diff --git a/src/backend/mod.rs b/src/backend/mod.rs
index b5f5b40..d516f36 100644
--- a/src/backend/mod.rs
+++ b/src/backend/mod.rs
@@ -586,9 +586,12 @@ pub fn compile(module: &Module<'_>) -> anyhow::Result<Vec<u8>> {
     for (func, func_decl) in module.funcs.entries().skip(num_func_imports) {
         match func_decl {
             FuncDecl::Import(_, _) => anyhow::bail!("Import comes after func with body: {}", func),
-            FuncDecl::Lazy(sig, _, _) | FuncDecl::Body(sig, _, _) => {
+            FuncDecl::Lazy(sig, _, _)
+            | FuncDecl::Body(sig, _, _)
+            | FuncDecl::Expanded(sig, _, _, _) => {
                 funcs.function(sig.index() as u32);
             }
+            FuncDecl::None => panic!("FuncDecl::None at compilation time"),
         }
     }
     into_mod.section(&funcs);
@@ -707,6 +710,10 @@ pub fn compile(module: &Module<'_>) -> anyhow::Result<Vec<u8>> {
                     let data = &module.orig_bytes[reader.range()];
                     Ok(FuncOrRawBytes::Raw(data))
                 }
+                FuncDecl::Expanded(_, _name, range, _) => {
+                    let data = &module.orig_bytes[range.clone()];
+                    Ok(FuncOrRawBytes::Raw(data))
+                }
                 FuncDecl::Body(_, name, body) => {
                     log::debug!("Compiling {} \"{}\"", func, name);
                     WasmFuncBackend::new(body)?
@@ -714,6 +721,7 @@ pub fn compile(module: &Module<'_>) -> anyhow::Result<Vec<u8>> {
                         .map(|f| FuncOrRawBytes::Func(f))
                 }
                 FuncDecl::Import(_, _) => unreachable!("Should have skipped imports"),
+                FuncDecl::None => panic!("FuncDecl::None at compilation time"),
             }
         })
         .collect::<Result<Vec<FuncOrRawBytes<'_>>>>()?;
diff --git a/src/interp.rs b/src/interp.rs
index e9d5844..0f93f04 100644
--- a/src/interp.rs
+++ b/src/interp.rs
@@ -95,7 +95,8 @@ impl InterpContext {
                 assert_eq!(import.kind, ImportKind::Func(func));
                 return self.call_import(&import.name[..], args);
             }
-            FuncDecl::Body(_, _, body) => body,
+            FuncDecl::Body(_, _, body) | FuncDecl::Expanded(_, _, _, body) => body,
+            FuncDecl::None => panic!("FuncDecl::None in call()"),
         };
 
         log::trace!(
diff --git a/src/ir/display.rs b/src/ir/display.rs
index d0cb7ec..08ce720 100644
--- a/src/ir/display.rs
+++ b/src/ir/display.rs
@@ -230,7 +230,7 @@ impl<'a> Display for ModuleDisplay<'a> {
         }
         for (func, func_decl) in self.0.funcs.entries() {
             match func_decl {
-                FuncDecl::Body(sig, name, body) => {
+                FuncDecl::Body(sig, name, body) | FuncDecl::Expanded(sig, name, _, body) => {
                     writeln!(
                         f,
                         "  {} \"{}\": {} = # {}",
@@ -262,6 +262,9 @@ impl<'a> Display for ModuleDisplay<'a> {
                         sig_strs.get(&sig).unwrap()
                     )?;
                 }
+                FuncDecl::None => {
+                    writeln!(f, "  {}: none", func)?;
+                }
             }
         }
         for (loc, loc_data) in self.0.debug.source_locs.entries() {
diff --git a/src/ir/func.rs b/src/ir/func.rs
index 8d0c2a3..d1ea31e 100644
--- a/src/ir/func.rs
+++ b/src/ir/func.rs
@@ -5,12 +5,25 @@ use crate::frontend::parse_body;
 use crate::ir::SourceLoc;
 use crate::passes::Fuel;
 use anyhow::Result;
+use std::ops::Range;
 
-#[derive(Clone, Debug)]
+/// A declaration of a function: there is one `FuncDecl` per `Func`
+/// index.
+#[derive(Clone, Debug, Default)]
 pub enum FuncDecl<'a> {
+    /// An imported function.
     Import(Signature, String),
+    /// An un-expanded body that can be lazily expanded if needed.
     Lazy(Signature, String, wasmparser::FunctionBody<'a>),
+    /// Expanded body, but still "clean" (unchanged); range in
+    /// original Wasm lets us pass it straight through when converting
+    /// back to a Wasm module.
+    Expanded(Signature, String, Range<usize>, FunctionBody),
+    /// A modified or new function body that requires compilation.
     Body(Signature, String, FunctionBody),
+    /// A placeholder.
+    #[default]
+    None,
 }
 
 impl<'a> FuncDecl<'a> {
@@ -18,15 +31,18 @@ impl<'a> FuncDecl<'a> {
         match self {
             FuncDecl::Import(sig, ..) => *sig,
             FuncDecl::Lazy(sig, ..) => *sig,
+            FuncDecl::Expanded(sig, ..) => *sig,
             FuncDecl::Body(sig, ..) => *sig,
+            FuncDecl::None => panic!("No signature for FuncDecl::None"),
         }
     }
 
     pub fn parse(&mut self, module: &Module) -> Result<()> {
         match self {
             FuncDecl::Lazy(sig, name, body) => {
+                let range = body.range();
                 let body = parse_body(module, *sig, body)?;
-                *self = FuncDecl::Body(*sig, name.clone(), body);
+                *self = FuncDecl::Expanded(*sig, name.clone(), range, body);
                 Ok(())
             }
             _ => Ok(()),
@@ -34,6 +50,7 @@ impl<'a> FuncDecl<'a> {
     }
 
     pub fn optimize(&mut self, fuel: &mut Fuel) {
+        self.mark_dirty();
         match self {
             FuncDecl::Body(_, _, body) => {
                 body.optimize(fuel);
@@ -43,6 +60,7 @@ impl<'a> FuncDecl<'a> {
     }
 
     pub fn convert_to_max_ssa(&mut self) {
+        self.mark_dirty();
         match self {
             FuncDecl::Body(_, _, body) => {
                 body.convert_to_max_ssa();
@@ -51,14 +69,23 @@ impl<'a> FuncDecl<'a> {
         }
     }
 
+    pub fn mark_dirty(&mut self) {
+        let new = match std::mem::take(self) {
+            FuncDecl::Expanded(sig, name, _, body) => FuncDecl::Body(sig, name, body),
+            x => x,
+        };
+        *self = new;
+    }
+
     pub fn body(&self) -> Option<&FunctionBody> {
         match self {
-            FuncDecl::Body(_, _, body) => Some(body),
+            FuncDecl::Expanded(_, _, _, body) | FuncDecl::Body(_, _, body) => Some(body),
             _ => None,
         }
     }
 
     pub fn body_mut(&mut self) -> Option<&mut FunctionBody> {
+        self.mark_dirty();
         match self {
             FuncDecl::Body(_, _, body) => Some(body),
             _ => None,
@@ -67,17 +94,21 @@ impl<'a> FuncDecl<'a> {
 
     pub fn name(&self) -> &str {
         match self {
-            FuncDecl::Body(_, name, _) | FuncDecl::Lazy(_, name, _) | FuncDecl::Import(_, name) => {
-                &name[..]
-            }
+            FuncDecl::Body(_, name, _)
+            | FuncDecl::Lazy(_, name, _)
+            | FuncDecl::Expanded(_, name, _, _)
+            | FuncDecl::Import(_, name) => &name[..],
+            FuncDecl::None => panic!("No name for FuncDecl::None"),
         }
     }
 
     pub fn set_name(&mut self, new_name: &str) {
         match self {
-            FuncDecl::Body(_, name, _) | FuncDecl::Lazy(_, name, _) | FuncDecl::Import(_, name) => {
-                *name = new_name.to_owned()
-            }
+            FuncDecl::Body(_, name, _)
+            | FuncDecl::Lazy(_, name, _)
+            | FuncDecl::Expanded(_, name, _, _)
+            | FuncDecl::Import(_, name) => *name = new_name.to_owned(),
+            FuncDecl::None => panic!("No name for FuncDecl::None"),
         }
     }
 }