diff --git a/src/ion/data_structures.rs b/src/ion/data_structures.rs
index 9dd5cb3b..a81ee8fb 100644
--- a/src/ion/data_structures.rs
+++ b/src/ion/data_structures.rs
@@ -295,7 +295,7 @@ const fn no_bloat_capacity<T>() -> usize {
 #[derive(Clone, Debug)]
 pub struct SpillSet {
     pub slot: SpillSlotIndex,
-    pub reg_hint: PReg,
+    pub hint: PReg,
     pub class: RegClass,
     pub spill_bundle: LiveBundleIndex,
     pub required: bool,
@@ -594,7 +594,7 @@ pub struct PrioQueue {
 pub struct PrioQueueEntry {
     pub prio: u32,
     pub bundle: LiveBundleIndex,
-    pub reg_hint: PReg,
+    pub hint: PReg,
 }
 
 #[derive(Clone, Debug)]
@@ -664,11 +664,11 @@ impl<'a> ContainerComparator for PrioQueueComparator<'a> {
 
 impl PrioQueue {
     #[inline(always)]
-    pub fn insert(&mut self, bundle: LiveBundleIndex, prio: usize, reg_hint: PReg) {
+    pub fn insert(&mut self, bundle: LiveBundleIndex, prio: usize, hint: PReg) {
         self.heap.push(PrioQueueEntry {
             prio: prio as u32,
             bundle,
-            reg_hint,
+            hint,
         });
     }
 
@@ -679,7 +679,7 @@ impl PrioQueue {
 
     #[inline(always)]
     pub fn pop(&mut self) -> Option<(LiveBundleIndex, PReg)> {
-        self.heap.pop().map(|entry| (entry.bundle, entry.reg_hint))
+        self.heap.pop().map(|entry| (entry.bundle, entry.hint))
     }
 }
 
diff --git a/src/ion/merge.rs b/src/ion/merge.rs
index 3ca3a9c3..58a80aac 100644
--- a/src/ion/merge.rs
+++ b/src/ion/merge.rs
@@ -315,7 +315,7 @@ impl<'a, F: Function> Env<'a, F> {
                 slot: SpillSlotIndex::invalid(),
                 required: false,
                 class: reg.class(),
-                reg_hint: PReg::invalid(),
+                hint: PReg::invalid(),
                 spill_bundle: LiveBundleIndex::invalid(),
                 splits: 0,
                 range,
diff --git a/src/ion/moves.rs b/src/ion/moves.rs
index 11159a2a..95195238 100644
--- a/src/ion/moves.rs
+++ b/src/ion/moves.rs
@@ -892,8 +892,7 @@ impl<'a, F: Function> Env<'a, F> {
                 }
 
                 let resolved = parallel_moves.resolve();
-                let mut scratch_iter =
-                    RegTraversalIter::new(self.env, regclass, None, PReg::invalid(), 0);
+                let mut scratch_iter = RegTraversalIter::new(self.env, regclass, None, None, 0);
                 let mut dedicated_scratch = self.env.scratch_by_class[regclass as usize];
                 let key = LiveRangeKey::from_range(&CodeRange {
                     from: pos_prio.pos,
diff --git a/src/ion/process.rs b/src/ion/process.rs
index 982f7e93..6dba89cb 100644
--- a/src/ion/process.rs
+++ b/src/ion/process.rs
@@ -38,9 +38,9 @@ pub enum AllocRegResult<'a> {
 
 impl<'a, F: Function> Env<'a, F> {
     pub fn process_bundles(&mut self) -> Result<(), RegAllocError> {
-        while let Some((bundle, reg_hint)) = self.ctx.allocation_queue.pop() {
+        while let Some((bundle, hint)) = self.ctx.allocation_queue.pop() {
             self.ctx.output.stats.process_bundle_count += 1;
-            self.process_bundle(bundle, reg_hint)?;
+            self.process_bundle(bundle, hint)?;
         }
         self.ctx.output.stats.final_liverange_count = self.ranges.len();
         self.ctx.output.stats.final_bundle_count = self.bundles.len();
@@ -408,17 +408,14 @@ impl<'a, F: Function> Env<'a, F> {
         &mut self,
         bundle: LiveBundleIndex,
         mut split_at: ProgPoint,
-        reg_hint: PReg,
+        hint: PReg,
         // Do we trim the parts around the split and put them in the
         // spill bundle?
         mut trim_ends_into_spill_bundle: bool,
     ) {
         self.ctx.output.stats.splits += 1;
         trace!(
-            "split bundle {:?} at {:?} and requeue with reg hint (for first part) {:?}",
-            bundle,
-            split_at,
-            reg_hint,
+            "split bundle {bundle:?} at {split_at:?} and requeue with reg hint (for first part) {hint:?}"
         );
 
         // Split `bundle` at `split_at`, creating new LiveRanges and
@@ -432,7 +429,7 @@ impl<'a, F: Function> Env<'a, F> {
         // bundle. See the doc-comment on
         // `split_into_minimal_bundles()` above for more.
         if self.ctx.spillsets[spillset].splits >= MAX_SPLITS_PER_SPILLSET {
-            self.split_into_minimal_bundles(bundle, reg_hint);
+            self.split_into_minimal_bundles(bundle, hint);
             return;
         }
         self.ctx.spillsets[spillset].splits += 1;
@@ -457,7 +454,7 @@ impl<'a, F: Function> Env<'a, F> {
         // minimal-bundle splitting in this case as well.
         if bundle_end.prev().inst() == bundle_start.inst() {
             trace!(" -> spans only one inst; splitting into minimal bundles");
-            self.split_into_minimal_bundles(bundle, reg_hint);
+            self.split_into_minimal_bundles(bundle, hint);
             return;
         }
 
@@ -777,14 +774,14 @@ impl<'a, F: Function> Env<'a, F> {
             let prio = self.ctx.bundles[bundle].prio;
             self.ctx
                 .allocation_queue
-                .insert(bundle, prio as usize, reg_hint);
+                .insert(bundle, prio as usize, hint);
         }
         if self.ctx.bundles[new_bundle].ranges.len() > 0 {
             self.recompute_bundle_properties(new_bundle);
             let prio = self.ctx.bundles[new_bundle].prio;
             self.ctx
                 .allocation_queue
-                .insert(new_bundle, prio as usize, reg_hint);
+                .insert(new_bundle, prio as usize, hint);
         }
     }
 
@@ -818,7 +815,7 @@ impl<'a, F: Function> Env<'a, F> {
     /// the spill bundle; and then does minimal reservations of
     /// registers just at uses/defs and moves the "spilled" value
     /// into/out of them immediately.
-    pub fn split_into_minimal_bundles(&mut self, bundle: LiveBundleIndex, reg_hint: PReg) {
+    pub fn split_into_minimal_bundles(&mut self, bundle: LiveBundleIndex, hint: PReg) {
         assert_eq!(self.ctx.scratch_removed_lrs_vregs.len(), 0);
         self.ctx.scratch_removed_lrs.clear();
 
@@ -830,11 +827,7 @@ impl<'a, F: Function> Env<'a, F> {
             .get_or_create_spill_bundle(bundle, /* create_if_absent = */ true)
             .unwrap();
 
-        trace!(
-            "Splitting bundle {:?} into minimal bundles with reg hint {}",
-            bundle,
-            reg_hint
-        );
+        trace!("Splitting bundle {bundle:?} into minimal bundles with reg hint {hint:?}");
 
         let mut spill_uses = UseList::new_in(self.ctx.bump());
 
@@ -966,7 +959,7 @@ impl<'a, F: Function> Env<'a, F> {
                 let prio = self.ctx.bundles[bundle].prio;
                 self.ctx
                     .allocation_queue
-                    .insert(bundle, prio as usize, reg_hint);
+                    .insert(bundle, prio as usize, hint);
             }
         }
     }
@@ -974,19 +967,20 @@ impl<'a, F: Function> Env<'a, F> {
     pub fn process_bundle(
         &mut self,
         bundle: LiveBundleIndex,
-        reg_hint: PReg,
+        hint: PReg,
     ) -> Result<(), RegAllocError> {
         let class = self.ctx.spillsets[self.bundles[bundle].spillset].class;
+
         // Grab a hint from either the queue or our spillset, if any.
-        let mut hint_reg = if reg_hint != PReg::invalid() {
-            reg_hint
+        let mut hint = if hint != PReg::invalid() {
+            hint
         } else {
-            self.ctx.spillsets[self.bundles[bundle].spillset].reg_hint
+            self.ctx.spillsets[self.bundles[bundle].spillset].hint
         };
-        if self.ctx.pregs[hint_reg.index()].is_stack {
-            hint_reg = PReg::invalid();
+        if self.ctx.pregs[hint.index()].is_stack {
+            hint = PReg::invalid();
         }
-        trace!("process_bundle: bundle {:?} hint {:?}", bundle, hint_reg,);
+        trace!("process_bundle: bundle {bundle:?} hint {hint:?}");
 
         let req = match self.compute_requirement(bundle) {
             Ok(req) => req,
@@ -1002,7 +996,7 @@ impl<'a, F: Function> Env<'a, F> {
                 self.split_and_requeue_bundle(
                     bundle,
                     /* split_at_point = */ conflict.suggested_split_point(),
-                    reg_hint,
+                    hint,
                     /* trim_ends_into_spill_bundle = */
                     conflict.should_trim_edges_around_split(),
                 );
@@ -1078,7 +1072,9 @@ impl<'a, F: Function> Env<'a, F> {
                 + bundle.index();
 
             self.ctx.output.stats.process_bundle_reg_probe_start_any += 1;
-            for preg in RegTraversalIter::new(self.env, class, fixed_preg, hint_reg, scan_offset) {
+            for preg in
+                RegTraversalIter::new(self.env, class, fixed_preg, hint.as_valid(), scan_offset)
+            {
                 self.ctx.output.stats.process_bundle_reg_probes_any += 1;
                 let preg_idx = PRegIndex::new(preg.index());
                 trace!("trying preg {:?}", preg_idx);
@@ -1099,7 +1095,7 @@ impl<'a, F: Function> Env<'a, F> {
                     AllocRegResult::Allocated(alloc) => {
                         self.ctx.output.stats.process_bundle_reg_success_any += 1;
                         trace!(" -> allocated to any {:?}", preg_idx);
-                        self.ctx.spillsets[self.ctx.bundles[bundle].spillset].reg_hint =
+                        self.ctx.spillsets[self.ctx.bundles[bundle].spillset].hint =
                             alloc.as_reg().unwrap();
                         // Success, return scratch memory to context and finish
                         break 'outer;
diff --git a/src/ion/reg_traversal.rs b/src/ion/reg_traversal.rs
index eb4ef6b9..7ad4611d 100644
--- a/src/ion/reg_traversal.rs
+++ b/src/ion/reg_traversal.rs
@@ -79,14 +79,11 @@ impl<'a> RegTraversalIter<'a> {
         env: &'a MachineEnv,
         class: RegClass,
         fixed: Option<PReg>,
-        hint: PReg,
+        hint: Option<PReg>,
         offset: usize,
     ) -> Self {
-        let hint = if hint != PReg::invalid() {
-            Some(hint)
-        } else {
-            None
-        };
+        debug_assert!(fixed != Some(PReg::invalid()));
+        debug_assert!(hint != Some(PReg::invalid()));
 
         let class = class as u8 as usize;
         let preferred = Cursor::new(&env.preferred_regs_by_class[class], offset);
diff --git a/src/ion/spill.rs b/src/ion/spill.rs
index 61d8b70f..3e1f8d2f 100644
--- a/src/ion/spill.rs
+++ b/src/ion/spill.rs
@@ -30,7 +30,9 @@ impl<'a, F: Function> Env<'a, F> {
             }
 
             let class = self.ctx.spillsets[self.ctx.bundles[bundle].spillset].class;
-            let hint = self.ctx.spillsets[self.ctx.bundles[bundle].spillset].reg_hint;
+            let hint = self.ctx.spillsets[self.ctx.bundles[bundle].spillset]
+                .hint
+                .as_valid();
 
             // This may be an empty-range bundle whose ranges are not
             // sorted; sort all range-lists again here.
diff --git a/src/lib.rs b/src/lib.rs
index c37c3540..4d10e452 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -115,6 +115,7 @@ impl PReg {
     pub const MAX_BITS: usize = 6;
     pub const MAX: usize = (1 << Self::MAX_BITS) - 1;
     pub const NUM_INDEX: usize = 1 << (Self::MAX_BITS + 2); // including RegClass bits
+    pub const INVALID: u8 = ((RegClass::Int as u8) << Self::MAX_BITS) | (Self::MAX as u8);
 
     /// Create a new PReg. The `hw_enc` range is 6 bits.
     #[inline(always)]
@@ -162,7 +163,19 @@ impl PReg {
     /// data structures.
     #[inline(always)]
     pub const fn invalid() -> Self {
-        PReg::new(Self::MAX, RegClass::Int)
+        PReg {
+            bits: Self::INVALID,
+        }
+    }
+
+    /// Return a valid [`PReg`] or [`None`] if it is invalid.
+    #[inline(always)]
+    pub const fn as_valid(self) -> Option<Self> {
+        if self.bits == Self::INVALID {
+            None
+        } else {
+            Some(self)
+        }
     }
 }