[NFC] Assert we do not use --low-memory-unused when STACK_FIRST (#21291)

Assuming low memory was unused when the stack was first - which is where low
memory is - would be wrong. We never did that wrong thing as we only use that
optimization when not optimizing. Add an assertion + tests to prevent regressions.

test/test_other.py

@@ -63,12 +63,12 @@ def uses_canonical_tmp(func):
   test to satisfy the leak detector.
   """
   @wraps(func)
-  def decorated(self):
+  def decorated(self, *args, **kwargs):
     # Before running the test completely remove the canonical_tmp
     if os.path.exists(self.canonical_temp_dir):
       shutil.rmtree(self.canonical_temp_dir)
     try:
-      func(self)
+      func(self, *args, **kwargs)
     finally:
       # Make sure the test isn't lying about the fact that it uses
       # canonical_tmp
@@ -1206,6 +1206,25 @@ def test_wl_stackfirst(self):
     err = self.expect_fail(cmd + ['-sGLOBAL_BASE=1024'])
     self.assertContained('error: --stack-first is not compatible with -sGLOBAL_BASE', err)
 
+  @parameterized({
+    # In a simple -O0 build we do not set --low-memory-unused (as the stack is
+    # first, which is nice for debugging but bad for code size (larger globals)
+    # and bad for the low-memory-unused trick.
+    '': ([], False),
+    # When we optimize, we do.
+    'O2': (['-O2'], True),
+    # But a low global base prevents it.
+    'O2_GB_512': (['-O2', '-sGLOBAL_BASE=512'], False),
+    # A large-enough global base allows it.
+    'O2_GB_1024': (['-O2', '-sGLOBAL_BASE=1024'], True),
+    # Forcing the stack to be first in the linker prevents it.
+    'linker_flag': (['-O2', '-Wl,--stack-first'], False),
+  })
+  def test_binaryen_low_memory_unused(self, args, low_memory_unused):
+    cmd = [EMCC, test_file('hello_world.c'), '-v'] + args
+    err = self.run_process(cmd, stdout=PIPE, stderr=PIPE).stderr
+    self.assertContainedIf('--low-memory-unused ', err, low_memory_unused)
+
   def test_l_link(self):
     # Linking with -lLIBNAME and -L/DIRNAME should work, also should work with spaces
     create_file('main.c', '''

tools/link.py

@@ -357,8 +357,9 @@ def get_binaryen_passes(memfile):
   if optimizing:
     passes += [building.opt_level_to_str(settings.OPT_LEVEL, settings.SHRINK_LEVEL)]
   # when optimizing, use the fact that low memory is never used (1024 is a
-  # hardcoded value in the binaryen pass)
-  if optimizing and settings.GLOBAL_BASE >= 1024:
+  # hardcoded value in the binaryen pass). we also cannot do it when the stack
+  # is first, as then the stack is in the low memory that should be unused.
+  if optimizing and settings.GLOBAL_BASE >= 1024 and not settings.STACK_FIRST:
     passes += ['--low-memory-unused']
   if settings.AUTODEBUG:
     # adding '--flatten' here may make these even more effective