What is the purpose of branch prediction in a CPU?

The purpose of branch prediction in a CPU is to improve the overall performance and efficiency of the processor by reducing the impact of branch instructions on the pipeline.

Branch instructions are instructions that can change the normal sequential flow of instructions in a program, such as conditional branches (if-else statements) or loops. When a branch instruction is encountered, the CPU needs to determine the target address of the branch and fetch the correct instructions from that address. However, this process introduces a delay in the pipeline, as the CPU needs to wait for the branch instruction to be executed and the target address to be determined before fetching the correct instructions.

Branch prediction is a technique used by modern CPUs to mitigate this delay. It involves predicting the outcome of a branch instruction before it is actually executed, based on historical information and patterns. The CPU maintains a branch prediction table or cache that stores information about previous branch instructions and their outcomes. This information is used to make an educated guess about the outcome of a branch instruction.

If the prediction is correct, the CPU can continue fetching and executing instructions from the predicted target address, without waiting for the branch instruction to be fully executed. This helps to keep the pipeline filled with instructions and avoids the delay caused by waiting for the branch instruction.

However, if the prediction is incorrect, the CPU needs to discard the incorrectly fetched instructions and fetch the correct instructions from the actual target address. This is known as a branch misprediction and can result in a pipeline flush, which incurs a performance penalty. To minimize the impact of mispredictions, modern CPUs employ various techniques such as speculative execution, where both the predicted and actual outcomes of a branch are executed in parallel, and recovery mechanisms to quickly recover from mispredictions.

Overall, branch prediction plays a crucial role in improving the performance of CPUs by reducing the impact of branch instructions on the pipeline and allowing for more efficient instruction execution.