diff --git a/lib/cretonne/src/constant_hash.rs b/lib/cretonne/src/constant_hash.rs
index 08cc2944c8..a165654dce 100644
--- a/lib/cretonne/src/constant_hash.rs
+++ b/lib/cretonne/src/constant_hash.rs
@@ -24,8 +24,12 @@ pub trait Table<K: Copy + Eq> {
 /// The provided `hash` value must have been computed from `key` using the same hash function that
 /// was used to construct the table.
 ///
-/// Returns the table index containing the found entry, or `None` if no entry could be found.
-pub fn probe<K: Copy + Eq, T: Table<K> + ?Sized>(table: &T, key: K, hash: usize) -> Option<usize> {
+/// Returns `Ok(idx)` with the table index containing the found entry, or `Err(idx)` with the empty
+/// sentinel entry if no entry could be found.
+pub fn probe<K: Copy + Eq, T: Table<K> + ?Sized>(table: &T,
+                                                 key: K,
+                                                 hash: usize)
+                                                 -> Result<usize, usize> {
     debug_assert!(table.len().is_power_of_two());
     let mask = table.len() - 1;
 
@@ -36,8 +40,8 @@ pub fn probe<K: Copy + Eq, T: Table<K> + ?Sized>(table: &T, key: K, hash: usize)
         idx &= mask;
 
         match table.key(idx) {
-            None => return None,
-            Some(k) if k == key => return Some(idx),
+            None => return Err(idx),
+            Some(k) if k == key => return Ok(idx),
             _ => {}
         }
 
diff --git a/lib/cretonne/src/ir/instructions.rs b/lib/cretonne/src/ir/instructions.rs
index 808b7cc01a..279a7a37a9 100644
--- a/lib/cretonne/src/ir/instructions.rs
+++ b/lib/cretonne/src/ir/instructions.rs
@@ -85,10 +85,10 @@ impl FromStr for Opcode {
         }
 
         match probe::<&str, [Option<Opcode>]>(&OPCODE_HASH_TABLE, s, simple_hash(s)) {
-            None => Err("Unknown opcode"),
+            Err(_) => Err("Unknown opcode"),
             // We unwrap here because probe() should have ensured that the entry
             // at this index is not None.
-            Some(i) => Ok(OPCODE_HASH_TABLE[i].unwrap()),
+            Ok(i) => Ok(OPCODE_HASH_TABLE[i].unwrap()),
         }
     }
 }
diff --git a/lib/cretonne/src/isa/enc_tables.rs b/lib/cretonne/src/isa/enc_tables.rs
index 3467a2bdfd..b53e80036e 100644
--- a/lib/cretonne/src/isa/enc_tables.rs
+++ b/lib/cretonne/src/isa/enc_tables.rs
@@ -5,6 +5,7 @@
 use ir::{Type, Opcode, InstructionData};
 use isa::{Encoding, Legalize};
 use constant_hash::{Table, probe};
+use std::ops::Range;
 
 /// Level 1 hash table entry.
 ///
@@ -27,6 +28,14 @@ pub struct Level1Entry<OffT: Into<u32> + Copy> {
     pub offset: OffT,
 }
 
+impl<OffT: Into<u32> + Copy> Level1Entry<OffT> {
+    /// Get the level 2 table range indicated by this entry.
+    fn range(&self) -> Range<usize> {
+        let b = self.offset.into() as usize;
+        b..b + (1 << self.log2len)
+    }
+}
+
 impl<OffT: Into<u32> + Copy> Table<Type> for [Level1Entry<OffT>] {
     fn len(&self) -> usize {
         self.len()
@@ -83,20 +92,23 @@ pub fn lookup_enclist<OffT1, OffT2>(ctrl_typevar: Type,
           OffT2: Into<u32> + Copy
 {
     // TODO: The choice of legalization actions here is naive. This needs to be configurable.
-    probe(level1_table, ctrl_typevar, ctrl_typevar.index())
-        .ok_or_else(|| if ctrl_typevar.lane_type().bits() > 32 {
-                        Legalize::Narrow
-                    } else {
-                        Legalize::Expand
-                    })
-        .and_then(|l1idx| {
-                      let l1ent = &level1_table[l1idx];
-                      let l2off = l1ent.offset.into() as usize;
-                      let l2tab = &level2_table[l2off..l2off + (1 << l1ent.log2len)];
-                      probe(l2tab, opcode, opcode as usize)
-                          .map(|l2idx| l2tab[l2idx].offset.into() as usize)
-                          .ok_or(Legalize::Expand)
-                  })
+    match probe(level1_table, ctrl_typevar, ctrl_typevar.index()) {
+        Err(_) => {
+            // No level 1 entry for the type.
+            Err(if ctrl_typevar.lane_type().bits() > 32 {
+                    Legalize::Narrow
+                } else {
+                    Legalize::Expand
+                })
+        }
+        Ok(l1idx) => {
+            let l1ent = &level1_table[l1idx];
+            let l2tab = &level2_table[l1ent.range()];
+            probe(l2tab, opcode, opcode as usize)
+                .map(|l2idx| l2tab[l2idx].offset.into() as usize)
+                .map_err(|_| Legalize::Expand)
+        }
+    }
 }
 
 /// Encoding list entry.
diff --git a/lib/cretonne/src/settings.rs b/lib/cretonne/src/settings.rs
index 19b9fafc21..be5140ef34 100644
--- a/lib/cretonne/src/settings.rs
+++ b/lib/cretonne/src/settings.rs
@@ -83,8 +83,8 @@ impl Builder {
     /// Look up a descriptor by name.
     fn lookup(&self, name: &str) -> Result<(usize, detail::Detail)> {
         match probe(self.template, name, simple_hash(name)) {
-            None => Err(Error::BadName),
-            Some(entry) => {
+            Err(_) => Err(Error::BadName),
+            Ok(entry) => {
                 let d = &self.template.descriptors[self.template.hash_table[entry] as usize];
                 Ok((d.offset as usize, d.detail))
             }