Avoid use of EMCC_FORCE_STDLIBS in gen_struct_info.py (#13422)

Using `-nostdlib` instead means we can remove the special handling
in system_libs.py too.

Author: sbc100

tools/gen_struct_info.py

@@ -88,6 +88,7 @@
 sys.path.insert(1, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
 from tools import shared
+from tools import system_libs
 
 QUIET = (__name__ != '__main__')
 DEBUG = False
@@ -231,15 +232,20 @@ def inspect_headers(headers, cpp_opts):
 
   js_file = tempfile.mkstemp('.js')
 
+  # Check sanity early on before populating the cache with libcompiler_rt
+  # If we don't do this the parallel build of compiler_rt will run while holding the cache
+  # lock and with EM_EXCLUSIVE_CACHE_ACCESS set causing N processes to race to run sanity checks.
+  # While this is not in itself serious problem it is wasteful and noise on stdout.
+  # For the same reason we run this early in embuilder.py and emcc.py.
+  # TODO(sbc): If we can remove EM_EXCLUSIVE_CACHE_ACCESS then this would not longer be needed.
+  shared.check_sanity()
+
+  compiler_rt = system_libs.Library.get_usable_variations()['libcompiler_rt'].get_path()
+
   # Close all unneeded FDs.
   os.close(src_file[0])
   os.close(js_file[0])
 
-  # TODO(sbc): Switch to '-nostdlib -lcompiler_rt'
-  env = os.environ.copy()
-  env['EMCC_FORCE_STDLIBS'] = 'libcompiler_rt'
-  env['EMCC_ONLY_FORCED_STDLIBS'] = '1'
-
   info = []
   # Compile the program.
   show('Compiling generated code...')
@@ -248,6 +254,8 @@ def inspect_headers(headers, cpp_opts):
                                     '-O0',
                                     '-Werror',
                                     '-Wno-format',
+                                    '-nostdlib',
+                                    compiler_rt,
                                     '-I', shared.path_from_root(),
                                     '-s', 'BOOTSTRAPPING_STRUCT_INFO=1',
                                     '-s', 'STRICT',
@@ -267,7 +275,7 @@ def inspect_headers(headers, cpp_opts):
 
   show(shared.shlex_join(cmd))
   try:
-    subprocess.check_call(cmd, env=env)
+    subprocess.check_call(cmd)
   except subprocess.CalledProcessError as e:
     sys.stderr.write('FAIL: Compilation failed!: %s\n' % e.cmd)
     sys.exit(1)

tools/system_libs.py

@@ -1363,7 +1363,7 @@ def handle_reverse_deps(input_files, only_forced):
   # If we are only doing forced stdlibs, then we don't know the actual
   # symbols we need, and must assume all of deps_info must be exported.
   # Note that this might cause warnings on exports that do not exist.
-  if only_forced and not shared.Settings.BOOTSTRAPPING_STRUCT_INFO:
+  if only_forced:
     for key, value in deps_info.deps_info.items():
       for dep in value:
         shared.Settings.EXPORTED_FUNCTIONS.append(mangle_c_symbol_name(dep))
@@ -1453,8 +1453,7 @@ def add_library(lib):
     add_library(system_libs_map[forced])
 
   if only_forced:
-    if not shared.Settings.BOOTSTRAPPING_STRUCT_INFO:
-      add_library(system_libs_map['libc_rt_wasm'])
+    add_library(system_libs_map['libc_rt_wasm'])
     add_library(system_libs_map['libcompiler_rt'])
   else:
     if shared.Settings.AUTO_NATIVE_LIBRARIES: