5 files changed, 36 insertions, 4 deletions

diff --git a/src/tools/wasm-emscripten-finalize.cpp b/src/tools/wasm-emscripten-finalize.cpp
index fc55a8d83..8e37d60f4 100644
--- a/src/tools/wasm-emscripten-finalize.cpp
+++ b/src/tools/wasm-emscripten-finalize.cpp
@@ -203,9 +203,21 @@ int main(int argc, const char* argv[]) {
     Fatal() << "Need to specify an infile\n";
   }
 
+  // We will write the modified wasm if the user asked us to, either by
+  // specifying an output file or requesting text output (which goes to stdout
+  // by default).
+  auto writeOutput = outfile.size() > 0 || !emitBinary;
+
   Module wasm;
   ModuleReader reader;
   reader.setDWARF(DWARF);
+  if (!writeOutput) {
+    // If we are not writing the output then all we are doing is simple parsing
+    // of metadata from global parts of the wasm such as imports and exports. In
+    // that case, it is unnecessary to parse function contents which are the
+    // great bulk of the work, and we can skip all that.
+    reader.setSkipFunctionBodies(true);
+  }
   try {
     reader.read(infile, wasm, inputSourceMapFilename);
   } catch (ParseException& p) {
@@ -303,9 +315,7 @@ int main(int argc, const char* argv[]) {
   BYN_TRACE_WITH_TYPE("emscripten-dump", "Module after:\n");
   BYN_DEBUG_WITH_TYPE("emscripten-dump", std::cerr << wasm << '\n');
 
-  // Write the modified wasm if the user asked us to, either by specifying an
-  // output file, or requesting text output (which goes to stdout by default).
-  if (outfile.size() > 0 || !emitBinary) {
+  if (writeOutput) {
     Output output(outfile, emitBinary ? Flags::Binary : Flags::Text);
     ModuleWriter writer;
     writer.setDebugInfo(debugInfo);
diff --git a/src/wasm-binary.h b/src/wasm-binary.h
index 326c37bf0..453bcb9af 100644
--- a/src/wasm-binary.h
+++ b/src/wasm-binary.h
@@ -1307,6 +1307,7 @@ class WasmBinaryBuilder {
   std::istream* sourceMap;
   std::pair<uint32_t, Function::DebugLocation> nextDebugLocation;
   bool DWARF = false;
+  bool skipFunctionBodies = false;
 
   size_t pos = 0;
   Index startIndex = -1;
@@ -1324,6 +1325,9 @@ public:
       nextDebugLocation(0, {0, 0, 0}), debugLocation() {}
 
   void setDWARF(bool value) { DWARF = value; }
+  void setSkipFunctionBodies(bool skipFunctionBodies_) {
+    skipFunctionBodies = skipFunctionBodies_;
+  }
   void read();
   void readUserSection(size_t payloadLen);
 
diff --git a/src/wasm-io.h b/src/wasm-io.h
index fbc8947c6..04fe90b51 100644
--- a/src/wasm-io.h
+++ b/src/wasm-io.h
@@ -35,6 +35,11 @@ public:
 
   void setProfile(IRProfile profile_) { profile = profile_; }
 
+  // TODO: add support for this in the text format as well
+  void setSkipFunctionBodies(bool skipFunctionBodies_) {
+    skipFunctionBodies = skipFunctionBodies_;
+  }
+
   // read text
   void readText(std::string filename, Module& wasm);
   // read binary
@@ -53,6 +58,8 @@ private:
 
   IRProfile profile = IRProfile::Normal;
 
+  bool skipFunctionBodies = false;
+
   void readStdin(Module& wasm, std::string sourceMapFilename);
 
   void readBinaryData(std::vector<char>& input,
diff --git a/src/wasm/wasm-binary.cpp b/src/wasm/wasm-binary.cpp
index 1dac17e93..52aa4a6e0 100644
--- a/src/wasm/wasm-binary.cpp
+++ b/src/wasm/wasm-binary.cpp
@@ -2137,7 +2137,17 @@ void WasmBinaryBuilder::readFunctions() {
       assert(controlFlowStack.empty());
       assert(letStack.empty());
       assert(depth == 0);
-      func->body = getBlockOrSingleton(func->sig.results);
+      if (!skipFunctionBodies) {
+        func->body = getBlockOrSingleton(func->sig.results);
+      } else {
+        // When skipping the function body we need to put something valid in
+        // their place so we validate. An unreachable is always acceptable
+        // there.
+        func->body = Builder(wasm).makeUnreachable();
+
+        // Skip reading the contents.
+        pos = endOfFunction;
+      }
       assert(depth == 0);
       assert(breakStack.empty());
       assert(breakTargetNames.empty());
diff --git a/src/wasm/wasm-io.cpp b/src/wasm/wasm-io.cpp
index 2fb4af838..f2c5af6db 100644
--- a/src/wasm/wasm-io.cpp
+++ b/src/wasm/wasm-io.cpp
@@ -51,6 +51,7 @@ void ModuleReader::readBinaryData(std::vector<char>& input,
   std::unique_ptr<std::ifstream> sourceMapStream;
   WasmBinaryBuilder parser(wasm, input);
   parser.setDWARF(DWARF);
+  parser.setSkipFunctionBodies(skipFunctionBodies);
   if (sourceMapFilename.size()) {
     sourceMapStream = make_unique<std::ifstream>();
     sourceMapStream->open(sourceMapFilename);