diff --git a/ykpack/src/types.rs b/ykpack/src/types.rs
index 2b05d48dd..4425b0e19 100644
--- a/ykpack/src/types.rs
+++ b/ykpack/src/types.rs
@@ -92,6 +92,8 @@ impl Display for BasicBlock {
 #[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Clone)]
 pub enum Statement {
     Nop,
+    /// This is a special instruction used only in SSA generation.
+    SsaEntryDefs(Vec<LocalIndex>),
     Assign(Place, Rvalue),
     Unimplemented, // FIXME
 }
@@ -102,12 +104,61 @@ impl Display for Statement {
     }
 }
 
+impl Statement {
+    pub fn uses_vars_mut(&mut self) -> Vec<&mut LocalIndex> {
+        match self {
+            Statement::Nop | Statement::Unimplemented | Statement::SsaEntryDefs(_) => vec![],
+            Statement::Assign(_, rv) => rv.uses_vars_mut(),
+        }
+    }
+
+    pub fn defs_vars_mut(&mut self) -> Vec<&mut LocalIndex> {
+        match self {
+            Statement::Nop | Statement::Unimplemented => vec![],
+            Statement::Assign(p, _) => p.defs_vars_mut(),
+            Statement::SsaEntryDefs(ref mut vs) => vs.iter_mut().collect(),
+        }
+    }
+
+    pub fn is_phi(&self) -> bool {
+        if let Statement::Assign(_, Rvalue::Phi(..)) = self {
+            return true;
+        }
+        false
+    }
+
+    pub fn phi_arg_mut(&mut self, j: usize) -> Option<&mut LocalIndex> {
+        if let Statement::Assign(_, Rvalue::Phi(ps)) = self {
+            if let Place::Local(ref mut l) = ps[j] {
+                return Some(l);
+            }
+        }
+        None
+    }
+}
+
 #[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Clone)]
 pub enum Place {
     Local(LocalIndex),
     Unimplemented, // FIXME
 }
 
+impl Place {
+    fn uses_vars_mut(&mut self) -> Vec<&mut LocalIndex> {
+        match self {
+            Place::Local(l) => vec![l],
+            Place::Unimplemented => vec![],
+        }
+    }
+
+    fn defs_vars_mut(&mut self) -> Vec<&mut LocalIndex> {
+        match self {
+            Place::Local(l) => vec![l],
+            Place::Unimplemented => vec![],
+        }
+    }
+}
+
 #[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Clone)]
 pub enum Rvalue {
     Place(Place),
@@ -115,6 +166,23 @@ pub enum Rvalue {
     Unimplemented, // FIXME
 }
 
+impl Rvalue {
+    fn uses_vars_mut(&mut self) -> Vec<&mut LocalIndex> {
+        match self {
+            Rvalue::Place(p) => p.uses_vars_mut(),
+            Rvalue::Phi(ps) => {
+                let mut res = Vec::new();
+                ps.iter_mut().fold(&mut res, |r, p| {
+                    r.extend(p.uses_vars_mut());
+                    r
+                });
+                res
+            }
+            Rvalue::Unimplemented => vec![],
+        }
+    }
+}
+
 /// A call target.
 #[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Clone)]
 pub enum CallOperand {
@@ -135,7 +203,10 @@ pub enum Terminator {
     },
     Resume,
     Abort,
-    Return,
+    Return {
+        // Because TIR is in SSA, we have to say which SSA variable to return.
+        local: LocalIndex
+    },
     Unreachable,
     Drop {
         target_bb: BasicBlockIndex,
@@ -167,6 +238,25 @@ impl Display for Terminator {
     }
 }
 
+impl Terminator {
+    pub fn uses_vars_mut(&mut self) -> Vec<&mut LocalIndex> {
+        match self {
+            Terminator::GeneratorDrop
+            | Terminator::DropAndReplace { .. }
+            | Terminator::Drop { .. }
+            | Terminator::Unreachable
+            | Terminator::Goto { .. }
+            | Terminator::Resume
+            | Terminator::Abort => Vec::new(),
+            Terminator::SwitchInt { .. } => Vec::new(), // FIXME has a condition which will use.
+            Terminator::Return { local: ref mut v } => vec![v],
+            Terminator::Call { .. } => Vec::new(), // FIXME, may use a local variable.
+            Terminator::Assert { .. } => Vec::new(), // FIXME has a condition var.
+            Terminator::Yield { .. } => Vec::new(), // FIXME check semantics of this terminator.
+        }
+    }
+}
+
 /// The top-level pack type.
 #[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Clone)]
 pub enum Pack {
@@ -179,3 +269,38 @@ impl Display for Pack {
         write!(f, "{}", mir)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{Place, Rvalue, Statement};
+
+    #[test]
+    fn assign_uses_vars_mut() {
+        let mut s = Statement::Assign(Place::Local(42), Rvalue::Place(Place::Local(43)));
+        assert_eq!(s.uses_vars_mut(), vec![&mut 43]);
+    }
+
+    #[test]
+    fn assign_defs_vars_mut() {
+        let mut s = Statement::Assign(Place::Local(42), Rvalue::Place(Place::Local(43)));
+        assert_eq!(s.defs_vars_mut(), vec![&mut 42]);
+    }
+
+    #[test]
+    fn phi_uses_vars_mut() {
+        let mut s = Statement::Assign(
+            Place::Local(44),
+            Rvalue::Phi(vec![Place::Local(100), Place::Local(200)]),
+        );
+        assert_eq!(s.uses_vars_mut(), vec![&mut 100, &mut 200]);
+    }
+
+    #[test]
+    fn phi_defs_vars_mut() {
+        let mut s = Statement::Assign(
+            Place::Local(44),
+            Rvalue::Phi(vec![Place::Local(100), Place::Local(200)]),
+        );
+        assert_eq!(s.defs_vars_mut(), vec![&mut 44]);
+    }
+}