Link system libraries by default

ChangeLog.md

@@ -18,12 +18,18 @@ See docs/process.md for how version tagging works.
 Current Trunk
 -------------
 - Honor `CACHE` setting in config file as an alternative to `EM_CACHE`
-  environment variable or `--cache` commandline flag.
+  environment variable.
 - Remove `--cache` command line arg.  The `CACHE` config setting and the
   `EM_CACHE` environment variable can be used to control this.
 - Compiling to a file with no suffix will now generate an executable (JS) rather
   than an object file.  This means simple cases like `emcc -o foo foo.c` do the
   expected thing and generate an executable.
+- System libraries such as libc and libc++ are now included by default at
+  link time rather than selectively included based on the symbols used in the
+  input object files.  For small programs that don't use any system libraries
+  this might result in slightly slower link times with the old fastcomp
+  backend.  In order to exclude these libraries build with `-nostdlib` and/or
+  `-nostdlib++`.
 
 1.39.15: 05/06/2020
 -------------------

emcc.py

@@ -2078,6 +2078,8 @@ def check_human_readable_list(items):
         clang_compiler = CC
 
       def is_link_flag(flag):
+        if flag.startswith('-nostdlib'):
+          return True
         return flag.startswith(('-l', '-L', '-Wl,'))
 
       compile_args = [a for a in newargs if a and not is_link_flag(a)]
@@ -2296,7 +2298,9 @@ def compile_source_file(i, input_file):
          not shared.Settings.SIDE_MODULE: # shared libraries/side modules link no C libraries, need them in parent
         extra_files_to_link = system_libs.get_ports(shared.Settings)
         if '-nostdlib' not in newargs and '-nodefaultlibs' not in newargs:
-          extra_files_to_link += system_libs.calculate([f for _, f in sorted(temp_files)] + extra_files_to_link, in_temp, stdout_=None, stderr_=None, forced=forced_stdlibs)
+          # TODO(sbc): Only set link_as_cxx if use_cxx
+          link_as_cxx = '-nostdlib++' not in newargs
+          extra_files_to_link += system_libs.calculate([f for _, f in sorted(temp_files)] + extra_files_to_link, in_temp, link_as_cxx, forced=forced_stdlibs)
         linker_inputs += extra_files_to_link
 
     # exit block 'calculate system libraries'