Prep for upstream llvm change to main mangling. NFC

This change should allow https://reviews.llvm.org/D75277 to land
without breaking emscripten.

In the long run I hope to remove all of this in favor of calling
`_start` from JS, but that can happen later, and could be user-visible
if folks are calling `main` themselves.

emcc.py

@@ -1724,6 +1724,9 @@ def phase_linker_setup(options, state, newargs, user_settings):
       assert not settings.EXPORTED_FUNCTIONS
       settings.EXPORTED_FUNCTIONS = ['_main']
 
+  if '_main' in settings.EXPORTED_FUNCTIONS:
+    settings.EXPORTED_FUNCTIONS.append('___main_argc_argv')
+
   if settings.STANDALONE_WASM:
     # In STANDALONE_WASM mode we either build a command or a reactor.
     # See https://github.com/WebAssembly/WASI/blob/main/design/application-abi.md

emscripten.py

@@ -297,6 +297,11 @@ def emscript(in_wasm, out_wasm, outfile_js, memfile):
 
   metadata = finalize_wasm(in_wasm, out_wasm, memfile)
 
+  if '__main_argc_argv' in metadata['exports']:
+    settings.MANGLED_MAIN = 1
+    metadata['exports'].remove('__main_argc_argv')
+    metadata['exports'].append('main')
+
   update_settings_glue(metadata)
 
   if not settings.WASM_BIGINT and metadata['emJsFuncs']:
@@ -778,6 +783,8 @@ def make_export_wrappers(exports, delay_assignment):
   wrappers = []
   for name in exports:
     mangled = asmjs_mangle(name)
+    if settings.MANGLED_MAIN and name == 'main':
+      name = '__main_argc_argv'
     # The emscripten stack functions are called very early (by writeStackCookie) before
     # the runtime is initialized so we can't create these wrappers that check for
     # runtimeInitialized.
@@ -817,8 +824,6 @@ def create_receiving(exports):
   if not settings.DECLARE_ASM_MODULE_EXPORTS:
     return ''
 
-  exports_that_are_not_initializers = [x for x in exports if x != building.WASM_CALL_CTORS]
-
   receiving = []
 
   # with WASM_ASYNC_COMPILATION that asm object may not exist at this point in time
@@ -833,8 +838,12 @@ def create_receiving(exports):
       # var asm = output.instance.exports;
       # _main = asm["_main"];
       generate_dyncall_assignment = settings.DYNCALLS and '$dynCall' in settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE
+      exports_that_are_not_initializers = [x for x in exports if x != building.WASM_CALL_CTORS]
+
       for s in exports_that_are_not_initializers:
         mangled = asmjs_mangle(s)
+        if settings.MANGLED_MAIN and s == 'main':
+          s = '__main_argc_argv'
         dynCallAssignment = ('dynCalls["' + s.replace('dynCall_', '') + '"] = ') if generate_dyncall_assignment and mangled.startswith('dynCall_') else ''
         receiving += [dynCallAssignment + mangled + ' = asm["' + s + '"];']
     else:

src/library_dylink.js

@@ -444,7 +444,7 @@ var LibraryDylink = {
 
       // Export native export on the Module object.
       // TODO(sbc): Do all users want this?  Should we skip this by default?
-      var module_sym = asmjsMangle(sym);
+      var module_sym = asmjsMangle(sym == '__main_argc_argv' ? 'main' : sym);
       if (!Module.hasOwnProperty(module_sym)) {
         Module[module_sym] = exports[sym];
       }

src/settings_internal.js

@@ -227,3 +227,5 @@ var TRANSPILE_TO_ES5 = false;
 // A copy of the default the default INCOMING_MODULE_JS_API. (Soon to
 // include additional items).
 var ALL_INCOMING_MODULE_JS_API = []
+
+var MANGLED_MAIN = false;

system/lib/standalone/__main_argc_argv.c

@@ -5,17 +5,13 @@
  * found in the LICENSE file.
  */
 
-// This file should no longer be needed once we land the llvm-side change to
-// switch to using __main_argc_argv:
-// TODO(https://reviews.llvm.org/D75277)
-
-// See https://github.com/CraneStation/wasi-libc/pull/152
-
-#include <assert.h>
-
-int main(int argc, char *argv[]);
+// New compilers define `__main_argc_argv`. If that doesn't exist, we
+// may get called here. Old compilers define `main` expecting an
+// argv/argc, so call that.
+// TODO: Remove this layer when we no longer have to support old compilers.
+int __legacy_main(int argc, char *argv[]) __asm("main");
 
 __attribute__((__weak__))
 int __main_argc_argv(int argc, char *argv[]) {
-  return main(argc, argv);
+  return __legacy_main(argc, argv);
 }

tools/building.py

@@ -233,7 +233,7 @@ def llvm_backend_args():
     # When 'main' has a non-standard signature, LLVM outlines its content out to
     # '__original_main'. So we add it to the allowed list as well.
     if 'main' in settings.EXCEPTION_CATCHING_ALLOWED:
-      settings.EXCEPTION_CATCHING_ALLOWED += ['__original_main']
+      settings.EXCEPTION_CATCHING_ALLOWED += ['__original_main', '__main_argc_argv']
     allowed = ','.join(settings.EXCEPTION_CATCHING_ALLOWED)
     args += ['-emscripten-cxx-exceptions-allowed=' + allowed]