How branch predictor and branch target buffer co-exist?

Question

My question is how they co-exist and work together in modern CPU architecture?

Answer 1

You've got it slightly reversed. On every fetch you index into your branch predictor, which tells you whether the instruction that you have just received will be decoded into a taken branch. If not, you fetch the next sequential address. But if your branch predictor says that it will be a taken branch, you don't know which instruction to fetch next, since you haven't decoded this instruction yet. So in order to not waste cycles waiting for the branch to resolve, you would use a Branch Target Buffer(or BTB). A BTB stores previous addresses where branch redirected the control flow. Using this mechanism you are trying to predict where the control flow will be redirected this time. This technique has 100% success rate for unconditional branches, function calls, and returns when paired with a Return Address Stack. On conditional branches the success rate is slightly lower, but is still really good given high temporal locality of branch targets. As an example you could consider a backwards branch of a loop, which will always branch to the same location.

When the branch instruction is actually resolved (usually in Decode or Execute stage of the pipeline, depending on the implementation), you will adjust the values in both the branch predictor and the BTB in order to have more up to date information for future predictions.

Here is a pictorial explanation how BTB lookup and update happen:

http://www-ee.eng.hawaii.edu/~tep/EE461/Notes/ILP/buffer.html