The performance of a processor is influenced by many factors. Branch prediction plays a decisive role in this. This technique is used by modern CPUs to speed up the execution of program code by predicting which path a program branch will take.
What is Branch Prediction?
Program code consists of a series of instructions that are executed in a specific order. However, sometimes there are branches where the execution can take different paths depending on certain conditions. An example of a branch is a conditional statement such as “if-else”, where depending on the fulfillment of a condition, either the code in the “if” branch or the code in the “else” branch is executed. Branch prediction now tries to guess in advance which branch will be taken before this is actually determined.
if(foo > 42) {
// Execute if block
foo = 0;
}
else {
// Execute else block
foo = 1;
}Why is Branch Prediction important?
Running programs takes time and resources, and branches can cause delays. If the processor has to wait for a branch to resolve, this can lead to so-called pipeline stalls, where the processor is not used efficiently. In order for the pipeline to always be maximally utilized and thus most efficient, stalls are to be avoided as a matter of urgency. At this point Branch Prediction tries to minimize the stalls by trying to predict which path will be taken and filling up the pipeline with the next instruction accordingly early. Without Branch Prediction, the pipeline in the example would not be filled with new instructions until the if query, because the value of foo is not clear. Depending on the length of the pipeline, these clock cycles would then remain unused.
How does Branch Prediction work?
Modern CPUs use various techniques to perform branch prediction. A common dynamic method is to use so-called Branch History Tables (BHT) or Branch Target Buffers (BTB). These special hardware structures store information about previous branches and their execution paths. This information is used to make a prediction for future branches. The “two-bit counter” or “Gshare algorithm” analyzes past branches and adjusts its predictions accordingly. The processor then uses these predictions to prepare the next instruction and efficiently fill the pipeline.
Advantages and disadvantages of Branch Prediction
Branch prediction is an extremely effective technique to increase CPU performance. It helps reduce pipeline stalls and minimize the number of clock cycles needed to resolve branches. However, if the prediction is not correct, this can lead to “branch misprediction” where the pipeline has to be reset. This can happen especially with complex branch structures or if the behavior of the program changes during runtime. Predication can help the CPU to use branch prediction more effectively.