5.2.1. Branch predictor

Branch prediction in the processor is dynamic and is based around a global history prediction scheme. In addition, there is extra logic to handle predictions that thrash and to predict the end of long loops.

The global history scheme is an adaptive predictor that learns the behavior of branches during execution, identifying them based on the historical pattern of behavior of the preceding branches. For each pattern of branch behavior, the history table holds a 2-bit hint value. The 2-bit hint indicates if the next branch must be predicted taken or predicted not-taken based on the behavior of previous branches. The history table contains 256 entries.

For loops beyond a certain number of iterations, the branch history is not large enough to learn the history and predict the loop exit. The PFU includes logic to count the number of iterations (up to 31) of a loop, and thereby predict the not-taken branch that exits the loop. If the number of iterations taken exceeds 31, the loop branch is never predicted as not-taken.

If multiple branch histories index into the same hint value, this can cause thrashing in the history table and reduce accuracy of the branch predictor. Logic in the branch predictor detects these cases and provides some hysteresis for the hint value.

For direct branches, the target address is calculated statically from the instruction encoding and the program counter. For indirect branches, the hint value predicts if the branch is taken or not-taken, and the return stack can sometimes be used to predict the target address. When the destination of a branch cannot be calculated statically, or popped from the return stack, PFU assumes the branch to be not-taken.

The PFU updates the history for each occurrence of a branch when the DPU indicates how the branch was resolved.

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