diff --git a/cranelift/src/libcretonne/cfg.rs b/cranelift/src/libcretonne/cfg.rs
index 4fc2f1b292..8c423c23ac 100644
--- a/cranelift/src/libcretonne/cfg.rs
+++ b/cranelift/src/libcretonne/cfg.rs
@@ -95,6 +95,7 @@ impl ControlFlowGraph {
 #[cfg(test)]
 mod tests {
     use instructions::*;
+    use entity_map::EntityRef;
     use entities::{Ebb, Inst, NO_VALUE};
     use repr::Function;
     use super::*;
diff --git a/cranelift/src/libcretonne/entities.rs b/cranelift/src/libcretonne/entities.rs
index c87ed6a818..33b41e5273 100644
--- a/cranelift/src/libcretonne/entities.rs
+++ b/cranelift/src/libcretonne/entities.rs
@@ -19,6 +19,7 @@
 //! The entity references all implement the `Display` trait in a way that matches the textual IL
 //! format.
 
+use entity_map::EntityRef;
 use std::default::Default;
 use std::fmt::{self, Display, Formatter};
 use std::u32;
@@ -27,12 +28,18 @@ use std::u32;
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, PartialOrd, Ord)]
 pub struct Ebb(u32);
 
-impl Ebb {
-    pub fn new(index: usize) -> Ebb {
+impl EntityRef for Ebb {
+    fn new(index: usize) -> Self {
         assert!(index < (u32::MAX as usize));
         Ebb(index as u32)
     }
 
+    fn index(self) -> usize {
+        self.0 as usize
+    }
+}
+
+impl Ebb {
     /// Create a new EBB reference from its number. This corresponds to the ebbNN representation.
     pub fn with_number(n: u32) -> Option<Ebb> {
         if n < u32::MAX {
@@ -41,10 +48,6 @@ impl Ebb {
             None
         }
     }
-
-    pub fn index(&self) -> usize {
-        self.0 as usize
-    }
 }
 
 /// Display an `Ebb` reference as "ebb12".
@@ -67,13 +70,13 @@ impl Default for Ebb {
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, PartialOrd, Ord)]
 pub struct Inst(u32);
 
-impl Inst {
-    pub fn new(index: usize) -> Inst {
+impl EntityRef for Inst {
+    fn new(index: usize) -> Self {
         assert!(index < (u32::MAX as usize));
         Inst(index as u32)
     }
 
-    pub fn index(&self) -> usize {
+    fn index(self) -> usize {
         self.0 as usize
     }
 }
@@ -188,13 +191,13 @@ impl Default for Value {
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 pub struct StackSlot(u32);
 
-impl StackSlot {
-    pub fn new(index: usize) -> StackSlot {
+impl EntityRef for StackSlot {
+    fn new(index: usize) -> StackSlot {
         assert!(index < (u32::MAX as usize));
         StackSlot(index as u32)
     }
 
-    pub fn index(&self) -> usize {
+    fn index(self) -> usize {
         self.0 as usize
     }
 }
@@ -219,13 +222,13 @@ impl Default for StackSlot {
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 pub struct JumpTable(u32);
 
-impl JumpTable {
-    pub fn new(index: usize) -> JumpTable {
+impl EntityRef for JumpTable {
+    fn new(index: usize) -> JumpTable {
         assert!(index < (u32::MAX as usize));
         JumpTable(index as u32)
     }
 
-    pub fn index(&self) -> usize {
+    fn index(self) -> usize {
         self.0 as usize
     }
 }
@@ -250,6 +253,7 @@ impl Default for JumpTable {
 mod tests {
     use super::*;
     use std::u32;
+    use entity_map::EntityRef;
 
     #[test]
     fn value_with_number() {
diff --git a/cranelift/src/libcretonne/entity_map.rs b/cranelift/src/libcretonne/entity_map.rs
index 3e1f16a595..fbf8109d9f 100644
--- a/cranelift/src/libcretonne/entity_map.rs
+++ b/cranelift/src/libcretonne/entity_map.rs
@@ -41,11 +41,15 @@ impl<K, V> EntityMap<K, V>
         }
     }
 
+    /// Check if `k` is a valid key in the map.
+    pub fn is_valid(&self, k: K) -> bool {
+        k.index() < self.elems.len()
+    }
+
     /// Ensure that `k` is a valid key but adding default entries if necesssary.
     pub fn ensure(&mut self, k: K) {
-        let idx = k.index();
-        if idx >= self.elems.len() {
-            self.elems.resize(idx + 1, V::default())
+        if !self.is_valid(k) {
+            self.elems.resize(k.index() + 1, V::default())
         }
     }
 
@@ -106,7 +110,9 @@ mod tests {
         let r2 = E(2);
         let mut m = EntityMap::new();
 
+        assert!(!m.is_valid(r0));
         m[r2] = 3;
+        assert!(m.is_valid(r1));
         m[r1] = 5;
 
         assert_eq!(m[r1], 5);
diff --git a/cranelift/src/libcretonne/repr.rs b/cranelift/src/libcretonne/repr.rs
index 5a36115fbd..27de00ea66 100644
--- a/cranelift/src/libcretonne/repr.rs
+++ b/cranelift/src/libcretonne/repr.rs
@@ -1,6 +1,7 @@
 //! Representation of Cretonne IL functions.
 
 use types::{Type, FunctionName, Signature, VOID};
+use entity_map::EntityRef;
 use entities::{Ebb, NO_EBB, Inst, NO_INST, Value, NO_VALUE, ExpandedValue, StackSlot};
 use instructions::*;
 use std::fmt::{self, Display, Formatter};