Allow -u flag to pass through to the linker (#23679)

`-u` is one of the few linker flags that is also a compiler flag, and
the compiler is supposed to pass it through. This is similar to `-z`,
for example.

Fixes: #23677

Author: sbc100

emcc.py

@@ -93,7 +93,7 @@
     '-iprefix', '-iwithprefix', '-iwithprefixbefore',
     '-isysroot', '-imultilib', '-A', '-isystem', '-iquote',
     '-install_name', '-compatibility_version', '-mllvm',
-    '-current_version', '-I', '-L', '-include-pch',
+    '-current_version', '-I', '-L', '-include-pch', '-u',
     '-undefined', '-target', '-Xlinker', '-Xclang', '-z'
 }
 
@@ -703,7 +703,7 @@ def phase_parse_arguments(state):
     settings.WARN_DEPRECATED = 0
 
   for i in range(len(newargs)):
-    if newargs[i] in ('-l', '-L', '-I', '-z', '--js-library', '-o', '-x'):
+    if newargs[i] in ('-l', '-L', '-I', '-z', '--js-library', '-o', '-x', '-u'):
       # Scan for flags that can be written as either one or two arguments
       # and normalize them to the single argument form.
       if newargs[i] == '--js-library':
@@ -788,7 +788,7 @@ def get_next_arg():
         add_link_arg(flag)
     elif arg == '-Xlinker':
       add_link_arg(get_next_arg())
-    elif arg == '-s' or arg.startswith(('-l', '-L', '--js-library=', '-z')):
+    elif arg == '-s' or arg.startswith(('-l', '-L', '--js-library=', '-z', '-u')):
       add_link_arg(arg)
     elif not arg.startswith('-o') and arg not in ('-nostdlib', '-nostartfiles', '-nolibc', '-nodefaultlibs', '-s'):
       # All other flags are for the compiler

test/test_other.py

@@ -12375,6 +12375,29 @@ def test_linker_flags_pass_through(self):
     err = self.run_process([EMCC, test_file('hello_world.c'), '-zfoo'], stderr=PIPE).stderr
     self.assertContained('wasm-ld: warning: unknown -z value: foo', err)
 
+  def test_linker_flags_pass_through_u(self):
+    # Create a static library that contains a reference to an undefined symbol: bar
+    create_file('test.c', '''
+    extern int bar();
+    int foo() {
+      return bar();
+    }
+    ''')
+    self.run_process([EMCC, '-c', 'test.c'])
+    self.run_process([EMAR, 'crs', 'libtest.a', 'test.o'])
+    # This test uses --no-gc-sections otherwise the foo symbol never actually gets included
+    # in the link.
+    cmd = [EMCC, test_file('hello_world.c'), '-Wl,--no-gc-sections']
+
+    # Verify that the library is normally ignored and that the link succeeds
+    self.run_process(cmd + ['libtest.a'])
+    # Adding `-u baz` makes no diffeence
+    self.run_process(cmd + ['-ubaz', 'libtest.a'])
+
+    # But adding `-ufoo` should fail because it loads foo, which depends on bar
+    err = self.expect_fail(cmd + ['-ufoo', 'libtest.a'])
+    self.assertContained('wasm-ld: error: libtest.a(test.o): undefined symbol: bar', err)
+
   def test_linker_flags_unused(self):
     err = self.run_process([EMCC, test_file('hello_world.c'), '-c', '-lbar'], stderr=PIPE).stderr
     self.assertContained("warning: -lbar: 'linker' input unused [-Wunused-command-line-argument]", err)