diff --git a/src/bitvec.rs b/src/bitvec.rs
index 46a56ff..5c2cc2f 100644
--- a/src/bitvec.rs
+++ b/src/bitvec.rs
@@ -177,7 +177,7 @@ impl<'a> std::iter::Iterator for AdaptiveMapIter<'a> {
 
 /// A conceptually infinite-length bitvector that allows bitwise operations and
 /// iteration over set bits efficiently.
-#[derive(Clone, Debug)]
+#[derive(Clone)]
 pub struct BitVec {
     elems: AdaptiveMap,
 }
@@ -275,6 +275,13 @@ impl Iterator for SetBitsIter {
     }
 }
 
+impl std::fmt::Debug for BitVec {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        let vals = self.iter().collect::<Vec<_>>();
+        write!(f, "{:?}", vals)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::BitVec;
diff --git a/src/fuzzing/func.rs b/src/fuzzing/func.rs
index 5e8bff8..ae8dcce 100644
--- a/src/fuzzing/func.rs
+++ b/src/fuzzing/func.rs
@@ -130,7 +130,7 @@ impl Function for Func {
         &self.reftype_vregs[..]
     }
 
-    fn is_move(&self, insn: Inst) -> Option<(VReg, VReg)> {
+    fn is_move(&self, _: Inst) -> Option<(VReg, VReg)> {
         None
     }
 
diff --git a/src/ion/mod.rs b/src/ion/mod.rs
index a93ded7..0a5a816 100644
--- a/src/ion/mod.rs
+++ b/src/ion/mod.rs
@@ -267,7 +267,6 @@ struct Env<'a, F: Function> {
     env: &'a MachineEnv,
     cfginfo: CFGInfo,
     liveins: Vec<BitVec>,
-    livein_parents: Vec<Vec<Block>>,
     /// Blockparam outputs: from-vreg, (end of) from-block, (start of)
     /// to-block, to-vreg. The field order is significant: these are sorted so
     /// that a scan over vregs, then blocks in each range, can scan in
@@ -664,7 +663,6 @@ impl<'a, F: Function> Env<'a, F> {
             cfginfo,
 
             liveins: vec![],
-            livein_parents: vec![],
             blockparam_outs: vec![],
             blockparam_ins: vec![],
             blockparam_allocs: vec![],
@@ -1016,23 +1014,13 @@ impl<'a, F: Function> Env<'a, F> {
     }
 
     fn is_live_in(&mut self, block: Block, vreg: VRegIndex) -> bool {
-        if self.liveins[block.index()].get(vreg.index()) {
-            return true;
-        }
-        for &parent in &self.livein_parents[block.index()] {
-            if self.liveins[parent.index()].get(vreg.index()) {
-                self.liveins[block.index()].set(vreg.index(), true);
-                return true;
-            }
-        }
-        false
+        self.liveins[block.index()].get(vreg.index())
     }
 
     fn compute_liveness(&mut self) {
         // Create initial LiveIn bitsets.
         for _ in 0..self.func.blocks() {
             self.liveins.push(BitVec::new());
-            self.livein_parents.push(vec![]);
         }
 
         let mut num_ranges = 0;
@@ -1428,7 +1416,7 @@ impl<'a, F: Function> Env<'a, F> {
                 );
                 log::debug!(" -> loop range {:?}", loop_range);
                 for &loopblock in loop_blocks {
-                    self.livein_parents[loopblock.index()].push(block);
+                    self.liveins[loopblock.index()].or(&live);
                 }
                 for vreg in live.iter() {
                     log::debug!(
diff --git a/src/lib.rs b/src/lib.rs
index 0232813..437e6ba 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -779,11 +779,25 @@ pub enum InstPosition {
 }
 
 /// A program point: a single point before or after a given instruction.
-#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct ProgPoint {
     bits: u32,
 }
 
+impl std::fmt::Debug for ProgPoint {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(
+            f,
+            "progpoint{}{}",
+            self.inst().index(),
+            match self.pos() {
+                InstPosition::Before => "-pre",
+                InstPosition::After => "-post",
+            }
+        )
+    }
+}
+
 impl ProgPoint {
     pub fn new(inst: Inst, pos: InstPosition) -> Self {
         let bits = ((inst.0 as u32) << 1) | (pos as u8 as u32);