Don't run ATMAINS unless there is _main function exported (#13901)

Fixes: #13791

emscripten.py

@@ -125,6 +125,8 @@ def update_settings_glue(metadata, DEBUG):
     if shared.Settings.INITIAL_TABLE == -1:
       shared.Settings.INITIAL_TABLE = metadata['tableSize'] + 1
 
+  shared.Settings.HAS_MAIN = shared.Settings.MAIN_MODULE or shared.Settings.STANDALONE_WASM or '_main' in shared.Settings.IMPLEMENTED_FUNCTIONS
+
   # When using dynamic linking the main function might be in a side module.
   # To be safe assume they do take input parametes.
   shared.Settings.MAIN_READS_PARAMS = metadata['mainReadsParams'] or shared.Settings.MAIN_MODULE
@@ -142,7 +144,8 @@ def update_settings_glue(metadata, DEBUG):
 
 def apply_static_code_hooks(forwarded_json, code):
   code = shared.do_replace(code, '<<< ATINITS >>>', str(forwarded_json['ATINITS']))
-  code = shared.do_replace(code, '<<< ATMAINS >>>', str(forwarded_json['ATMAINS']))
+  if shared.Settings.HAS_MAIN:
+    code = shared.do_replace(code, '<<< ATMAINS >>>', str(forwarded_json['ATMAINS']))
   if shared.Settings.EXIT_RUNTIME:
     code = shared.do_replace(code, '<<< ATEXITS >>>', str(forwarded_json['ATEXITS']))
   return code

src/jsifier.js

@@ -15,14 +15,6 @@ var addedLibraryItems = {};
 // Each such proxied function is identified via an ordinal number (this is not the same namespace as function pointers in general).
 var proxiedFunctionTable = ["null" /* Reserve index 0 for an undefined function*/];
 
-// 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.
-// Also set for STANDALONE_WASM since the _start function is needed to call
-// static ctors, even if there is no user main.
-var HAS_MAIN = ('_main' in IMPLEMENTED_FUNCTIONS) || MAIN_MODULE || STANDALONE_WASM;
-
 // Mangles the given C/JS side function name to assembly level function name (adds an underscore)
 function mangleCSymbolName(f) {
   return f[0] == '$' ? f.substr(1) : '_' + f;

src/postamble.js

@@ -319,7 +319,9 @@ function run(args) {
 
     initRuntime();
 
+#if HAS_MAIN
     preMain();
+#endif
 
 #if MODULARIZE
     readyPromiseResolve(Module);

src/postamble_minimal.js

@@ -7,7 +7,7 @@
 // === Auto-generated postamble setup entry stuff ===
 {{{ exportRuntime() }}}
 
-#if hasExportedFunction('_main') // Only if user is exporting a C main(), we will generate a run() function that can be used to launch main.
+#if HAS_MAIN // Only if user is exporting a C main(), we will generate a run() function that can be used to launch main.
 function run() {
 #if MEMORYPROFILER
   emscriptenMemoryProfiler.onPreloadComplete();

src/preamble.js

@@ -395,6 +395,7 @@ function initRuntime() {
   callRuntimeCallbacks(__ATINIT__);
 }
 
+#if HAS_MAIN
 function preMain() {
 #if STACK_OVERFLOW_CHECK
   checkStackCookie();
@@ -405,6 +406,7 @@ function preMain() {
   <<< ATMAINS >>>
   callRuntimeCallbacks(__ATMAIN__);
 }
+#endif
 
 function exitRuntime() {
 #if STACK_OVERFLOW_CHECK

src/settings_internal.js

@@ -185,3 +185,11 @@ var GENERATE_SOURCE_MAP = 0;
 var STACK_BASE = 0;
 var STACK_MAX = 0;
 var HEAP_BASE = 0;
+
+// 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.
+// Also set for STANDALONE_WASM since the _start function is needed to call
+// static ctors, even if there is no user main.
+var HAS_MAIN = 0;

src/shell_minimal.js

@@ -108,7 +108,7 @@ function ready() {
 #if MODULARIZE && EXPORT_READY_PROMISE
   readyPromiseResolve(Module);
 #endif // MODULARIZE
-#if INVOKE_RUN && hasExportedFunction('_main')
+#if INVOKE_RUN && HAS_MAIN
 #if USE_PTHREADS
   if (!ENVIRONMENT_IS_PTHREAD) {
 #endif