wasm-emscripten-finalize: Add mainReadsParams metadata (#2247)

The new flag indicates whether main reads the argc/argv parameters. If it does not, we can avoid emitting code to generate those arguments in the JS, which is not trivial in small programs - it requires some string conversion code.

Nicely the existing test inputs were enough for testing this (see outputs).

This depends on an emscripten change to land first, as emscripten.py asserts on metadata fields it doesn't recognize.

1 files changed, 17 insertions, 1 deletions

diff --git a/src/wasm/wasm-emscripten.cpp b/src/wasm/wasm-emscripten.cpp
index a3433bf9f..3ee3e4424 100644
--- a/src/wasm/wasm-emscripten.cpp
+++ b/src/wasm/wasm-emscripten.cpp
@@ -1042,7 +1042,23 @@ std::string EmscriptenGlueGenerator::generateEmscriptenMetadata(
   wasm.features.iterFeatures([&](FeatureSet::Feature f) {
     meta << nextElement() << "\"--enable-" << FeatureSet::toString(f) << '"';
   });
-  meta << "\n  ]\n";
+  meta << "\n  ],\n";
+
+  auto mainReadsParams = false;
+  if (auto* exp = wasm.getExportOrNull("main")) {
+    if (exp->kind == ExternalKind::Function) {
+      auto* main = wasm.getFunction(exp->value);
+      mainReadsParams = true;
+      // If main does not read its parameters, it will just be a stub that
+      // calls __original_main (which has no parameters).
+      if (auto* call = main->body->dynCast<Call>()) {
+        if (call->operands.empty()) {
+          mainReadsParams = false;
+        }
+      }
+    }
+  }
+  meta << "  \"mainReadsParams\": " << int(mainReadsParams) << '\n';
 
   meta << "}\n";