diff --git a/cranelift/peepmatic/crates/automata/src/output_impls.rs b/cranelift/peepmatic/crates/automata/src/output_impls.rs
index 6701f7d867..308c466d5b 100644
--- a/cranelift/peepmatic/crates/automata/src/output_impls.rs
+++ b/cranelift/peepmatic/crates/automata/src/output_impls.rs
@@ -57,6 +57,43 @@ where
     }
 }
 
+impl<T> Output for Box<[T]>
+where
+    T: Clone + Eq + Hash,
+{
+    fn empty() -> Self {
+        vec![].into_boxed_slice()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    fn prefix(a: &Self, b: &Self) -> Self {
+        a.iter()
+            .cloned()
+            .zip(b.iter().cloned())
+            .take_while(|(a, b)| a == b)
+            .map(|(a, _)| a)
+            .collect()
+    }
+
+    fn difference(a: &Self, b: &Self) -> Self {
+        let i = a
+            .iter()
+            .zip(b.iter())
+            .position(|(a, b)| a != b)
+            .unwrap_or(cmp::min(a.len(), b.len()));
+        a[i..].to_vec().into_boxed_slice()
+    }
+
+    fn concat(a: &Self, b: &Self) -> Self {
+        let mut c = a.clone().to_vec();
+        c.extend(b.iter().cloned());
+        c.into_boxed_slice()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use crate::Output;
diff --git a/cranelift/peepmatic/crates/runtime/src/optimizations.rs b/cranelift/peepmatic/crates/runtime/src/optimizations.rs
index 4ed66a8a12..ff838ff720 100644
--- a/cranelift/peepmatic/crates/runtime/src/optimizations.rs
+++ b/cranelift/peepmatic/crates/runtime/src/optimizations.rs
@@ -29,7 +29,7 @@ pub struct PeepholeOptimizations {
 
     /// The underlying automata for matching optimizations' left-hand sides, and
     /// building up the corresponding right-hand side.
-    pub automata: Automaton<MatchResult, MatchOp, Vec<Action>>,
+    pub automata: Automaton<MatchResult, MatchOp, Box<[Action]>>,
 }
 
 impl PeepholeOptimizations {
diff --git a/cranelift/peepmatic/src/automatize.rs b/cranelift/peepmatic/src/automatize.rs
index 5b96339f9f..de053c5574 100644
--- a/cranelift/peepmatic/src/automatize.rs
+++ b/cranelift/peepmatic/src/automatize.rs
@@ -6,10 +6,10 @@ use peepmatic_runtime::linear;
 /// Construct an automaton from a set of linear optimizations.
 pub fn automatize(
     opts: &linear::Optimizations,
-) -> Automaton<linear::MatchResult, linear::MatchOp, Vec<linear::Action>> {
+) -> Automaton<linear::MatchResult, linear::MatchOp, Box<[linear::Action]>> {
     debug_assert!(crate::linear_passes::is_sorted_lexicographically(opts));
 
-    let mut builder = Builder::<linear::MatchResult, linear::MatchOp, Vec<linear::Action>>::new();
+    let mut builder = Builder::<linear::MatchResult, linear::MatchOp, Box<[linear::Action]>>::new();
 
     for opt in &opts.optimizations {
         let mut insertion = builder.insert();
@@ -22,7 +22,7 @@ pub fn automatize(
                 insertion.set_state_data(inc.operation);
             }
 
-            insertion.next(inc.expected, inc.actions.clone());
+            insertion.next(inc.expected, inc.actions.clone().into_boxed_slice());
         }
         insertion.finish();
     }
diff --git a/cranelift/peepmatic/src/dot_fmt.rs b/cranelift/peepmatic/src/dot_fmt.rs
index 1d854d6561..6939577147 100644
--- a/cranelift/peepmatic/src/dot_fmt.rs
+++ b/cranelift/peepmatic/src/dot_fmt.rs
@@ -17,7 +17,7 @@ use std::num::NonZeroU16;
 #[derive(Debug)]
 pub(crate) struct PeepholeDotFmt<'a>(pub(crate) &'a PathInterner, pub(crate) &'a IntegerInterner);
 
-impl DotFmt<linear::MatchResult, linear::MatchOp, Vec<linear::Action>> for PeepholeDotFmt<'_> {
+impl DotFmt<linear::MatchResult, linear::MatchOp, Box<[linear::Action]>> for PeepholeDotFmt<'_> {
     fn fmt_transition(
         &self,
         w: &mut impl Write,
@@ -73,7 +73,7 @@ impl DotFmt<linear::MatchResult, linear::MatchOp, Vec<linear::Action>> for Peeph
         writeln!(w, "</font>")
     }
 
-    fn fmt_output(&self, w: &mut impl Write, actions: &Vec<linear::Action>) -> io::Result<()> {
+    fn fmt_output(&self, w: &mut impl Write, actions: &Box<[linear::Action]>) -> io::Result<()> {
         use linear::Action::*;
 
         if actions.is_empty() {
@@ -84,7 +84,7 @@ impl DotFmt<linear::MatchResult, linear::MatchOp, Vec<linear::Action>> for Peeph
 
         let p = p(self.0);
 
-        for a in actions {
+        for a in actions.iter() {
             match a {
                 GetLhs { path } => write!(w, "get-lhs @ {}<br/>", p(path))?,
                 UnaryUnquote { operator, operand } => {