3 files changed, 47 insertions, 8 deletions

diff --git a/src/interp/interp.cc b/src/interp/interp.cc
index ca8ff59d..3d5ee30e 100644
--- a/src/interp/interp.cc
+++ b/src/interp/interp.cc
@@ -144,11 +144,27 @@ Module* Environment::FindModule(string_view name) {
 }
 
 Module* Environment::FindRegisteredModule(string_view name) {
-  auto iter = registered_module_bindings_.find(name.to_string());
-  if (iter == registered_module_bindings_.end()) {
-    return nullptr;
+  bool retry = false;
+  while (true) {
+    auto iter = registered_module_bindings_.find(name.to_string());
+    if (iter != registered_module_bindings_.end()) {
+      return modules_[iter->second.index].get();
+    }
+
+    if (retry) {
+      // If you return true from on_unknown_module, you must add the module
+      // using AppendHostModule().
+      assert(false);
+      break;
+    }
+
+    if (on_unknown_module && on_unknown_module(this, name)) {
+      retry = true;
+      continue;
+    }
+    break;
   }
-  return modules_[iter->second.index].get();
+  return nullptr;
 }
 
 Thread::Options::Options(uint32_t value_stack_size, uint32_t call_stack_size)
diff --git a/src/interp/interp.h b/src/interp/interp.h
index 1a98ccfb..1230b927 100644
--- a/src/interp/interp.h
+++ b/src/interp/interp.h
@@ -537,6 +537,11 @@ class Environment {
   void Disassemble(Stream* stream, IstreamOffset from, IstreamOffset to);
   void DisassembleModule(Stream* stream, Module*);
 
+  // Called when a module name isn't found in registered_module_bindings_. If
+  // you want to provide a module with this name, call AppendHostModule() with
+  // this name and return true.
+  std::function<bool(Environment*, string_view name)> on_unknown_module;
+
  private:
   friend class Thread;
 
diff --git a/src/tools/wasm-interp.cc b/src/tools/wasm-interp.cc
index cb5808d8..c08a4382 100644
--- a/src/tools/wasm-interp.cc
+++ b/src/tools/wasm-interp.cc
@@ -46,6 +46,7 @@ static Thread::Options s_thread_options;
 static Stream* s_trace_stream;
 static bool s_run_all_exports;
 static bool s_host_print;
+static bool s_dummy_import_func;
 static Features s_features;
 
 static std::unique_ptr<FileStream> s_log_stream;
@@ -106,6 +107,11 @@ static void ParseOptions(int argc, char** argv) {
                    "Include an importable function named \"host.print\" for "
                    "printing to stdout",
                    []() { s_host_print = true; });
+  parser.AddOption(
+      "dummy-import-func",
+      "Provide a dummy implementation of all imported functions. The function "
+      "will log the call and return an appropriate zero value.",
+      []() { s_dummy_import_func = true; });
 
   parser.AddArgument("filename", OptionParser::ArgumentCount::One,
                      [](const char* argument) { s_infile = argument; });
@@ -169,7 +175,7 @@ static interp::Result PrintCallback(const HostFunc* func,
 
 static void InitEnvironment(Environment* env) {
   if (s_host_print) {
-    HostModule* host_module = env->AppendHostModule("host");
+    auto* host_module = env->AppendHostModule("host");
     host_module->on_unknown_func_export =
         [](Environment* env, HostModule* host_module, string_view name,
            Index sig_index) -> Index {
@@ -177,9 +183,21 @@ static void InitEnvironment(Environment* env) {
         return kInvalidIndex;
       }
 
-      std::pair<HostFunc*, Index> pair =
-          host_module->AppendFuncExport(name, sig_index, PrintCallback);
-      return pair.second;
+      return host_module->AppendFuncExport(name, sig_index, PrintCallback)
+          .second;
+    };
+  }
+
+  if (s_dummy_import_func) {
+    env->on_unknown_module = [](Environment* env, string_view name) {
+      auto* host_module = env->AppendHostModule(name);
+      host_module->on_unknown_func_export =
+          [](Environment* env, HostModule* host_module, string_view name,
+             Index sig_index) -> Index {
+        return host_module->AppendFuncExport(name, sig_index, PrintCallback)
+            .second;
+      };
+      return true;
     };
   }
 }