Tournament Predictor

As we discussed in the PShare and GShare section, designers desired to use two predictors to handle different cases of branches - correlated branches and branches that were self similar. But how do we determine which predictor will be used for a program counter (instruction) if we don't know what type of branch it is ahead of time?

The high-level representation below explains how the GShare and PShare predictors are both indexed by the program counter and each predictor provides a predicted branch outcome for that program counter value. At the same time, we use the program counter to index into a meta-predictor table that has a 2BC at each index. We use the meta-predictor to keep track of which shared predictor gives us the correct answer for the program counter the most often.