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.

src/ion/mod.rs

@@ -2570,6 +2570,17 @@ impl<'a, F: Function> Env<'a, F> {
                     } else {
                         n_regs
                     };
+                    // Heuristic: start the scan for an available
+                    // register at an offset influenced both by our
+                    // location in the code and by the bundle we're
+                    // considering. This has the effect of spreading
+                    // demand more evenly across registers.
+                    let scan_offset = self.ranges[self.bundles[bundle.index()].first_range.index()]
+                        .range
+                        .from
+                        .inst
+                        .index()
+                        + bundle.index();
                     for i in 0..loop_count {
                         // The order in which we try registers is somewhat complex:
                         // - First, if there is a hint, we try that.
@@ -2587,7 +2598,7 @@ impl<'a, F: Function> Env<'a, F> {
                             (0, Some(hint_reg)) => hint_reg,
                             (i, Some(hint_reg)) => {
                                 let reg = self.env.regs_by_class[class as u8 as usize]
-                                    [(i - 1 + bundle.index()) % n_regs];
+                                    [(i - 1 + scan_offset) % n_regs];
                                 if reg == hint_reg {
                                     continue;
                                 }
@@ -2595,7 +2606,7 @@ impl<'a, F: Function> Env<'a, F> {
                             }
                             (i, None) => {
                                 self.env.regs_by_class[class as u8 as usize]
-                                    [(i + bundle.index()) % n_regs]
+                                    [(i + scan_offset) % n_regs]
                             }
                         };