Allow user override of default-enabled compile-time wasm features (#22966)

The signext feature is enabled by default by clang and can be overridden
with the -msign-ext/-mno-sign-ext flags at compile time. However it
cannot be individually controlled at link time (it can only be lowered
by setting the min browser version). This PR allows -mno-sign-ext,
-msign-ext (and likewise -mbulk-memory) to be used at link time as well,
and override the browser version control.

Author: dschuff

emcc.py

@@ -46,6 +46,7 @@
 from tools import cache
 from tools.settings import default_setting, user_settings, settings, MEM_SIZE_SETTINGS, COMPILE_TIME_SETTINGS
 from tools.utils import read_file, removeprefix
+from tools import feature_matrix
 
 logger = logging.getLogger('emcc')
 
@@ -1403,8 +1404,18 @@ def consume_arg_file():
       settings.WASM_EXCEPTIONS = 0
     elif arg == '-mbulk-memory':
       settings.BULK_MEMORY = 1
+      feature_matrix.enable_feature(feature_matrix.Feature.BULK_MEMORY,
+                                    '-mbulk-memory',
+                                    override=True)
     elif arg == '-mno-bulk-memory':
       settings.BULK_MEMORY = 0
+      feature_matrix.disable_feature(feature_matrix.Feature.BULK_MEMORY)
+    elif arg == '-msign-ext':
+      feature_matrix.enable_feature(feature_matrix.Feature.SIGN_EXT,
+                                    '-msign-ext',
+                                    override=True)
+    elif arg == '-mno-sign-ext':
+      feature_matrix.disable_feature(feature_matrix.Feature.SIGN_EXT)
     elif arg == '-fexceptions':
       # TODO Currently -fexceptions only means Emscripten EH. Switch to wasm
       # exception handling by default when -fexceptions is given when wasm

test/test_other.py

@@ -10434,23 +10434,37 @@ def compile(flags):
     verify_features_sec('multivalue', True)
     verify_features_sec('reference-types', True)
 
+    # Disable a feature
+    compile(['-mno-sign-ext', '-c'])
+    verify_features_sec('sign-ext', False)
+    # Disabling overrides default browser versions
+    compile(['-mno-sign-ext'])
+    verify_features_sec_linked('sign-ext', False)
+    # Disabling overrides manual browser versions
+    compile(['-sMIN_SAFARI_VERSION=150000', '-mno-sign-ext'])
+    # Disable via browser selection
+    compile(['-sMIN_FIREFOX_VERSION=61'])
+    verify_features_sec_linked('sign-ext', False)
+    # Manual enable overrides browser version
+    compile(['-sMIN_FIREFOX_VERSION=61', '-msign-ext'])
+    verify_features_sec_linked('sign-ext', 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'])
+    compile(['-sMIN_SAFARI_VERSION=150000'])
     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)
+    compile(['-sMIN_SAFARI_VERSION=150000', '-mno-bulk-memory'])
+    # -mno-bulk-memory at link time overrides MIN_SAFARI_VERSION
+    verify_features_sec_linked('bulk-memory', False)
 
   def test_js_preprocess(self):
     # Use stderr rather than stdout here because stdout is redirected to the output JS file itself.

tools/feature_matrix.py

@@ -39,6 +39,8 @@ class Feature(IntEnum):
 
 
 default_features = {Feature.SIGN_EXT, Feature.MUTABLE_GLOBALS}
+disable_override_features = set()
+enable_override_features = set()
 
 min_browser_versions = {
   Feature.NON_TRAPPING_FPTOINT: {
@@ -93,6 +95,11 @@ class Feature(IntEnum):
 
 
 def caniuse(feature):
+  if feature in disable_override_features:
+    return False
+  if feature in enable_override_features:
+    return True
+
   min_versions = min_browser_versions[feature]
 
   def report_missing(setting_name):
@@ -114,10 +121,12 @@ def report_missing(setting_name):
   return True
 
 
-def enable_feature(feature, reason):
+def enable_feature(feature, reason, override=False):
   """Updates default settings for browser versions such that the given
   feature is available everywhere.
   """
+  if override:
+    enable_override_features.add(feature)
   for name, min_version in min_browser_versions[feature].items():
     name = f'MIN_{name.upper()}_VERSION'
     if settings[name] < min_version:
@@ -132,6 +141,12 @@ def enable_feature(feature, reason):
         setattr(settings, name, min_version)
 
 
+def disable_feature(feature):
+  """Allow the user to disable a feature that would otherwise be on by default.
+  """
+  disable_override_features.add(feature)
+
+
 # apply minimum browser version defaults based on user settings. if
 # a user requests a feature that we know is only supported in browsers
 # from a specific version and above, we can assume that browser version.