diff --git a/lib/cretonne/src/lib.rs b/lib/cretonne/src/lib.rs
index 6445d7866b..67694e139f 100644
--- a/lib/cretonne/src/lib.rs
+++ b/lib/cretonne/src/lib.rs
@@ -14,6 +14,7 @@ pub mod isa;
 pub mod cfg;
 pub mod dominator_tree;
 pub mod entity_map;
+pub mod sparse_map;
 pub mod settings;
 pub mod verifier;
 pub mod regalloc;
diff --git a/lib/cretonne/src/sparse_map.rs b/lib/cretonne/src/sparse_map.rs
new file mode 100644
index 0000000000..5194d8a760
--- /dev/null
+++ b/lib/cretonne/src/sparse_map.rs
@@ -0,0 +1,297 @@
+//! Sparse mapping of entity references to larger value types.
+//!
+//! This module provides a `SparseMap` data structure which implements a sparse mapping from an
+//! `EntityRef` key to a value type that may be on the larger side. This implementation is based on
+//! the paper:
+//!
+//! > Briggs, Torczon, *An efficient representation for sparse sets*,
+//!   ACM Letters on Programming Languages and Systems, Volume 2, Issue 1-4, March-Dec. 1993.
+//!
+//! A `SparseMap<K, V>` map provides:
+//!
+//! - Memory usage equivalent to `EntityMap<K, u32>` + `Vec<V>`, so much smaller than
+//!   `EntityMap<K, V>` for sparse mappings of larger `V` types.
+//! - Constant time lookup, slightly slower than `EntityMap`.
+//! - A very fast, constant time `clear()` operation.
+//! - Fast insert and erase operations.
+//! - Stable iteration that is as fast as a `Vec<V>`.
+//!
+//! # Compared to `EntityMap`
+//!
+//! When should we use a `SparseMap` instead of a secondary `EntityMap`? First of all, `SparseMap`
+//! does not provide the functionality of a primary `EntityMap` which can allocate and assign
+//! entity references to objects as they are pushed onto the map. It is only the secondary
+//! entity maps that can be replaced with a `SparseMap`.
+//!
+//! - A secondary entity map requires its values to implement `Default`, and it is a bit loose
+//!   about creating new mappings to the default value. It doesn't distinguish clearly between an
+//!   unmapped key and one that maps to the default value. `SparseMap` does not require `Default`
+//!   values, and it tracks accurately if a key has been mapped or not.
+//! - Iterating over the contants of an `EntityMap` is linear in the size of the *key space*, while
+//!   iterating over a `SparseMap` is linear in the number of elements in the mapping. This is an
+//!   advantage precisely when the mapping is sparse.
+//! - `SparseMap::clear()` is constant time and super-fast. `EntityMap::clear()` is linear in the
+//!   size of the key space. (Or, rather the required `resize()` call following the `clear()` is).
+//! - `SparseMap` requires the values to implement `SparseMapValue<K>` which means that they must
+//!   contain their own key.
+
+use entity_map::{EntityRef, EntityMap};
+use std::mem;
+use std::u32;
+
+/// Trait for extracting keys from values stored in a `SparseMap`.
+///
+/// All values stored in a `SparseMap` must keep track of their own key in the map and implement
+/// this trait to provide access to the key.
+pub trait SparseMapValue<K> {
+    /// Get the key of this sparse map value. This key is not alowed to change while the value
+    /// is a member of the map.
+    fn key(&self) -> K;
+}
+
+/// A sparse mapping of entity references.
+pub struct SparseMap<K, V>
+    where K: EntityRef,
+          V: SparseMapValue<K>
+{
+    sparse: EntityMap<K, u32>,
+    dense: Vec<V>,
+}
+
+impl<K, V> SparseMap<K, V>
+    where K: EntityRef,
+          V: SparseMapValue<K>
+{
+    /// Create a new empty mapping.
+    pub fn new() -> Self {
+        SparseMap {
+            sparse: EntityMap::new(),
+            dense: Vec::new(),
+        }
+    }
+
+    /// Returns the number of elements in the map.
+    pub fn len(&self) -> usize {
+        self.dense.len()
+    }
+
+    /// Returns true is the map contains no elements.
+    pub fn is_empty(&self) -> bool {
+        self.dense.is_empty()
+    }
+
+    /// Returns a reference to the value corresponding to the key.
+    pub fn get(&self, key: K) -> Option<&V> {
+        if let Some(idx) = self.sparse.get(key).cloned() {
+            if let Some(entry) = self.dense.get(idx as usize) {
+                if entry.key() == key {
+                    return Some(entry);
+                }
+            }
+        }
+        None
+    }
+
+    /// Returns a mutable reference to the value corresponding to the key.
+    ///
+    /// Note that the returned value must not be mutated in a way that would change its key. This
+    /// would invalidate the sparse set data structure.
+    pub fn get_mut(&mut self, key: K) -> Option<&mut V> {
+        if let Some(idx) = self.sparse.get(key).cloned() {
+            if let Some(entry) = self.dense.get_mut(idx as usize) {
+                if entry.key() == key {
+                    return Some(entry);
+                }
+            }
+        }
+        None
+    }
+
+    /// Return the index into `dense` of the value corresponding to `key`.
+    fn index(&self, key: K) -> Option<usize> {
+        if let Some(idx) = self.sparse.get(key).cloned() {
+            let idx = idx as usize;
+            if let Some(entry) = self.dense.get(idx) {
+                if entry.key() == key {
+                    return Some(idx);
+                }
+            }
+        }
+        None
+    }
+
+    /// Insert a value into the map.
+    ///
+    /// If the map did not have this key present, `None` is returned.
+    ///
+    /// If the map did have this key present, the value is updated, and the old value is returned.
+    ///
+    /// It is not necessary to provide a key since the value knows its own key already.
+    pub fn insert(&mut self, value: V) -> Option<V> {
+        let key = value.key();
+
+        // Replace the existing entry for `key` if there is one.
+        if let Some(entry) = self.get_mut(key) {
+            return Some(mem::replace(entry, value));
+        }
+
+        // There was no previous entry for `key`. Add it to the end of `dense`.
+        let idx = self.dense.len();
+        assert!(idx <= u32::MAX as usize, "SparseMap overflow");
+        self.dense.push(value);
+        *self.sparse.ensure(key) = idx as u32;
+        None
+    }
+
+    /// Remove a value from the map and return it.
+    pub fn remove(&mut self, key: K) -> Option<V> {
+        if let Some(idx) = self.index(key) {
+            let back = self.dense.pop().unwrap();
+
+            // Are we popping the back of `dense`?
+            if idx == self.dense.len() {
+                return Some(back);
+            }
+
+            // We're removing an element from the middle of `dense`.
+            // Replace the element at `idx` with the back of `dense`.
+            // Repair `sparse` first.
+            self.sparse[back.key()] = idx as u32;
+            return Some(mem::replace(&mut self.dense[idx], back));
+        }
+
+        // Nothing to remove.
+        None
+    }
+}
+
+/// Any `EntityRef` can be used as a sparse map value representing itself.
+impl<T> SparseMapValue<T> for T
+    where T: EntityRef
+{
+    fn key(&self) -> T {
+        *self
+    }
+}
+
+/// A sparse set of entity references.
+///
+/// Any type that implements `EntityRef` can be used as a sparse set value too.
+pub type SparseSet<T> = SparseMap<T, T>;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use entity_map::EntityRef;
+    use ir::Inst;
+
+    // Mock key-value object for testing.
+    #[derive(PartialEq, Eq, Debug)]
+    struct Obj(Inst, &'static str);
+
+    impl SparseMapValue<Inst> for Obj {
+        fn key(&self) -> Inst {
+            self.0
+        }
+    }
+
+    #[test]
+    fn empty_immutable_map() {
+        let i1 = Inst::new(1);
+        let map: SparseMap<Inst, Obj> = SparseMap::new();
+
+        assert!(map.is_empty());
+        assert_eq!(map.len(), 0);
+        assert_eq!(map.get(i1), None);
+    }
+
+    #[test]
+    fn single_entry() {
+        let i0 = Inst::new(0);
+        let i1 = Inst::new(1);
+        let i2 = Inst::new(2);
+        let mut map = SparseMap::new();
+
+        assert!(map.is_empty());
+        assert_eq!(map.len(), 0);
+        assert_eq!(map.get(i1), None);
+        assert_eq!(map.get_mut(i1), None);
+        assert_eq!(map.remove(i1), None);
+
+        assert_eq!(map.insert(Obj(i1, "hi")), None);
+        assert!(!map.is_empty());
+        assert_eq!(map.len(), 1);
+        assert_eq!(map.get(i0), None);
+        assert_eq!(map.get(i1), Some(&Obj(i1, "hi")));
+        assert_eq!(map.get(i2), None);
+        assert_eq!(map.get_mut(i0), None);
+        assert_eq!(map.get_mut(i1), Some(&mut Obj(i1, "hi")));
+        assert_eq!(map.get_mut(i2), None);
+
+        assert_eq!(map.remove(i0), None);
+        assert_eq!(map.remove(i2), None);
+        assert_eq!(map.remove(i1), Some(Obj(i1, "hi")));
+        assert_eq!(map.len(), 0);
+        assert_eq!(map.get(i1), None);
+        assert_eq!(map.get_mut(i1), None);
+        assert_eq!(map.remove(i0), None);
+        assert_eq!(map.remove(i1), None);
+        assert_eq!(map.remove(i2), None);
+    }
+
+    #[test]
+    fn multiple_entries() {
+        let i0 = Inst::new(0);
+        let i1 = Inst::new(1);
+        let i2 = Inst::new(2);
+        let i3 = Inst::new(3);
+        let mut map = SparseMap::new();
+
+        assert_eq!(map.insert(Obj(i2, "foo")), None);
+        assert_eq!(map.insert(Obj(i1, "bar")), None);
+        assert_eq!(map.insert(Obj(i0, "baz")), None);
+
+        assert_eq!(map.len(), 3);
+        assert_eq!(map.get(i0), Some(&Obj(i0, "baz")));
+        assert_eq!(map.get(i1), Some(&Obj(i1, "bar")));
+        assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
+        assert_eq!(map.get(i3), None);
+
+        // Remove front object, causing back to be swapped down.
+        assert_eq!(map.remove(i1), Some(Obj(i1, "bar")));
+        assert_eq!(map.len(), 2);
+        assert_eq!(map.get(i0), Some(&Obj(i0, "baz")));
+        assert_eq!(map.get(i1), None);
+        assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
+        assert_eq!(map.get(i3), None);
+
+        // Reinsert something at a previously used key.
+        assert_eq!(map.insert(Obj(i1, "barbar")), None);
+        assert_eq!(map.len(), 3);
+        assert_eq!(map.get(i0), Some(&Obj(i0, "baz")));
+        assert_eq!(map.get(i1), Some(&Obj(i1, "barbar")));
+        assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
+        assert_eq!(map.get(i3), None);
+
+        // Replace an entry.
+        assert_eq!(map.insert(Obj(i0, "bazbaz")), Some(Obj(i0, "baz")));
+        assert_eq!(map.len(), 3);
+        assert_eq!(map.get(i0), Some(&Obj(i0, "bazbaz")));
+        assert_eq!(map.get(i1), Some(&Obj(i1, "barbar")));
+        assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
+        assert_eq!(map.get(i3), None);
+    }
+
+    #[test]
+    fn entity_set() {
+        let i0 = Inst::new(0);
+        let i1 = Inst::new(1);
+        let mut set = SparseSet::new();
+
+        assert_eq!(set.insert(i0), None);
+        assert_eq!(set.insert(i0), Some(i0));
+        assert_eq!(set.insert(i1), None);
+        assert_eq!(set.get(i0), Some(&i0));
+        assert_eq!(set.get(i1), Some(&i1));
+    }
+}