diff --git a/ptx/src/translate.rs b/ptx/src/translate.rs
index ac78cbe..e0f69a7 100644
--- a/ptx/src/translate.rs
+++ b/ptx/src/translate.rs
@@ -151,9 +151,9 @@ fn ssa_legalize(
     bbs: Vec<BasicBlock>,
     doms: &Vec<BBIndex>,
     dom_fronts: &Vec<HashSet<BBIndex>>,
-) {
+) -> Vec<Vec<PhiDef>> {
     let phis = gather_phi_sets(&func, max_id, &bbs, dom_fronts);
-    apply_ssa_renaming(func, &bbs, doms, max_id, &phis);
+    apply_ssa_renaming(func, &bbs, doms, max_id, &phis)
 }
 
 // "Modern Compiler Implementation in Java" - Algorithm 19.7
@@ -163,9 +163,9 @@ fn apply_ssa_renaming(
     doms: &[BBIndex],
     max_id: spirv::Word,
     old_phi: &[HashSet<spirv::Word>],
-) {
+) -> Vec<Vec<PhiDef>> {
     let mut dom_tree = vec![Vec::new(); bbs.len()];
-    for (bb, idom) in doms.iter().enumerate() {
+    for (bb, idom) in doms.iter().enumerate().skip(1) {
         dom_tree[idom.0].push(BBIndex(bb));
     }
     let mut old_dst_id = vec![Vec::new(); bbs.len()];
@@ -178,7 +178,7 @@ fn apply_ssa_renaming(
         .iter()
         .map(|ids| {
             ids.iter()
-                .map(|id| (*id, Vec::new()))
+                .map(|id| (*id, (u32::max_value(), HashSet::new())))
                 .collect::<HashMap<_, _>>()
         })
         .collect::<Vec<_>>();
@@ -190,7 +190,7 @@ fn apply_ssa_renaming(
         if let Some((BBIndex(bb), dom_succ_idx)) = state.last_mut() {
             let bb = *bb;
             if *dom_succ_idx == 0 {
-                rename_phi_dst(max_id, &mut ssa_state, &mut new_phi[bb]);
+                rename_phi_dst(&mut ssa_state, &mut new_phi[bb]);
                 rename_bb_body(&mut ssa_state, func, bbs, BBIndex(bb));
                 for BBIndex(succ_idx) in bbs[bb].succ.iter() {
                     rename_succesor_phi_src(&ssa_state, &mut new_phi[*succ_idx]);
@@ -207,22 +207,32 @@ fn apply_ssa_renaming(
             break;
         }
     }
+    new_phi
+        .into_iter()
+        .map(|map| {
+            map.into_iter()
+                .map(|(_, (new_id, defs))| PhiDef {
+                    dst: new_id,
+                    src: defs,
+                })
+                .collect::<Vec<_>>()
+        })
+        .collect::<Vec<_>>()
+}
+
+// before ssa-renaming every phi is x <- phi(x,x,x,x)
+#[derive(Debug, PartialEq)]
+struct PhiDef {
+    dst: spirv::Word,
+    src: HashSet<spirv::Word>,
 }
 
 fn rename_phi_dst(
-    max_old_id: spirv::Word,
     rewriter: &mut SSARewriteState,
-    phi: &mut HashMap<spirv::Word, Vec<spirv::Word>>,
+    phi: &mut HashMap<spirv::Word, (spirv::Word, HashSet<spirv::Word>)>,
 ) {
-    let old_keys = phi
-        .keys()
-        .copied()
-        .filter(|id| *id <= max_old_id)
-        .collect::<Vec<_>>();
-    for k in old_keys.into_iter() {
-        let remapped_id = rewriter.redefine(k);
-        let values = phi.remove(&k).unwrap();
-        phi.insert(remapped_id, values);
+    for (old_k, (new_k, _)) in phi.iter_mut() {
+        *new_k = rewriter.redefine(*old_k);
     }
 }
 
@@ -245,10 +255,10 @@ fn rename_bb_body(
 
 fn rename_succesor_phi_src(
     ssa_state: &SSARewriteState,
-    phi: &mut HashMap<spirv::Word, Vec<spirv::Word>>,
+    phi: &mut HashMap<spirv::Word, (spirv::Word, HashSet<spirv::Word>)>,
 ) {
-    for (id, v) in phi.iter_mut() {
-        v.push(ssa_state.get(*id));
+    for (id, (_, v)) in phi.iter_mut() {
+        v.insert(ssa_state.get(*id));
     }
 }
 
@@ -295,9 +305,10 @@ struct SSARewriteState {
 
 impl SSARewriteState {
     fn new(max: spirv::Word) -> Self {
-        let stack = vec![Vec::new(); max as usize];
+        let len = max + 1;
+        let stack = (0..len).map(|x| vec![x + len]).collect::<Vec<_>>();
         SSARewriteState {
-            next: max + 1,
+            next: 2 * len,
             stack,
         }
     }
@@ -318,40 +329,50 @@ impl SSARewriteState {
     }
 }
 
-// "Modern Compiler Implementation in Java" - Algorithm 19.6
+// "Engineering a Compiler" - Figure 9.9
+// Calculates semi-pruned phis
 fn gather_phi_sets(
     func: &[Statement],
     max_id: spirv::Word,
-    bbs: &[BasicBlock],
+    cfg: &[BasicBlock],
     dom_fronts: &[HashSet<BBIndex>],
 ) -> Vec<HashSet<spirv::Word>> {
-    let mut result = vec![HashSet::new(); bbs.len()];
-    let mut bb_dst_definitions = vec![HashSet::new(); bbs.len()];
-    let mut def_sites = vec![(HashSet::new(), Vec::new()); (max_id as usize) + 1];
-    for bb in 0..bbs.len() {
-        let bb = BBIndex(bb);
-        for s in get_bb_body(func, bbs, bb) {
-            s.for_dst_id(&mut |id| {
-                bb_dst_definitions[bb.0].insert(id);
-                let (ref mut set, ref mut stack) = def_sites[id as usize];
-                if set.insert(bb) {
-                    stack.push(bb);
+    let mut result = vec![HashSet::new(); cfg.len()];
+    let mut globals = HashSet::new();
+    let mut blocks = vec![(Vec::new(), HashSet::new()); (max_id as usize) + 1];
+    for bb in 0..cfg.len() {
+        let mut var_kill = HashSet::new();
+        let mut visitor = |is_dst, id: &u32| {
+            if is_dst {
+                var_kill.insert(*id);
+                let (ref mut stack, ref mut set) = blocks[*id as usize];
+                stack.push(BBIndex(bb));
+                set.insert(BBIndex(bb));
+            } else {
+                if !var_kill.contains(id) {
+                    globals.insert(*id);
                 }
-            });
+            }
+        };
+        for s in get_bb_body(func, cfg, BBIndex(bb)) {
+            match s {
+                Statement::Instruction(pred, inst) => {
+                    pred.as_ref().map(|p| p.visit_id(&mut visitor));
+                    inst.visit_id(&mut visitor);
+                }
+                Statement::Label(_) => (),
+            }
         }
     }
-    for (id, to_work) in def_sites.iter_mut().enumerate() {
-        let id = id as spirv::Word;
-        let (ref mut set, ref mut stack) = to_work;
+    for id in globals {
+        let (ref mut work_stack, ref mut work_set) = &mut blocks[id as usize];
         loop {
-            if let Some(bb) = stack.pop() {
-                set.remove(&bb);
-                for y_bb in &dom_fronts[bb.0] {
-                    if result[y_bb.0].insert(id) {
-                        if !bb_dst_definitions[y_bb.0].contains(&id) {
-                            if set.insert(*y_bb) {
-                                stack.push(*y_bb);
-                            }
+            if let Some(bb) = work_stack.pop() {
+                work_set.remove(&bb);
+                for d_bb in &dom_fronts[bb.0] {
+                    if result[d_bb.0].insert(id) {
+                        if work_set.insert(*d_bb) {
+                            work_stack.push(*d_bb);
                         }
                     }
                 }
@@ -596,6 +617,16 @@ impl Statement {
         }
     }
 
+    fn visit_id<F: FnMut(bool, &spirv::Word)>(&self, f: &mut F) {
+        match self {
+            Statement::Label(id) => f(true, id),
+            Statement::Instruction(pred, inst) => {
+                pred.as_ref().map(|p| p.visit_id(f));
+                inst.visit_id(f);
+            }
+        }
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut spirv::Word)>(&mut self, f: &mut F) {
         match self {
             Statement::Label(id) => f(true, id),
@@ -615,6 +646,10 @@ impl<T> ast::PredAt<T> {
         }
     }
 
+    fn visit_id<F: FnMut(bool, &T)>(&self, f: &mut F) {
+        f(false, &self.label)
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut T)>(&mut self, f: &mut F) {
         f(false, &mut self.label)
     }
@@ -642,6 +677,23 @@ impl<T> ast::Instruction<T> {
         }
     }
 
+    fn visit_id<F: FnMut(bool, &T)>(&self, f: &mut F) {
+        match self {
+            ast::Instruction::Ld(_, a) => a.visit_id(f),
+            ast::Instruction::Mov(_, a) => a.visit_id(f),
+            ast::Instruction::Mul(_, a) => a.visit_id(f),
+            ast::Instruction::Add(_, a) => a.visit_id(f),
+            ast::Instruction::Setp(_, a) => a.visit_id(f),
+            ast::Instruction::SetpBool(_, a) => a.visit_id(f),
+            ast::Instruction::Not(_, a) => a.visit_id(f),
+            ast::Instruction::Cvt(_, a) => a.visit_id(f),
+            ast::Instruction::Shl(_, a) => a.visit_id(f),
+            ast::Instruction::St(_, a) => a.visit_id(f),
+            ast::Instruction::Bra(_, a) => a.visit_id(f),
+            ast::Instruction::Ret(_) => (),
+        }
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut T)>(&mut self, f: &mut F) {
         match self {
             ast::Instruction::Ld(_, a) => a.visit_id_mut(f),
@@ -718,6 +770,10 @@ impl<T> ast::Arg1<T> {
         ast::Arg1 { src: f(self.src) }
     }
 
+    fn visit_id<F: FnMut(bool, &T)>(&self, f: &mut F) {
+        f(false, &self.src);
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut T)>(&mut self, f: &mut F) {
         f(false, &mut self.src);
     }
@@ -731,6 +787,11 @@ impl<T> ast::Arg2<T> {
         }
     }
 
+    fn visit_id<F: FnMut(bool, &T)>(&self, f: &mut F) {
+        f(true, &self.dst);
+        self.src.visit_id(f);
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut T)>(&mut self, f: &mut F) {
         f(true, &mut self.dst);
         self.src.visit_id_mut(f);
@@ -751,6 +812,11 @@ impl<T> ast::Arg2Mov<T> {
         }
     }
 
+    fn visit_id<F: FnMut(bool, &T)>(&self, f: &mut F) {
+        f(true, &self.dst);
+        self.src.visit_id(f);
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut T)>(&mut self, f: &mut F) {
         f(true, &mut self.dst);
         self.src.visit_id_mut(f);
@@ -772,6 +838,12 @@ impl<T> ast::Arg3<T> {
         }
     }
 
+    fn visit_id<F: FnMut(bool, &T)>(&self, f: &mut F) {
+        f(true, &self.dst);
+        self.src1.visit_id(f);
+        self.src2.visit_id(f);
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut T)>(&mut self, f: &mut F) {
         f(true, &mut self.dst);
         self.src1.visit_id_mut(f);
@@ -795,6 +867,13 @@ impl<T> ast::Arg4<T> {
         }
     }
 
+    fn visit_id<F: FnMut(bool, &T)>(&self, f: &mut F) {
+        f(true, &self.dst1);
+        self.dst2.as_ref().map(|i| f(true, i));
+        self.src1.visit_id(f);
+        self.src2.visit_id(f);
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut T)>(&mut self, f: &mut F) {
         f(true, &mut self.dst1);
         self.dst2.as_mut().map(|i| f(true, i));
@@ -821,6 +900,14 @@ impl<T> ast::Arg5<T> {
         }
     }
 
+    fn visit_id<F: FnMut(bool, &T)>(&self, f: &mut F) {
+        f(true, &self.dst1);
+        self.dst2.as_ref().map(|i| f(true, i));
+        self.src1.visit_id(f);
+        self.src2.visit_id(f);
+        self.src3.visit_id(f);
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut T)>(&mut self, f: &mut F) {
         f(true, &mut self.dst1);
         self.dst2.as_mut().map(|i| f(true, i));
@@ -846,6 +933,14 @@ impl<T> ast::Operand<T> {
         }
     }
 
+    fn visit_id<F: FnMut(bool, &T)>(&self, f: &mut F) {
+        match self {
+            ast::Operand::Reg(i) => f(false, i),
+            ast::Operand::RegOffset(i, _) => f(false, i),
+            ast::Operand::Imm(_) => (),
+        }
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut T)>(&mut self, f: &mut F) {
         match self {
             ast::Operand::Reg(i) => f(false, i),
@@ -863,6 +958,13 @@ impl<T> ast::MovOperand<T> {
         }
     }
 
+    fn visit_id<F: FnMut(bool, &T)>(&self, f: &mut F) {
+        match self {
+            ast::MovOperand::Op(o) => o.visit_id(f),
+            ast::MovOperand::Vec(_, _) => (),
+        }
+    }
+
     fn visit_id_mut<F: FnMut(bool, &mut T)>(&mut self, f: &mut F) {
         match self {
             ast::MovOperand::Op(o) => o.visit_id_mut(f),
@@ -1202,7 +1304,7 @@ mod tests {
     }
 
     // page 403
-    const fig_19_4: &'static str = "{
+    const FIG_19_4: &'static str = "{
         mov.u32     i, 1;
         mov.u32     j, 1;
         mov.u32     k, 0;
@@ -1226,8 +1328,8 @@ mod tests {
     }";
 
     #[test]
-    fn gather_phi_sets_fig_19_4() {
-        let func = fig_19_4;
+    fn get_basic_blocks_fig_19_4() {
+        let func = FIG_19_4;
         let mut errors = Vec::new();
         let ast = ptx::FunctionBodyParser::new()
             .parse(&mut errors, func)
@@ -1348,7 +1450,6 @@ mod tests {
         );
     }
 
-    // page 403
     #[test]
     fn dominance_frontiers_fig_19_4() {
         let cfg = cfg_fig_19_4();
@@ -1369,4 +1470,45 @@ mod tests {
         ];
         assert_eq!(dom_fronts, should);
     }
+
+    #[test]
+    fn gather_phi_sets_fig_19_4() {
+        let func = FIG_19_4;
+        let mut errors = Vec::new();
+        let fn_ast = ptx::FunctionBodyParser::new()
+            .parse(&mut errors, func)
+            .unwrap();
+        assert_eq!(errors.len(), 0);
+        let (normalized_ids, max_id) = normalize_identifiers(fn_ast);
+        let bbs = get_basic_blocks(&normalized_ids);
+        let rpostorder = to_reverse_postorder(&bbs);
+        let doms = immediate_dominators(&bbs, &rpostorder);
+        let dom_fronts = dominance_frontiers(&bbs, &doms);
+        let phi = gather_phi_sets(&normalized_ids, max_id, &bbs, &dom_fronts);
+        assert_eq!(
+            phi,
+            vec![
+                HashSet::new(),
+                to_hashset(vec![1, 2]),
+                HashSet::new(),
+                HashSet::new(),
+                HashSet::new(),
+                to_hashset(vec![1, 2]),
+                HashSet::new()
+            ]
+        );
+    }
+
+    fn to_hashset<T: std::hash::Hash + Eq>(v: Vec<T>) -> HashSet<T> {
+        v.into_iter().collect::<HashSet<T>>()
+    }
+
+    fn assert_dst_unique(func: &[Statement]) {
+        let mut seen = HashSet::new();
+        for s in func {
+            s.for_dst_id(&mut |id| {
+                assert!(seen.insert(id));
+            });
+        }
+    }
 }