What is CPU branch prediction?

Branch prediction is a technique used in CPU design that attempts to guess the outcome of a conditional operation and prepare for the most likely result. A digital circuit that performs this operation is known as a branch predictor. It is an important component of modern CPU architectures, such as the x86.

What if branch prediction is wrong?

The branch that is guessed to be the most likely is then fetched and speculatively executed. If it is later detected that the guess was wrong, then the speculatively executed or partially executed instructions are discarded and the pipeline starts over with the correct branch, incurring a delay.

What are the types of branch prediction?

Branch prediction schemes are of two types: static branch schemes and dynamic branch schemes. branch scheme (hardware techniques) is based on the hardware and it assembles the information during the run-time of the program.

What is 1bit branch prediction?

How is 1-bit branch prediction implemented in hardware? – Each table entry contains one prediction bit for that branch, e.g., o for predict not taken (N), 1 for predict taken (T). The prediction bit is used to predict the branch outcome. It is updated after the branch’s actual outcome is known.

How good is branch prediction?

7 Answers. Branch prediction is pretty darned good these days. But that doesn’t mean the penalty of branches can be eliminated. In typical code, you probably get well over 99\% correct predictions, and yet the performance hit can still be significant.

What is a branch prediction buffer?

Branch prediction buffers contain prediction about whether the next branch will be taken (T) or not (NT), but it does not supply the target PC value. A Branch Target Buffer (BTB) does this. Instr address Predicted PC. BTB is a cache that holds. (instr addr, predicted PC)

What will be the penalty if branch predictor predicts wrong result?

When the prediction is right, that is, when the branch is not taken, there is no penalty to be paid. On the other hand, when the prediction is wrong, one bubble is created and the next instruction is fetched from the target address.

What is branch prediction logic?

Branch prediction logic: In this scheme, a prediction is made for the branch instruction currently in the pipeline. The prediction will either be taken or not taken. If the prediction is true then the pipeline will not be flushed and no clock cycles will be lost.

Where does branch prediction happen?

In order to avoid wasting cycles on frontend pipeline bubbles, each taken jump needs a BTB hit. This branch prediction must happen very early in the CPU pipeline, before instruction decode is finished. This same mechanism is needed for any taken branch, whether it’s unconditional, conditional or a function call.