Add implicit wasm feature disabling based on min browser versions

With https://reviews.llvm.org/D125728 about to land in LLVM we have four
features which will be enabled by default.  In order to allow users to
continue to target older browsers we want to turn these features off by
default when users opt into older browser support.

emcc.py

@@ -50,6 +50,7 @@
 from tools import wasm2c
 from tools import webassembly
 from tools import config
+from tools import feature_matrix
 from tools.settings import settings, MEM_SIZE_SETTINGS, COMPILE_TIME_SETTINGS
 from tools.utils import read_file, write_file, read_binary
 
@@ -829,7 +830,7 @@ def get_target_flags():
 
 
 def get_clang_flags(user_args):
-  flags = get_target_flags()
+  flags = get_target_flags() + feature_matrix.get_feature_flags()
 
   # if exception catching is disabled, we can prevent that code from being
   # generated in the frontend

test/test_other.py

@@ -10612,14 +10612,38 @@ def fail(args, details):
   def test_output_to_nowhere(self):
     self.run_process([EMXX, test_file('hello_world.cpp'), '-o', os.devnull, '-c'])
 
-  # Test that passing -sMIN_X_VERSION=-1 on the command line will result in browser X being not supported at all.
+  # Test that passing -sMIN_X_VERSION=-1 on the command line will result in browser X being not
+  # supported at all.
   # I.e. -sMIN_X_VERSION=-1 is equal to -sMIN_X_VERSION=Infinity
   def test_drop_support_for_browser(self):
     # Test that -1 means "not supported"
     self.run_process([EMCC, test_file('test_html5_core.c'), '-sMIN_IE_VERSION=-1'])
     self.assertContained('allowsDeferredCalls: true', read_file('a.out.js'))
     self.assertNotContained('allowsDeferredCalls: JSEvents.isInternetExplorer()', read_file('a.out.js'))
 
+  def test_browser_version_auto_features(self):
+    def get_enabled_features(filename):
+      with webassembly.Module(filename) as o:
+        features = o.parse_features_section()
+      return [x[1] for x in features if x[0] == '+']
+
+    # By default we expect to see some features disabled since our min browser versions
+    # don't allow them.
+    self.run_process([EMCC, '-mcpu=bleeding-edge', '-c', test_file('hello_world.c')])
+    enabled = get_enabled_features('hello_world.o')
+    self.assertNotContained('nontrapping-fptoint', enabled)
+    self.assertNotContained('mutable-globals', enabled)
+
+    # If we not disable all support for Edge/Safari/Firefox those feature should now be
+    # enabled by the -mcpu=bleeding-edge.
+    self.run_process([EMCC, '-mcpu=bleeding-edge', '-c', test_file('hello_world.c'),
+                      '-sMIN_EDGE_VERSION=-1',
+                      '-sMIN_SAFARI_VERSION=-1',
+                      '-sMIN_FIREFOX_VERSION=-1'])
+    enabled = get_enabled_features('hello_world.o')
+    self.assertContained('nontrapping-fptoint', enabled)
+    self.assertContained('mutable-globals', enabled)
+
   def test_errno_type(self):
     create_file('errno_type.c', '''
 #include <errno.h>

tools/feature_matrix.py

@@ -0,0 +1,80 @@
+# Copyright 2022 The Emscripten Authors.  All rights reserved.
+# Emscripten is available under two separate licenses, the MIT license and the
+# University of Illinois/NCSA Open Source License.  Both these licenses can be
+# found in the LICENSE file.
+
+"""Utilities for mapping browser versions to llvm feature flags."""
+
+import logging
+from enum import IntEnum, auto
+
+from .settings import settings
+
+logger = logging.getLogger('feature_matrix')
+
+
+class Feature(IntEnum):
+  NON_TRAPPING_FPTOINT = auto()
+  SIGN_EXT = auto()
+  BULK_MEMORY = auto()
+  MUTABLE_GLOBALS = auto()
+
+
+min_browser_versions = {
+  Feature.NON_TRAPPING_FPTOINT: {
+    'chrome': 75,
+    'firefox': 65,
+    'safari': 150000,
+  },
+  Feature.SIGN_EXT: {
+    'chrome': 74,
+    'firefox': 62,
+    'safari': 141000,
+  },
+  Feature.BULK_MEMORY: {
+    'chrome': 75,
+    'firefox': 79,
+    'safari': 150000,
+  },
+  Feature.MUTABLE_GLOBALS: {
+    'chrome': 74,
+    'firefox': -1, # Firefox has always supported mutable globals apparently
+    'safari': 120000,
+  },
+}
+
+
+def caniuse(feature):
+  min_versions = min_browser_versions[feature]
+  if settings.MIN_CHROME_VERSION < min_versions['chrome']:
+    return False
+  if settings.MIN_FIREFOX_VERSION < min_versions['firefox']:
+    return False
+  if settings.MIN_SAFARI_VERSION < min_versions['safari']:
+    return False
+  # IE and Edge don't support any non-MVP features
+  if settings.MIN_IE_VERSION != 0x7FFFFFFF or settings.MIN_EDGE_VERSION != 0x7FFFFFFF:
+    return False
+  return True
+
+
+def get_feature_flags():
+  flags = []
+
+  if not caniuse(Feature.NON_TRAPPING_FPTOINT):
+    logger.debug('adding -mno-nontrapping-fptoint due to target browser selection')
+    flags.append('-mno-nontrapping-fptoint')
+
+  if not caniuse(Feature.SIGN_EXT):
+    logger.debug('adding -mnosign-ext due to target browser selection')
+    flags.append('-mno-sign-ext')
+
+  if not caniuse(Feature.BULK_MEMORY):
+    logger.debug('adding -mnobulk-memory due to target browser selection')
+    flags.append('-mno-bulk-memory')
+
+  if not caniuse(Feature.MUTABLE_GLOBALS):
+    logger.debug('adding -mnomutable-globals due to target browser selection')
+    flags.append('-mno-mutable-globals')
+
+  return flags

tools/settings.py

@@ -69,6 +69,13 @@
     'WASM_OBJECT_FILES',
     'WASM_WORKERS',
 
+    # These are needed at compile time to calulate feature flags
+    'MIN_CHROME_VERSION',
+    'MIN_FIREFOX_VERSION',
+    'MIN_SAFARI_VERSION',
+    'MIN_IE_VERSION',
+    'MIN_EDGE_VERSION',
+
     # Internal settings used during compilation
     'EXCEPTION_CATCHING_ALLOWED',
     'WASM_EXCEPTIONS',