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.

As part of this change we are bumping the default minimum browser
versions for Safari and Firefox to the minimum version needed to
supported these 4 features which are about to be enabled by default:

- firefix: 65 -> 79
- safari: 12 - 15

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

src/settings.js

@@ -1732,7 +1732,7 @@ var AUTO_NATIVE_LIBRARIES = true;
 // Firefox ESR 60.5 (Firefox 65) was released on 2019-01-29.
 // MAX_INT (0x7FFFFFFF, or -1) specifies that target is not supported.
 // [link]
-var MIN_FIREFOX_VERSION = 65;
+var MIN_FIREFOX_VERSION = 79;
 
 // Specifies the oldest version of desktop Safari to target. Version is encoded
 // in MMmmVV, e.g. 70101 denotes Safari 7.1.1.
@@ -1743,7 +1743,7 @@ var MIN_FIREFOX_VERSION = 65;
 // see https://github.com/emscripten-core/emscripten/pull/7191.
 // MAX_INT (0x7FFFFFFF, or -1) specifies that target is not supported.
 // [link]
-var MIN_SAFARI_VERSION = 120000;
+var MIN_SAFARI_VERSION = 150000;
 
 // Specifies the oldest version of Internet Explorer to target. E.g. pass -s
 // MIN_IE_VERSION = 11 to drop support for IE 10 and older.

tools/feature_matrix.py

@@ -0,0 +1,70 @@
+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
+  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 +69,11 @@
     'WASM_OBJECT_FILES',
     'WASM_WORKERS',
 
+    # These are needed at compile time to calulate feature flags
+    'MIN_CHROME_VERSION',
+    'MIN_FIREFOX_VERSION',
+    'MIN_SAFARI_VERSION',
+
     # Internal settings used during compilation
     'EXCEPTION_CATCHING_ALLOWED',
     'WASM_EXCEPTIONS',