Don't emit code to call main if there is no main (#5850)

also emit it if we are a dynamically-linkable module, since then `main` may be in other code we will link in.

Author: kripken

src/jsifier.js

@@ -25,9 +25,14 @@ var proxiedFunctionTable = ["null" /* Reserve index 0 for an undefined function*
 // map: pair(sig, syncOrAsync) -> function body
 var proxiedFunctionInvokers = {};
 
+// used internally. set when there is a main() function.
+// also set when in a linkable module, as the main() function might
+// arrive from a dynamically-linked library, and not necessarily
+// the current compilation unit.
+var HAS_MAIN = ('_main' in IMPLEMENTED_FUNCTIONS) || MAIN_MODULE || SIDE_MODULE;
+
 // JSifier
 function JSify(data, functionsOnly) {
-  //B.start('jsifier');
   var mainPass = !functionsOnly;
 
   var itemsDict = { type: [], GlobalVariableStub: [], functionStub: [], function: [], GlobalVariable: [], GlobalVariablePostSet: [] };
@@ -569,6 +574,7 @@ function JSify(data, functionsOnly) {
     if (DETERMINISTIC) {
       print(read('deterministic.js'));
     }
+
     var postFile = BUILD_AS_SHARED_LIB || SIDE_MODULE ? 'postamble_sharedlib.js' : 'postamble.js';
     var postParts = processMacros(preprocess(read(postFile), postFile)).split('{{GLOBAL_VARS}}');
     print(postParts[0]);

src/postamble.js

@@ -163,6 +163,7 @@ dependenciesFulfilled = function runCaller() {
   if (!Module['calledRun']) dependenciesFulfilled = runCaller; // try this again later, after new deps are fulfilled
 }
 
+#if HAS_MAIN
 Module['callMain'] = Module.callMain = function callMain(args) {
 #if ASSERTIONS
   assert(runDependencies == 0, 'cannot call main when async dependencies remain! (listen on __ATMAIN__)');
@@ -237,6 +238,7 @@ Module['callMain'] = Module.callMain = function callMain(args) {
     calledMain = true;
   }
 }
+#endif // HAS_MAIN
 
 {{GLOBAL_VARS}}
 
@@ -280,7 +282,9 @@ function run(args) {
 
     if (Module['onRuntimeInitialized']) Module['onRuntimeInitialized']();
 
+#if HAS_MAIN
     if (Module['_main'] && shouldRunNow) Module['callMain'](args);
+#endif // HAS_MAIN
 
     postRun();
   }
@@ -381,6 +385,7 @@ if (Module['preInit']) {
   }
 }
 
+#if HAS_MAIN
 // shouldRunNow refers to calling main(), not run().
 #if INVOKE_RUN
 var shouldRunNow = true;
@@ -390,6 +395,7 @@ var shouldRunNow = false;
 if (Module['noInitialRun']) {
   shouldRunNow = false;
 }
+#endif // HAS_MAIN
 
 #if NO_EXIT_RUNTIME
 Module["noExitRuntime"] = true;