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Heuristic improvement: reg-scan offset by inst location.
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.
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