We currently use a heuristic that our scan for an available PReg
starts at an index into the register list that rotates with the bundle
index. This is a simple way to distribute contention across the whole
register file more evenly and avoid repeating less-likely-to-succeed
reg-map probes to lower-numbered registers for every bundle.
After some experimentation with different options (queue that
dynamically puts registers at end after allocating, various
ways of mixing/hashing indices, etc.), adding the *instruction offset*
(of the start of the first range in the bundle) as well gave the best
results. This is very simple and gives us a likely better-than-random
conflict avoidance because ranges tend to be local, so rotating
through registers as we scan down the list of instructions seems like
a very natural strategy.
On the tests used by our `cargo bench` benchmark, this reduces regfile
probes for the largest (459 instruction) benchmark from 1538 to 829,
i.e., approximately by half, and results in an 11% allocation speedup.
