diff --git a/src/libcretonne/entity_map.rs b/src/libcretonne/entity_map.rs
new file mode 100644
index 0000000000..3e1f16a595
--- /dev/null
+++ b/src/libcretonne/entity_map.rs
@@ -0,0 +1,130 @@
+//! Densely numbered entity references as mapping keys.
+//!
+//! This module defines an `EntityRef` trait that should be implemented by reference types wrapping
+//! a small integer index. The `EntityMap` data structure uses the dense index space to implement a
+//! map with a vector.
+
+use std::vec::Vec;
+use std::default::Default;
+use std::marker::PhantomData;
+use std::ops::{Index, IndexMut};
+
+/// A type wrapping a small integer index should implement `EntityRef` so it can be used as the key
+/// of an `EntityMap`.
+pub trait EntityRef: Copy {
+    /// Create a new entity reference from a small integer.
+    /// This should crash if the requested index is not representable.
+    fn new(usize) -> Self;
+
+    /// Get the index that was used to create this entity reference.
+    fn index(self) -> usize;
+}
+
+/// A mapping `K -> V` for densely indexed entity references.
+pub struct EntityMap<K, V>
+    where K: EntityRef,
+          V: Clone + Default
+{
+    elems: Vec<V>,
+    unused: PhantomData<K>,
+}
+
+impl<K, V> EntityMap<K, V>
+    where K: EntityRef,
+          V: Clone + Default
+{
+    /// Create a new empty map.
+    pub fn new() -> Self {
+        EntityMap {
+            elems: Vec::new(),
+            unused: PhantomData,
+        }
+    }
+
+    /// 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())
+        }
+    }
+
+    /// Append `v` to the mapping, assigning a new key which is returned.
+    pub fn push(&mut self, v: V) -> K {
+        let k = K::new(self.elems.len());
+        self.elems.push(v);
+        k
+    }
+}
+
+/// Immutable indexing into an `EntityMap`.
+/// The indexed value must have been accessed mutably previously, or the key passed to `ensure()`.
+impl<K, V> Index<K> for EntityMap<K, V>
+    where K: EntityRef,
+          V: Clone + Default
+{
+    type Output = V;
+
+    fn index(&self, k: K) -> &V {
+        &self.elems[k.index()]
+    }
+}
+
+/// Mutable indexing into an `EntityMap`.
+/// The map is resized automatically if the key has not been used before.
+impl<K, V> IndexMut<K> for EntityMap<K, V>
+    where K: EntityRef,
+          V: Clone + Default
+{
+    fn index_mut(&mut self, k: K) -> &mut V {
+        self.ensure(k);
+        &mut self.elems[k.index()]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // EntityRef impl for testing.
+    #[derive(Clone, Copy)]
+    struct E(u32);
+
+    impl EntityRef for E {
+        fn new(i: usize) -> Self {
+            E(i as u32)
+        }
+        fn index(self) -> usize {
+            self.0 as usize
+        }
+    }
+
+    #[test]
+    fn basic() {
+        let r0 = E(0);
+        let r1 = E(1);
+        let r2 = E(2);
+        let mut m = EntityMap::new();
+
+        m[r2] = 3;
+        m[r1] = 5;
+
+        assert_eq!(m[r1], 5);
+        assert_eq!(m[r2], 3);
+
+        let shared = &m;
+        assert_eq!(shared[r0], 0);
+        assert_eq!(shared[r1], 5);
+        assert_eq!(shared[r2], 3);
+    }
+
+    #[test]
+    fn push() {
+        let mut m = EntityMap::new();
+        let k1: E = m.push(12);
+        let k2 = m.push(33);
+
+        assert_eq!(m[k1], 12);
+        assert_eq!(m[k2], 33);
+    }
+}
diff --git a/src/libcretonne/lib.rs b/src/libcretonne/lib.rs
index f0bf55af39..55d601d38b 100644
--- a/src/libcretonne/lib.rs
+++ b/src/libcretonne/lib.rs
@@ -15,3 +15,5 @@ pub mod instructions;
 pub mod repr;
 pub mod write;
 pub mod cfg;
+
+pub mod entity_map;