Speculation

Rationale

Speculation is one of the few areas where the Mill favours higher energy consumption, because the performance gains are so great. Unneeded computation still costs energy, but since the Mill architecure is very wide issue and has a lot of functional units, it can exploit a lot of instrucion level parallelism. It saves time. In general purpose code the problem usually is to find that much ILP, because there are so many branches. Most branches only exist to avoid unwanted side effects under certain circumstances.

The Mill has devised a few ways to avoid the unwanted side effects, which means a lot less of the branches in a program are hard barriers to ILP. Phasing is one of the ways. Software Pipelining of loops also makes extensive use of the NaR and the None Metadata tags for this purpose.

Speculation increases ILP across branch boundaries independently of loops. NaR and None and pick enable if-conversion on a massive scale on the Mill, removing branches altogether from the code by utilizing (meta)data flow instead of control flow. And even without pick but with condition-coded parallel branches the ILP is massively increased.

Media

Presentation on Metadata and Speculation by Ivan Godard - Slides