Reply To: Prediction

The Mill doesn’t put “expects” in the code, because the entropy cost of the field in the encoding then would have to be paid on every use of such instructions. Instead “expects” go in the loadable prediction table in the load module. The effect as far as initial prediction quality is the same, but the predictor table is promptly overwritten by actual experience history.

Also, “expects” in the code only can supply taken/not taken information, which arrives much too late to be useful in guiding decode. Instead the Mill predictor carries “where to next” information so that it can run-ahead of fetch and thereby avoid code fetch stalls.

Will is right that anything the program can do to help guide the process is useful and will be incorporated in the program for better prediction. The difference is in how that added information is represented, and where.