STANDALONE_WASM: separate crt1 for commands and reactors

Unlike without STANDALONE_WASM users are expected to either define
a main or explicitly opt out.
This change and paves to way to moving to llvm's new main mangling
method: https://reviews.llvm.org/D70700

Fixes:  #9640

Author: sbc100

src/modules.js

@@ -161,7 +161,7 @@ var LibraryManager = {
     }
 
     if (BOOTSTRAPPING_STRUCT_INFO) {
-      libraries = ['library_formatString.js'];
+      libraries = ['library_formatString.js', 'library_stack_trace.js'];
     }
 
     // Deduplicate libraries to avoid processing any library file multiple times

src/postamble.js

@@ -156,7 +156,11 @@ function callMain(args) {
 #endif
 
 #if STANDALONE_WASM
+#if EXPECT_MAIN
   var entryFunction = Module['__start'];
+#else
+  var entryFunction = Module['__initialize'];
+#endif
 #else
   var entryFunction = Module['_main'];
 #endif

system/lib/libc/crt1.c

@@ -17,9 +17,7 @@
 
 extern void __wasm_call_ctors(void) __attribute__((weak));
 
-// TODO(sbc): We shouldn't even link this file if there is no main:
-// https://github.com/emscripten-core/emscripten/issues/9640
-extern int main(int argc, char** argv) __attribute__((weak));
+extern int main(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.
@@ -66,10 +64,6 @@ void _start(void) {
     __wasm_call_ctors();
   }
 
-  if (!main) {
-    return;
-  }
-
   int r = __original_main();
 
   exit(r);

system/lib/libc/crt1_reactor.c

@@ -0,0 +1,14 @@
+/*
+ * Copyright 2020 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+extern void __wasm_call_ctors(void) __attribute__((weak));
+
+void _initialize(void) {
+  if (__wasm_call_ctors) {
+    __wasm_call_ctors();
+  }
+}

tests/other/metadce/mem_no_main_O3_STANDALONE_WASM.exports

@@ -0,0 +1,3 @@
+_initialize
+foo
+memory

tests/other/metadce/mem_no_main_O3_STANDALONE_WASM_noentry.exports

@@ -1595,7 +1595,9 @@ def test_iostream_ctors(self):
 }
 ''', 'bugfree code')
 
-  @also_with_standalone_wasm
+  # Marked as impure since the WASI reactor modules (modules without main)
+  # are not yet suppored by the wasm engines we test against.
+  @also_with_impure_standalone_wasm
   def test_ctors_no_main(self):
     self.emcc_args.append('--no-entry')
     self.do_run_in_out_file_test('tests', 'core', 'test_ctors_no_main')
@@ -8852,7 +8854,6 @@ def test_safe_stack_dylink(self):
       }
     ''', ['abort(stack overflow)', '__handle_stack_overflow'], assert_returncode=None)
 
-  @also_with_standalone_wasm
   def test_undefined_main(self):
     if self.get_setting('LLD_REPORT_UNDEFINED'):
       self.skipTest('LLD_REPORT_UNDEFINED does not allow implicit undefined main')

tests/other/metadce/mem_no_main_O3_STANDALONE_WASM.funcs renamed to tests/other/metadce/mem_no_main_O3_STANDALONE_WASM_noentry.funcs

@@ -8290,7 +8290,7 @@ def strip_numeric_suffixes(funcname):
     'O3': (['-O3'], [], [],          85), # noqa
     'Os': (['-Os'], [], [],          85), # noqa
     'Oz': (['-Oz'], [], [],          85), # noqa
-    'Os_mr': (['-Os', '-s', 'MINIMAL_RUNTIME'], [], [], 85), # noqa
+    'Os_mr': (['-Os', '-s', 'MINIMAL_RUNTIME', '--no-entry'], [], [], 85), # noqa
   })
   @no_fastcomp()
   def test_metadce_minimal(self, *args):
@@ -8379,7 +8379,7 @@ def test_metadce_hello_fastcomp(self, *args):
     'O3_standalone_narg': ('mem_no_argv.c', ['-O3', '-s', 'STANDALONE_WASM'],
                            [], [], 6309),         # noqa
     # without main, no support code for argc/argv is emitted either
-    'O3_standalone_lib':  ('mem_no_main.c', ['-O3', '-s', 'STANDALONE_WASM'],
+    'O3_standalone_lib':  ('mem_no_main.c', ['-O3', '-s', 'STANDALONE_WASM', '--no-entry'],
                            [], [], 6309),         # noqa
     # Growth support code is in JS, no significant change in the wasm
     'O3_grow':            ('mem.c', ['-O3', '-s', 'ALLOW_MEMORY_GROWTH'],

tests/other/metadce/mem_no_main_O3_STANDALONE_WASM.imports renamed to tests/other/metadce/mem_no_main_O3_STANDALONE_WASM_noentry.imports

@@ -1582,9 +1582,12 @@ def lld_flags_for_executable(external_symbol_list):
         '-z', 'stack-size=%s' % Settings.TOTAL_STACK,
         '--initial-memory=%d' % Settings.INITIAL_MEMORY,
       ]
-      use_start_function = Settings.STANDALONE_WASM
 
-      if not use_start_function:
+      if Settings.STANDALONE_WASM:
+        # when Settings.EXPECT_MAIN is set we fall back to wasm-ld default of _start
+        if not Settings.EXPECT_MAIN:
+          cmd += ['--entry=_initialize']
+      else:
         if Settings.EXPECT_MAIN and not Settings.IGNORE_MISSING_MAIN:
           cmd += ['--entry=main']
         else:

tests/other/metadce/mem_no_main_O3_STANDALONE_WASM.sent renamed to tests/other/metadce/mem_no_main_O3_STANDALONE_WASM_noentry.sent

@@ -884,6 +884,24 @@ def can_build(self):
     return super(crt1, self).can_build() and shared.Settings.WASM_BACKEND
 
 
+class crt1_reactor(MuslInternalLibrary):
+  name = 'crt1_reactor'
+  cflags = ['-O2']
+  src_dir = ['system', 'lib', 'libc']
+  src_files = ['crt1_reactor.c']
+
+  force_object_files = True
+
+  def get_ext(self):
+    return '.o'
+
+  def can_use(self):
+    return super(crt1_reactor, self).can_use() and shared.Settings.STANDALONE_WASM
+
+  def can_build(self):
+    return super(crt1_reactor, self).can_build() and shared.Settings.WASM_BACKEND
+
+
 class libc_extras(NoBCLibrary, MuslInternalLibrary):
   """This library is separate from libc itself for fastcomp only so that the
   constructor it contains can be DCE'd.  Such tricks are not needed wih the
@@ -1557,7 +1575,10 @@ def add_library(lib):
     libs_to_link.append((lib.get_path(), need_whole_archive))
 
   if shared.Settings.STANDALONE_WASM:
-    add_library(system_libs_map['crt1'])
+    if shared.Settings.EXPECT_MAIN:
+      add_library(system_libs_map['crt1'])
+    else:
+      add_library(system_libs_map['crt1_reactor'])
 
   for forced in force_include:
     add_library(system_libs_map[forced])