Add a --standalone-wasm flag to wasm-emscripten-finalize (#2333)

The flag indicates that we want to run the wasm by itself, without JS support. In that case we don't emit JS dynCalls etc., and we also emit a wasi _start if there is a main, i.e., we try to use the current conventions in the wasm-only space.

3 files changed, 38 insertions, 1 deletions

diff --git a/src/tools/wasm-emscripten-finalize.cpp b/src/tools/wasm-emscripten-finalize.cpp
index ea69cabbc..8a246e5d2 100644
--- a/src/tools/wasm-emscripten-finalize.cpp
+++ b/src/tools/wasm-emscripten-finalize.cpp
@@ -50,6 +50,8 @@ int main(int argc, const char* argv[]) {
   bool legalizeJavaScriptFFI = true;
   bool checkStackOverflow = false;
   uint64_t globalBase = INVALID_BASE;
+  bool standaloneWasm = false;
+
   ToolOptions options("wasm-emscripten-finalize",
                       "Performs Emscripten-specific transforms on .wasm files");
   options
@@ -135,6 +137,14 @@ int main(int argc, const char* argv[]) {
          [&checkStackOverflow](Options* o, const std::string&) {
            checkStackOverflow = true;
          })
+    .add("--standalone-wasm",
+         "",
+         "Emit a wasm file that does not depend on JS, as much as possible,"
+         " using wasi and other standard conventions etc. where possible",
+         Options::Arguments::Zero,
+         [&standaloneWasm](Options* o, const std::string&) {
+           standaloneWasm = true;
+         })
     .add_positional("INFILE",
                     Options::Arguments::One,
                     [&infile](Options* o, const std::string& argument) {
@@ -233,7 +243,13 @@ int main(int argc, const char* argv[]) {
     }
   }
 
-  generator.generateDynCallThunks();
+  if (standaloneWasm) {
+    // Export a standard wasi "_start" method.
+    generator.exportWasiStart();
+  } else {
+    // If not standalone wasm then JS is relevant and we need dynCalls.
+    generator.generateDynCallThunks();
+  }
 
   // Legalize the wasm.
   {
diff --git a/src/wasm-emscripten.h b/src/wasm-emscripten.h
index 1758aa23a..9dc82bc1a 100644
--- a/src/wasm-emscripten.h
+++ b/src/wasm-emscripten.h
@@ -56,6 +56,8 @@ public:
 
   void enforceStackLimit();
 
+  void exportWasiStart();
+
   // Emits the data segments to a file. The file contains data from address base
   // onwards (we must pass in base, as we can't tell it from the wasm - the
   // first segment may start after a run of zeros, but we need those zeros in
diff --git a/src/wasm/wasm-emscripten.cpp b/src/wasm/wasm-emscripten.cpp
index 978afa296..3aa6f5647 100644
--- a/src/wasm/wasm-emscripten.cpp
+++ b/src/wasm/wasm-emscripten.cpp
@@ -22,6 +22,7 @@
 #include "asmjs/shared-constants.h"
 #include "ir/function-type-utils.h"
 #include "ir/import-utils.h"
+#include "ir/literal-utils.h"
 #include "ir/module-utils.h"
 #include "shared-constants.h"
 #include "wasm-builder.h"
@@ -1210,4 +1211,22 @@ void EmscriptenGlueGenerator::separateDataSegments(Output* outfile,
   wasm.memory.segments.clear();
 }
 
+void EmscriptenGlueGenerator::exportWasiStart() {
+  // If main exists, export a function to call it per the wasi standard.
+  Name main = "main";
+  if (!wasm.getFunctionOrNull(main)) {
+    return;
+  }
+  Name _start = "_start";
+  Builder builder(wasm);
+  auto* body = builder.makeDrop(builder.makeCall(
+    main,
+    {LiteralUtils::makeZero(i32, wasm), LiteralUtils::makeZero(i32, wasm)},
+    i32));
+  auto* func =
+    builder.makeFunction(_start, std::vector<wasm::Type>{}, none, {}, body);
+  wasm.addFunction(func);
+  wasm.addExport(builder.makeExport(_start, _start, ExternalKind::Function));
+}
+
 } // namespace wasm