STANDALONE_WASM: separate crt1 for commands and reactors (#11167)

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

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_noentry.exports

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

tests/test_core.py

@@ -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')
@@ -8852,22 +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')
-    if self.get_setting('STRICT'):
-      self.skipTest('STRICT does not allow implicit undefined main')
-    # By default in emscripten we allow main to be undefined.  Its used when
-    # building library code that has no main.
-    # TODO(sbc): Simplify the code by making this an opt-in feature.
-    # https://github.com/emscripten-core/emscripten/issues/9640
-    src = '''
-    #include <emscripten.h>
-    EMSCRIPTEN_KEEPALIVE void foo() {}
-    '''
-    self.build(src, self.get_dir(), 'test.c')
-
   def test_fpic_static(self):
     self.emcc_args.append('-fPIC')
     self.do_run_in_out_file_test('tests', 'core', 'test_hello_world')
@@ -8909,16 +8895,23 @@ def test_minimal_runtime_emscripten_get_exported_function(self):
     self.emcc_args += ['-lexports.js', '-s', 'MINIMAL_RUNTIME=1']
     self.do_run_in_out_file_test('tests', 'core', 'test_get_exported_function')
 
-  def test_auto_detect_main(self):
-    if not self.get_setting('LLD_REPORT_UNDEFINED') and not self.get_setting('STRICT'):
+  # 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_undefined_main(self):
+    # Traditionally in emscripten we allow main to be undefined.  This allows programs with a main
+    # and libraries without a main to be compiled identically.
+    # However we are trying to move away from that model to a more explicit opt-out model. See:
+    # https://github.com/emscripten-core/emscripten/issues/9640
+    if not self.get_setting('LLD_REPORT_UNDEFINED') and not self.get_setting('STRICT') and not self.get_setting('STANDALONE_WASM'):
       self.do_run_in_out_file_test('tests', 'core', 'test_ctors_no_main')
 
       # Disabling IGNORE_MISSING_MAIN should cause link to fail due to missing main
       self.set_setting('IGNORE_MISSING_MAIN', 0)
       err = self.expect_fail([PYTHON, EMCC, path_from_root('tests', 'core', 'test_ctors_no_main.cpp')] + self.get_emcc_args())
       self.assertContained('error: entry symbol not defined (pass --no-entry to suppress): main', err)
 
-    # We can fix the error either by adding --no-entry or by setting EXPORTED_FUNCTIONS to empty
+    # If we pass --no-entry or set EXPORTED_FUNCTIONS to empty should never see any errors
     self.emcc_args.append('--no-entry')
     self.do_run_in_out_file_test('tests', 'core', 'test_ctors_no_main')
 

tests/test_other.py

@@ -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'],

tools/shared.py

@@ -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:

tools/system_libs.py

@@ -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])