Reply To: Prediction

The prediction in the patent addresses data prediction in OOO machines, which we are not 🙂 For data access, the Mill does not have a better answer than the patent. Instead, the Mill has changed the question, and the patent’s answer is irrelevant.

As for code prediction, the Mill innovations also change the question: we ask not whether a branch is taken, but where control flow will go in the future. This can be wrapped around any mechanism for recognizing similarity with the past; we don’t have anything novel in the predictor itself. Our initial predictor, for testing purposes, is the dumbest possible two saturating-bit local predictor, known for 30 years. But that predictor could be replaced by a global predictor, or a perceptron, or a … – regardless of the prediction mechanism, the Mill can run fetch ahead without needing to see the code.

Two-bit local predictors run 90-95% accurate (the best modern predictors run 98-99%), so ours will miss every 10 or 20 taken transfers. But that means that the Mill can run 10-20 EBBs ahead, more than enough to hide memory latency. Should be interesting 🙂