Explicitly set bulk memory and nontrapping-fptoint clang flags (#22751)

When clang starts enabling features by default that Emscripten's minimum
browser versions want disabled, Emscripten will need to explicitly
disable them. This PR does that; once clang rolls, we can update the
default versions and tests in a single commit if needed.

See also llvm/llvm-project#112049

Author: dschuff

emcc.py

@@ -384,6 +384,15 @@ def get_clang_flags(user_args):
     if '-mbulk-memory' not in user_args:
       flags.append('-mbulk-memory')
 
+  # In emscripten we currently disable bulk memory by default.
+  # This should be removed/updated when we als update the default browser targets.
+  if '-mbulk-memory' not in user_args and '-mno-bulk-memory' not in user_args:
+    # Bulk memory may be enabled via threads or directly via -s.
+    if not settings.BULK_MEMORY:
+      flags.append('-mno-bulk-memory')
+  if '-mnontrapping-fptoint' not in user_args and '-mno-nontrapping-fptoint' not in user_args:
+    flags.append('-mno-nontrapping-fptoint')
+
   if settings.RELOCATABLE and '-fPIC' not in user_args:
     flags.append('-fPIC')
 

test/test_other.py

@@ -10268,6 +10268,51 @@ def test_wasm_features_section(self, args):
     self.run_process([EMCC, test_file('hello_world.c'), '-O2'] + args)
     self.verify_custom_sec_existence('a.out.wasm', 'target_features', False)
 
+  def test_wasm_features(self):
+    # Test that wasm features are explicitly enabled or disabled based on target engine version
+    def verify_features_sec(feature, expect_in, linked=False):
+      with webassembly.Module('a.out.wasm' if linked else 'hello_world.o') as module:
+        features = module.get_target_features()
+      if expect_in:
+        self.assertTrue(feature in features and
+                        features[feature] == webassembly.TargetFeaturePrefix.USED,
+                        f'{feature} missing from wasm file')
+      else:
+        self.assertFalse(feature in features,
+                         f'{feature} unexpectedly found in wasm file')
+
+    def verify_features_sec_linked(feature, expect_in):
+      return verify_features_sec(feature, expect_in, linked=True)
+
+    def compile(flags):
+      self.run_process([EMCC, test_file('hello_world.c')] + flags)
+
+    compile(['-c'])
+    verify_features_sec('bulk-memory', False)
+    verify_features_sec('nontrapping-fptoint', False)
+    verify_features_sec('sign-ext', True)
+    verify_features_sec('mutable-globals', True)
+    verify_features_sec('multivalue', True)
+    verify_features_sec('reference-types', True)
+
+    compile(['-mnontrapping-fptoint', '-c'])
+    verify_features_sec('nontrapping-fptoint', True)
+
+    # BIGINT causes binaryen to not run, and keeps the target_features section after link
+    # Setting this SAFARI_VERSION should enable bulk memory because it links in emscripten_memcpy_bulkmem
+    # However it does not enable nontrapping-fptoint yet because it has no effect at compile time and
+    # no libraries include nontrapping yet.
+    compile(['-sMIN_SAFARI_VERSION=150000', '-sWASM_BIGINT'])
+    verify_features_sec_linked('sign-ext', True)
+    verify_features_sec_linked('mutable-globals', True)
+    verify_features_sec_linked('multivalue', True)
+    verify_features_sec_linked('bulk-memory', True)
+    verify_features_sec_linked('nontrapping-fptoint', False)
+
+    compile(['-sMIN_SAFARI_VERSION=150000', '-mno-bulk-memory', '-sWASM_BIGINT'])
+    # FIXME? -mno-bulk-memory at link time does not override MIN_SAFARI_VERSION. it probably should?
+    verify_features_sec_linked('bulk-memory', True)
+
   def test_js_preprocess(self):
     # Use stderr rather than stdout here because stdout is redirected to the output JS file itself.
     create_file('lib.js', '''

tools/webassembly.py

@@ -162,6 +162,12 @@ class DylinkType(IntEnum):
   IMPORT_INFO = 4
 
 
+class TargetFeaturePrefix(IntEnum):
+  USED = 0x2b
+  DISALLOWED = 0x2d
+  REQUIRED = 0x3d
+
+
 class InvalidWasmError(BaseException):
   pass
 
@@ -561,6 +567,18 @@ def get_function_type(self, idx):
       func_type = self.get_function_types()[idx - self.num_imported_funcs()]
     return self.get_types()[func_type]
 
+  def get_target_features(self):
+    section = self.get_custom_section('target_features')
+    self.seek(section.offset)
+    assert self.read_string() == 'target_features'
+    features = {}
+    self.read_byte() # ignore feature count
+    while self.tell() < section.offset + section.size:
+      prefix = TargetFeaturePrefix(self.read_byte())
+      feature = self.read_string()
+      features[feature] = prefix
+    return features
+
 
 def parse_dylink_section(wasm_file):
   with Module(wasm_file) as module: