Remove handling of legacy main mangling. (#17170)

Now that https://reviews.llvm.org/D75277 as landed in llvm we can
depend on the `__main_void` alias and the `__main_argv_argc` mangling.

This only effects STANDALONE_WASM mode.

Author: sbc100

src/jsifier.js

@@ -204,7 +204,11 @@ function ${name}(${args}) {
           return;
         }
         if (!isDefined(ident)) {
-          let msg = 'undefined symbol: ' + ident;
+          let undefinedSym = ident;
+          if (ident === '__main_argc_argv') {
+            undefinedSym = 'main';
+          }
+          let msg = 'undefined symbol: ' + undefinedSym;
           if (dependent) msg += ` (referenced by ${dependent})`;
           if (ERROR_ON_UNDEFINED_SYMBOLS) {
             error(msg);
@@ -216,7 +220,7 @@ function ${name}(${args}) {
           } else if (VERBOSE || WARN_ON_UNDEFINED_SYMBOLS) {
             warn(msg);
           }
-          if (ident === 'main' && STANDALONE_WASM) {
+          if (undefinedSym === 'main' && STANDALONE_WASM) {
             warn('To build in STANDALONE_WASM mode without a main(), use emcc --no-entry');
           }
         }

system/lib/libc/crt1.c

@@ -5,16 +5,12 @@
  * found in the LICENSE file.
  */
 
-// Much of this code comes from:
-// https://github.com/CraneStation/wasi-libc/blob/master/libc-bottom-half/crt/crt1.c
-// Converted malloc() calls to alloca() to avoid including malloc in all programs.
-
 #include <stdlib.h>
 #include <wasi/api.h>
 
 __attribute__((__weak__)) void __wasm_call_ctors(void);
 
-int __original_main(void);
+int __main_void(void);
 
 void _start(void) {
   if (__wasm_call_ctors) {
@@ -26,7 +22,7 @@ void _start(void) {
    * or our __original_main fallback in __original_main.c which handles
    * populating argv.
    */
-  int r = __original_main();
+  int r = __main_void();
 
   exit(r);
 }

system/lib/standalone/__main_argc_argv.c

@@ -9,17 +9,19 @@
 #include <wasi/api.h>
 #include <sysexits.h>
 
+// Much of this code comes from:
+// https://github.com/CraneStation/wasi-libc/blob/master/libc-bottom-half/crt/crt1.c
+// Converted malloc() calls to alloca() to avoid including malloc in all programs.
+
 int __main_argc_argv(int argc, char *argv[]);
 
-// If main() uses argc/argv, then no __original_main is emitted, and then
-// this definition is used, which loads those values and sends them to main.
-// If main() does not use argc/argv, then the compiler emits __original_main
-// and this definition is not necessary, which avoids the wasi calls for
-// getting the args.
-// If there is no main() at all, we don't need this definition, but it will get
-// linked in. However, _start checks for main()'s existence and only calls
-// __original_main() if it does, so this will not be called, which allows
-// LLVM LTO or the Binaryen optimizer to remove it.
+// If the application's `main` does not uses argc/argv, then it will be defined
+// (by llvm) with an __main_void alias, and therefore this function will not
+// be included, and `_start` will call the application's `__main_void` directly.
+//
+// If the application's `main` does use argc/argv, then _start will call this
+// function which which loads argv valud values and sends them to to the
+// application's `main` which gets mangled to `__main_argc_argv` by llvm.
 __attribute__((weak))
 int __main_void(void) {
   /* Fill in the arguments from WASI syscalls. */

system/lib/standalone/__main_void.c

@@ -1653,8 +1653,9 @@ def get_default_variation(cls, **kwargs):
   def get_files(self):
     files = files_in_path(
         path='system/lib/standalone',
-        filenames=['standalone.c', 'standalone_wasm_stdio.c', '__original_main.c',
-                   '__main_void.c', '__main_argc_argv.c'])
+        filenames=['standalone.c',
+                   'standalone_wasm_stdio.c',
+                   '__main_void.c'])
     files += files_in_path(
         path='system/lib/libc',
         filenames=['emscripten_memcpy.c'])