3 files changed, 37 insertions, 5 deletions
diff --git a/src/ir/module-splitting.cpp b/src/ir/module-splitting.cpp
index d6dca8994..9c5c66641 100644
--- a/src/ir/module-splitting.cpp
+++ b/src/ir/module-splitting.cpp
@@ -272,6 +272,9 @@ struct ModuleSplitter {
   // names.
   std::map<Name, Name> exportedPrimaryFuncs;
 
+  // Map placeholder indices to the names of the functions they replace.
+  std::map<size_t, Name> placeholderMap;
+
   // Initialization helpers
   static std::unique_ptr<Module> initSecondary(const Module& primary);
   static std::pair<std::set<Name>, std::set<Name>>
@@ -483,6 +486,7 @@ void ModuleSplitter::setupTablePatching() {
   // `importNamespace`.`index`.
   forEachElement(primary, [&](Name, Name, Index index, Name& elem) {
     if (secondaryFuncs.count(elem)) {
+      placeholderMap[index] = elem;
       auto* secondaryFunc = secondary.getFunction(elem);
       replacedElems[index] = secondaryFunc;
       auto placeholder = std::make_unique<Function>();
@@ -655,8 +659,9 @@ void ModuleSplitter::shareImportableItems() {
 
 } // anonymous namespace
 
-std::unique_ptr<Module> splitFunctions(Module& primary, const Config& config) {
-  return std::move(ModuleSplitter(primary, config).secondaryPtr);
+Results splitFunctions(Module& primary, const Config& config) {
+  ModuleSplitter split(primary, config);
+  return {std::move(split.secondaryPtr), std::move(split.placeholderMap)};
 }
 
 } // namespace ModuleSplitting
diff --git a/src/ir/module-splitting.h b/src/ir/module-splitting.h
index 479fba148..cfbd45b14 100644
--- a/src/ir/module-splitting.h
+++ b/src/ir/module-splitting.h
@@ -68,8 +68,13 @@ struct Config {
   bool minimizeNewExportNames = false;
 };
 
+struct Results {
+  std::unique_ptr<Module> secondary;
+  std::map<size_t, Name> placeholderMap;
+};
+
 // Returns the new secondary module and modifies the `primary` module in place.
-std::unique_ptr<Module> splitFunctions(Module& primary, const Config& config);
+Results splitFunctions(Module& primary, const Config& config);
 
 } // namespace ModuleSplitting
 
diff --git a/src/tools/wasm-split.cpp b/src/tools/wasm-split.cpp
index 4ac295d0e..261c62a68 100644
--- a/src/tools/wasm-split.cpp
+++ b/src/tools/wasm-split.cpp
@@ -61,6 +61,7 @@ struct WasmSplitOptions : ToolOptions {
   bool verbose = false;
   bool emitBinary = true;
   bool symbolMap = false;
+  bool placeholderMap = false;
 
   // TODO: Remove this. See the comment in wasm-binary.h.
   bool emitModuleNames = false;
@@ -180,6 +181,13 @@ WasmSplitOptions::WasmSplitOptions()
          {Mode::Split},
          Options::Arguments::Zero,
          [&](Options* o, const std::string& argument) { symbolMap = true; })
+    .add(
+      "--placeholdermap",
+      "",
+      "Write a file mapping placeholder indices to the function names.",
+      {Mode::Split},
+      Options::Arguments::Zero,
+      [&](Options* o, const std::string& argument) { placeholderMap = true; })
     .add("--import-namespace",
          "",
          "The namespace from which to import objects from the primary "
@@ -690,6 +698,15 @@ void writeSymbolMap(Module& wasm, std::string filename) {
   runner.run();
 }
 
+void writePlaceholderMap(const std::map<size_t, Name> placeholderMap,
+                         std::string filename) {
+  Output output(filename, Flags::Text);
+  auto& o = output.getStream();
+  for (auto pair : placeholderMap) {
+    o << pair.first << ':' << pair.second << '\n';
+  }
+}
+
 void splitModule(const WasmSplitOptions& options) {
   Module wasm;
   parseInput(wasm, options);
@@ -795,8 +812,8 @@ void splitModule(const WasmSplitOptions& options) {
     config.newExportPrefix = options.exportPrefix;
   }
   config.minimizeNewExportNames = !options.passOptions.debugInfo;
-  std::unique_ptr<Module> secondary =
-    ModuleSplitting::splitFunctions(wasm, config);
+  auto splitResults = ModuleSplitting::splitFunctions(wasm, config);
+  auto& secondary = splitResults.secondary;
 
   adjustTableSize(wasm, options.initialTableSize);
   adjustTableSize(*secondary, options.initialTableSize);
@@ -806,6 +823,11 @@ void splitModule(const WasmSplitOptions& options) {
     writeSymbolMap(*secondary, options.secondaryOutput + ".symbols");
   }
 
+  if (options.placeholderMap) {
+    writePlaceholderMap(splitResults.placeholderMap,
+                        options.primaryOutput + ".placeholders");
+  }
+
   // Set the names of the split modules. This can help differentiate them in
   // stack traces.
   if (options.emitModuleNames) {