Mill Computing, Inc. Forums The Mill Architecture Grab bag of questions Reply To: Grab bag of questions

Ivan Godard
Keymaster
Post count: 656

– 1) Exit prediction with dynamic addresses.
The cause of the exit transfer doesn’t matter for prediction, with the exception of returns. Most ebbs (hyperblocks) have several conditional ways to exit, one of which is taken; the predictor says where the taken one will go, in which cycle, and the kind of transfer (call/return/branch). The prediction itself doesn’t care whether it was something dynamic. Calls and returns use a predictor-maintained stack of return positions rather than an actual address in the prediction; the return prediction entry itself just says “return” and pops the entry.

Your question seems to assume that prediction is per transfer site, as in per vtable. It’s not; it’s per transferred-to location: if you got here somehow, then you will next get there. History prediction can use a chain (actually a hash) of heres. If a region of code contains a bunch of vtable transfers on the same object, the first will predict that the object will be of the same type (and transfer the same way) as the prior object – and miss if wrong. But the history will then kick in and subsequent transfers will predict the transfer sequence of the last time an object of the same type was processed. There’s a per-member tradeoff between the size of the predictor and the fanout of types that the predictor can handle.