diff --git a/crates/codegen/Cargo.toml b/crates/codegen/Cargo.toml
index cb98cbf6..41ed2826 100644
--- a/crates/codegen/Cargo.toml
+++ b/crates/codegen/Cargo.toml
@@ -26,3 +26,4 @@ indexmap = { version = "2.0" }
 
 [dev-dependencies]
 sonatina-parser = { path = "../parser", version = "0.0.3-alpha" }
+insta = { version = "1.41" }
diff --git a/crates/codegen/src/module_linker.rs b/crates/codegen/src/module_linker.rs
index 1464a65e..848b6499 100644
--- a/crates/codegen/src/module_linker.rs
+++ b/crates/codegen/src/module_linker.rs
@@ -1,3 +1,6 @@
+//! This module defines a module-level linking on sonatina-IR that links
+//! multiple sonatina modules into a single module.
+
 use std::mem;
 
 use cranelift_entity::entity_impl;
@@ -9,7 +12,7 @@ use sonatina_ir::{
     module::FuncRef,
     types::{CompoundType, CompoundTypeRef, StructData},
     visitor::VisitorMut,
-    Function, GlobalVariableRef, Linkage, Module, Signature, Type, Value,
+    GlobalVariableRef, Linkage, Module, Signature, Type, Value,
 };
 
 /// A struct represents a linked module, that is the result of the
@@ -51,6 +54,10 @@ pub enum LinkError {
 }
 
 impl LinkedModule {
+    pub fn module(&self) -> &Module {
+        &self.module
+    }
+
     /// Links multiple modules into a single module.
     /// Returns a linked module and a list of module references.
     /// The order of module references are the same as the input modules.
@@ -94,7 +101,7 @@ impl LinkedModule {
 /// An entity representing a module reference.
 /// This is used to identify a module in the linked module for mapping from
 /// the source module reference to the linked module reference.
-#[derive(Clone, PartialEq, Eq, Copy, Hash, PartialOrd, Ord)]
+#[derive(Debug, Clone, PartialEq, Eq, Copy, Hash, PartialOrd, Ord)]
 pub struct ModuleRef(pub u32);
 entity_impl!(ModuleRef);
 
@@ -198,6 +205,7 @@ struct ModuleLinker {
 
     module_ref_map: DashMap<ModuleRef, RefMap>,
 
+    /// Modules to be linked.
     modules: IndexMap<ModuleRef, Module>,
 }
 
@@ -234,7 +242,7 @@ impl ModuleLinker {
         }
     }
 
-    /// Registers module as a source module to be linked.
+    /// Registers a module as a source module to be linked.
     fn register_module(&mut self, module: Module) -> ModuleRef {
         let next_id = self.module_ref_map.len();
         let module_ref = ModuleRef(next_id as u32);
@@ -251,20 +259,49 @@ impl ModuleLinker {
         // Links all references in the source modules to the linked module.
         self.link_refs(&module_refs)?;
 
-        // Updates the references in the function body.
-        self.update_funcs(&module_refs);
-
         let modules = mem::take(&mut self.modules);
         // Move functions to the linked module.
         for (module_ref, module) in modules {
             let ref_map = self.module_ref_map.get(&module_ref).unwrap();
-
-            module.func_store.par_into_for_each(|func_ref, func| {
-                let linkage = func.dfg.ctx.func_sig(func_ref, |sig| sig.linkage());
-                if linkage.is_external() {
+            module.func_store.par_into_for_each(|func_ref, mut func| {
+                // If linkage is external, we don't need to move the function definition to the
+                // linked module.
+                if func
+                    .dfg
+                    .ctx
+                    .func_sig(func_ref, |sig| sig.linkage())
+                    .is_external()
+                {
                     return;
                 }
 
+                // Updates module context to the linked module.
+                func.dfg.ctx = self.builder.ctx.clone();
+
+                // Updates references in values to the linked module.
+                func.dfg.values.values_mut().for_each(|value| {
+                    ref_map.update_value(value);
+                });
+
+                // Updates the references in instructions to the linked module.
+                struct InstUpdater<'a> {
+                    ref_map: &'a Ref<'a, ModuleRef, RefMap>,
+                }
+                impl VisitorMut for InstUpdater<'_> {
+                    fn visit_func_ref(&mut self, item: &mut FuncRef) {
+                        *item = self.ref_map.lookup_func(*item);
+                    }
+
+                    fn visit_ty(&mut self, item: &mut Type) {
+                        *item = self.ref_map.lookup_type(*item);
+                    }
+                }
+                let mut visitor = InstUpdater { ref_map: &ref_map };
+                func.dfg
+                    .insts
+                    .values_mut()
+                    .for_each(|inst| inst.accept_mut(&mut visitor));
+
                 let linked_func_ref = ref_map.lookup_func(func_ref);
                 self.builder.func_store.update(linked_func_ref, func);
             })
@@ -479,12 +516,13 @@ impl ModuleLinker {
 
             // Validates the linkage and update the linked gv if needed.
             // The allowed combinations are:
-            // (SourceLinkage, LinkedLinkage) = (External, Public) or (Public, External).
+            // (SourceLinkage, LinkedLinkage) = (External, Public), (Public, External) or
+            // (External, External).
             //
             // Also, in case of LinkedLinkage is External, we need to update it to
             // a Public.
             match (gv_data.linkage, linked_gv_data.linkage) {
-                (Linkage::External, Linkage::Public) => {}
+                (Linkage::External, Linkage::Public) | (Linkage::External, Linkage::External) => {}
                 (Linkage::Public, Linkage::External) => {
                     s.update_linkage(linked_gv_ref, Linkage::Public);
                 }
@@ -544,11 +582,11 @@ impl ModuleLinker {
                 });
             }
 
-            Ok(sig.linkage())
+            Ok(linked_sig.linkage())
         })?;
 
         match (sig.linkage(), linked_func_linkage) {
-            (Linkage::External, Linkage::Public) => {}
+            (Linkage::External, Linkage::Public) | (Linkage::External, Linkage::External) => {}
             (Linkage::Public, Linkage::External) => {
                 self.builder
                     .ctx
@@ -564,45 +602,4 @@ impl ModuleLinker {
         ref_map.func_mapping.insert(func_ref, linked_func_ref);
         Ok(linked_func_ref)
     }
-
-    fn update_funcs(&self, module_refs: &[ModuleRef]) {
-        for module_ref in module_refs {
-            let module = self.modules.get(module_ref).unwrap();
-            module
-                .func_store
-                .par_for_each(|_, func| self.update_func(*module_ref, func));
-        }
-    }
-
-    fn update_func(&self, module_ref: ModuleRef, func: &mut Function) {
-        let ref_map = self.module_ref_map.get(&module_ref).unwrap();
-
-        // Updates module context to the linked module.
-        func.dfg.ctx = self.builder.ctx.clone();
-
-        // Updates values to the linked module.
-        func.dfg.values.values_mut().for_each(|value| {
-            ref_map.update_value(value);
-        });
-
-        // Updates the instructions to the linked module.
-        struct InstUpdater<'a> {
-            ref_map: Ref<'a, ModuleRef, RefMap>,
-        }
-        impl VisitorMut for InstUpdater<'_> {
-            fn visit_func_ref(&mut self, item: &mut FuncRef) {
-                *item = self.ref_map.lookup_func(*item);
-            }
-
-            fn visit_ty(&mut self, item: &mut Type) {
-                *item = self.ref_map.lookup_type(*item);
-            }
-        }
-
-        let mut visitor = InstUpdater { ref_map };
-        func.dfg
-            .insts
-            .values_mut()
-            .for_each(|value| value.accept_mut(&mut visitor))
-    }
 }
diff --git a/crates/codegen/tests/common.rs b/crates/codegen/tests/common.rs
index e71ee38b..42b705be 100644
--- a/crates/codegen/tests/common.rs
+++ b/crates/codegen/tests/common.rs
@@ -19,3 +19,45 @@ pub fn parse_module(file_path: &str) -> ParsedModule {
         }
     }
 }
+
+// copied from fe test-utils
+/// A macro to assert that a value matches a snapshot.
+/// If the snapshot does not exist, it will be created in the same directory as
+/// the test file.
+#[macro_export]
+macro_rules! snap_test {
+    ($value:expr, $fixture_path: expr) => {
+        snap_test!($value, $fixture_path, None)
+    };
+
+    ($value:expr, $fixture_path: expr, $suffix: expr) => {
+        let mut settings = insta::Settings::new();
+        let fixture_path = ::std::path::Path::new($fixture_path);
+        let fixture_dir = fixture_path.parent().unwrap();
+        let fixture_name = fixture_path.file_stem().unwrap().to_str().unwrap();
+
+        settings.set_snapshot_path(fixture_dir);
+        settings.set_input_file($fixture_path);
+        settings.set_prepend_module_to_snapshot(false);
+        settings.set_omit_expression(true);
+
+        let suffix: Option<&str> = $suffix;
+        let name = if let Some(suffix) = suffix {
+            format!("{fixture_name}.{suffix}")
+        } else {
+            fixture_name.into()
+        };
+        settings.bind(|| {
+            insta::_macro_support::assert_snapshot(
+                (name, $value.as_str()).into(),
+                std::path::Path::new(env!("CARGO_MANIFEST_DIR")),
+                fixture_name,
+                module_path!(),
+                file!(),
+                line!(),
+                &$value,
+            )
+            .unwrap()
+        })
+    };
+}
diff --git a/crates/codegen/tests/linker.rs b/crates/codegen/tests/linker.rs
index 4340d308..a618bf99 100644
--- a/crates/codegen/tests/linker.rs
+++ b/crates/codegen/tests/linker.rs
@@ -1,20 +1,52 @@
 mod common;
-use std::{env, path::Path};
+use std::{
+    env,
+    path::{Path, PathBuf},
+};
 
 use common::parse_module;
 use sonatina_codegen::module_linker::LinkedModule;
+use sonatina_ir::ir_writer::ModuleWriter;
+
+fn fixture_dir() -> PathBuf {
+    let manifest_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
+    let manifest_dir = Path::new(&manifest_dir);
+    manifest_dir.join("tests/linker/fixtures")
+}
+
+macro_rules! test_ok {
+    ($name:ident) => {
+        #[test]
+        fn $name() {
+            let name = stringify!($name);
+            let dir = fixture_dir().join("link_ok");
+
+            let path_a = dir.join(format!("{name}_a.sntn"));
+            let path_b = dir.join(format!("{name}_b.sntn"));
+            let module_a = parse_module(path_a.to_str().unwrap());
+            let module_b = parse_module(path_b.to_str().unwrap());
+
+            let (linked, _) = LinkedModule::link(vec![module_a.module, module_b.module]).unwrap();
+            let mut writer = ModuleWriter::new(linked.module());
+            let module_text = writer.dump_string();
+
+            let snap_file_path = dir.join(format!("{name}.snap"));
+            snap_test!(module_text, snap_file_path.to_str().unwrap());
+        }
+    };
+}
+
+test_ok!(module);
 
 macro_rules! test_error {
     ($name:ident) => {
         #[test]
         fn $name() {
             let name = stringify!($name);
-            let manifest_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
-            let manifest_dir = Path::new(&manifest_dir);
-            let fixture_dir = manifest_dir.join("tests/linker/fixtures/errors");
-            let path_a = fixture_dir.join(format!("{name}_a.sntn"));
-            let path_b = fixture_dir.join(format!("{name}_b.sntn"));
+            let dir = fixture_dir().join("link_errors");
 
+            let path_a = dir.join(format!("{name}_a.sntn"));
+            let path_b = dir.join(format!("{name}_b.sntn"));
             let module_a = parse_module(path_a.to_str().unwrap());
             let module_b = parse_module(path_b.to_str().unwrap());
 
@@ -27,3 +59,4 @@ macro_rules! test_error {
 test_error!(struct_error);
 test_error!(gv_error);
 test_error!(func_error);
+test_error!(sig_error);
diff --git a/crates/codegen/tests/linker/fixtures/errors/func_error_a.sntn b/crates/codegen/tests/linker/fixtures/link_errors/func_error_a.sntn
similarity index 100%
rename from crates/codegen/tests/linker/fixtures/errors/func_error_a.sntn
rename to crates/codegen/tests/linker/fixtures/link_errors/func_error_a.sntn
diff --git a/crates/codegen/tests/linker/fixtures/errors/func_error_b.sntn b/crates/codegen/tests/linker/fixtures/link_errors/func_error_b.sntn
similarity index 100%
rename from crates/codegen/tests/linker/fixtures/errors/func_error_b.sntn
rename to crates/codegen/tests/linker/fixtures/link_errors/func_error_b.sntn
diff --git a/crates/codegen/tests/linker/fixtures/errors/gv_error_a.sntn b/crates/codegen/tests/linker/fixtures/link_errors/gv_error_a.sntn
similarity index 100%
rename from crates/codegen/tests/linker/fixtures/errors/gv_error_a.sntn
rename to crates/codegen/tests/linker/fixtures/link_errors/gv_error_a.sntn
diff --git a/crates/codegen/tests/linker/fixtures/errors/gv_error_b.sntn b/crates/codegen/tests/linker/fixtures/link_errors/gv_error_b.sntn
similarity index 100%
rename from crates/codegen/tests/linker/fixtures/errors/gv_error_b.sntn
rename to crates/codegen/tests/linker/fixtures/link_errors/gv_error_b.sntn
diff --git a/crates/codegen/tests/linker/fixtures/link_errors/sig_error_a.sntn b/crates/codegen/tests/linker/fixtures/link_errors/sig_error_a.sntn
new file mode 100644
index 00000000..f39beee1
--- /dev/null
+++ b/crates/codegen/tests/linker/fixtures/link_errors/sig_error_a.sntn
@@ -0,0 +1,3 @@
+target = "evm-ethereum-cancun"
+
+declare external %f(i64) -> i64;
diff --git a/crates/codegen/tests/linker/fixtures/link_errors/sig_error_b.sntn b/crates/codegen/tests/linker/fixtures/link_errors/sig_error_b.sntn
new file mode 100644
index 00000000..3cbdb05c
--- /dev/null
+++ b/crates/codegen/tests/linker/fixtures/link_errors/sig_error_b.sntn
@@ -0,0 +1,6 @@
+target = "evm-ethereum-cancun"
+
+func public %f(v0.i64) {
+    block0:
+        return;
+}
diff --git a/crates/codegen/tests/linker/fixtures/errors/struct_error_a.sntn b/crates/codegen/tests/linker/fixtures/link_errors/struct_error_a.sntn
similarity index 100%
rename from crates/codegen/tests/linker/fixtures/errors/struct_error_a.sntn
rename to crates/codegen/tests/linker/fixtures/link_errors/struct_error_a.sntn
diff --git a/crates/codegen/tests/linker/fixtures/errors/struct_error_b.sntn b/crates/codegen/tests/linker/fixtures/link_errors/struct_error_b.sntn
similarity index 100%
rename from crates/codegen/tests/linker/fixtures/errors/struct_error_b.sntn
rename to crates/codegen/tests/linker/fixtures/link_errors/struct_error_b.sntn
diff --git a/crates/codegen/tests/linker/fixtures/link_ok/module.snap b/crates/codegen/tests/linker/fixtures/link_ok/module.snap
new file mode 100644
index 00000000..3313ca1e
--- /dev/null
+++ b/crates/codegen/tests/linker/fixtures/link_ok/module.snap
@@ -0,0 +1,38 @@
+---
+source: crates/codegen/tests/linker.rs
+input_file: tests/linker/fixtures/link_ok/module.snap
+---
+target = evm-ethereum-cancun
+
+type @foo = {i8, i16, *i64};
+type @bar = {i8, [i8; 31]};
+
+global public const i256 $ZERO = 0
+global external const i256 $ONE
+
+declare func external %f_outer(i8) -> i8;
+
+func public %f_b(v0.i8, v1.i8) -> i8 {
+    block0:
+        v2.i8 = call %f_a v0;
+        v3.i8 = add v0 v2;
+        return;
+}
+
+func public %f_a(v0.i8) -> i8 {
+    block0:
+        return v0;
+}
+
+func private %types_a(v0.*i8, v1.[i8; 2], v2.[*i8; 2], v3.[[i8; 2]; 2], v4.@foo, v5.*@bar) -> i8 {
+    block0:
+        v6.i8 = mload v0 i8;
+        v7.i8 = call %f_b v6 1.i8;
+        call %f_outer;
+        return v6;
+}
+
+func private %types_b(v0.*i8, v1.[i8; 2], v2.[*i8; 2], v3.[[i8; 2]; 2], v4.@bar, v5.*@foo) {
+    block0:
+        return;
+}
diff --git a/crates/codegen/tests/linker/fixtures/link_ok/module_a.sntn b/crates/codegen/tests/linker/fixtures/link_ok/module_a.sntn
new file mode 100644
index 00000000..1100a702
--- /dev/null
+++ b/crates/codegen/tests/linker/fixtures/link_ok/module_a.sntn
@@ -0,0 +1,25 @@
+target = "evm-ethereum-cancun"
+
+type @foo = { i8, i16, *i64 };
+type @bar = <{ i8, [i8; 31] }>;
+
+declare external %f_b(i8, i8) -> i8;
+declare external %f_outer(i8) -> i8;
+
+
+global public const i256 $ZERO = 0
+global external const i256 $ONE
+
+
+func public %f_a(v0.i8) -> i8 {
+    block0:
+        return v0;
+}
+
+func private %types_a(v0.*i8, v1.[i8; 2], v2.[*i8; 2], v3.[[i8; 2]; 2], v4.@foo, v5.*@bar) -> i8 {
+    block0:
+        v6.i8 = mload v0 i8;
+        v7.i8 = call %f_b v6 1.i8;
+        call %f_outer;
+        return v6;
+}
diff --git a/crates/codegen/tests/linker/fixtures/link_ok/module_b.sntn b/crates/codegen/tests/linker/fixtures/link_ok/module_b.sntn
new file mode 100644
index 00000000..14eb796d
--- /dev/null
+++ b/crates/codegen/tests/linker/fixtures/link_ok/module_b.sntn
@@ -0,0 +1,24 @@
+target = "evm-ethereum-cancun"
+
+type @foo = { i8, i16, *i64 };
+type @bar = <{ i8, [i8; 31] }>;
+
+
+declare external %f_a(i8) -> i8;
+declare external %f_outer(i8) -> i8;
+
+global external const i256 $ZERO
+global external const i256 $ONE
+
+func public %f_b(v0.i8, v1.i8) -> i8 {
+    block0:
+        v2.i8 = call %f_a v0;
+        v3.i8 = add v0 v2;
+        return;
+}
+
+func private %types_b(v0.*i8, v1.[i8; 2], v2.[*i8; 2], v3.[[i8; 2]; 2], v4.@bar, v5.*@foo) {
+    block0:
+        return;
+}
+