and dynamic prediction, where the prediction is based on runtime behavior. Dynamic prediction techniques, like the two-level adaptive branch prediction, use more sophisticated algorithms and historical information to make accurate predictions.

Overall, branch prediction plays a crucial role in code optimization by reducing the impact of conditional branches on the execution time, allowing for more efficient and faster program execution.