Add sign-ext lowering pass when target browser doesn't support it

ChangeLog.md

@@ -28,7 +28,10 @@ See docs/process.md for more on how version tagging works.
   support these older engines you can use these settings
   (`-sMIN_SAFARI_VERSION=120000` and/or `-sMIN_EDGE_VERSION=44`) to revert to
   the previous defaults, which will result in the new proposals being disabled.
-  (#17690)
+  Note that in order to avoid support for the sign-extension emscripten uses
+  a binaryen pass, so targeting older browsers requires the running of wasm-opt
+  and is therefore incompatible with `ERROR_ON_WASM_CHANGES_AFTER_LINK` (i.e.
+  fast linking). (#17690)
 - Added `--reproduce` command line flag (or equivalently `EMCC_REPRODUCE`
   environment variable).  This options specifies the name of a tar file into
   which emscripten will copy all of the input files along with a response file

emcc.py

@@ -47,6 +47,7 @@
 from tools.response_file import substitute_response_files
 from tools.minimal_runtime_shell import generate_minimal_runtime_html
 import tools.line_endings
+from tools import feature_matrix
 from tools import js_manipulation
 from tools import wasm2c
 from tools import webassembly
@@ -466,21 +467,6 @@ def apply_user_settings():
       settings.LTO = 0 if value else 'full'
 
 
-# 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.
-def apply_min_browser_versions():
-
-  def default_min_browser_version(browser, version):
-    default_setting(f'MIN_{browser.upper()}_VERSION', version)
-
-  if settings.WASM_BIGINT:
-    default_min_browser_version('Safari', 150000)
-    default_min_browser_version('Edge', 79)
-    default_min_browser_version('Firefox', 68)
-    # Chrome has BigInt since v67 which is less than default min version.
-
-
 def is_ar_file_with_missing_index(archive_file):
   # We parse the archive header outselves because llvm-nm --print-armap is slower and less
   # reliable.
@@ -614,6 +600,11 @@ def get_binaryen_passes():
     passes += ['--safe-heap']
   if settings.MEMORY64 == 2:
     passes += ['--memory64-lowering']
+  # sign-ext is enabled by default by llvm.  If the target browser settings don't support
+  # this we lower it away here using a binaryen pass.
+  if not feature_matrix.caniuse(feature_matrix.Feature.SIGN_EXT):
+    logger.debug('lowering sign-ext feature due to incompatiable target browser engines')
+    passes += ['--signext-lowering']
   if optimizing:
     passes += ['--post-emscripten']
   if optimizing:
@@ -2748,7 +2739,7 @@ def get_full_import_name(name):
     if settings.WASM_EXCEPTIONS:
       settings.EXPORTED_FUNCTIONS += ['___cpp_exception', '___cxa_increment_exception_refcount', '___cxa_decrement_exception_refcount', '___thrown_object_from_unwind_exception']
 
-  apply_min_browser_versions()
+  feature_matrix.apply_min_browser_versions()
 
   if settings.SIDE_MODULE:
     # For side modules, we ignore all REQUIRED_EXPORTS that might have been added above.

src/settings.js

@@ -1968,7 +1968,9 @@ var SEPARATE_DWARF_URL = '';
 // not in others like split-dwarf).
 // When this flag is turned on, we error at link time if the build requires any
 // changes to the wasm after link. This can be useful in testing, for example.
-// [link]
+// Some example of features that require post-link wasm changes are:
+// - Lowering i64 to i32 pairs at the JS boundary (See WASM_BIGINT)
+// - Lowering sign-extnesion operation when targeting older browsers.
 var ERROR_ON_WASM_CHANGES_AFTER_LINK = false;
 
 // Abort on unhandled excptions that occur when calling exported WebAssembly

test/other/test_signext_lowering.c

@@ -0,0 +1,6 @@
+#include <stdio.h>
+#include <stdint.h>
+
+int main(int argc, char* argv[]) {
+  printf("%lld\n", (int64_t)argc);
+}

test/test_other.py

@@ -12724,3 +12724,32 @@ def test_reproduce(self):
     response = response.replace('\\', '/')
     response = response.replace(root, '<root>')
     self.assertTextDataIdentical(expected, response)
+
+  def test_signext_lowering(self):
+    cmd = [EMCC, test_file('other/test_signext_lowering.c'), '-sWASM_BIGINT']
+
+    # We expect ERROR_ON_WASM_CHANGES_AFTER_LINK to fail due lowering of sign-ext being
+    # required for safari 12.
+    output = self.expect_fail(cmd + ['-sERROR_ON_WASM_CHANGES_AFTER_LINK', '-sMIN_SAFARI_VERSION=120000'])
+    self.assertContained('error: changes to the wasm are required after link, but disallowed by ERROR_ON_WASM_CHANGES_AFTER_LINK', output)
+    self.assertContained('--signext-lowering', output)
+
+    # Same for firefox
+    output = self.expect_fail(cmd + ['-sERROR_ON_WASM_CHANGES_AFTER_LINK', '-sMIN_FIREFOX_VERSION=61'])
+    self.assertContained('error: changes to the wasm are required after link, but disallowed by ERROR_ON_WASM_CHANGES_AFTER_LINK', output)
+    self.assertContained('--signext-lowering', output)
+
+    # Same for chrome
+    output = self.expect_fail(cmd + ['-sERROR_ON_WASM_CHANGES_AFTER_LINK', '-sMIN_CHROME_VERSION=73'])
+    self.assertContained('error: changes to the wasm are required after link, but disallowed by ERROR_ON_WASM_CHANGES_AFTER_LINK', output)
+    self.assertContained('--signext-lowering', output)
+
+    # Running the same command with safari 14.1 should succeed because no lowering
+    # is required.
+    self.run_process(cmd + ['-sERROR_ON_WASM_CHANGES_AFTER_LINK', '-sMIN_SAFARI_VERSION=140100'])
+    self.assertEqual('1\n', self.run_js('a.out.js'))
+
+    # Running without ERROR_ON_WASM_CHANGES_AFTER_LINK but with safari 12
+    # should successfully lower sign-ext.
+    self.run_process(cmd + ['-sMIN_SAFARI_VERSION=120000', '-o', 'lowered.js'])
+    self.assertEqual('1\n', self.run_js('lowered.js'))

tools/feature_matrix.py

@@ -0,0 +1,88 @@
+# 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 webassembly features."""
+
+import logging
+from enum import IntEnum, auto
+
+from .settings import settings, user_settings
+
+logger = logging.getLogger('feature_matrix')
+
+
+class Feature(IntEnum):
+  NON_TRAPPING_FPTOINT = auto()
+  SIGN_EXT = auto()
+  BULK_MEMORY = auto()
+  MUTABLE_GLOBALS = auto()
+  JS_BIGINT_INTEGRATION = auto()
+
+
+default_features = {Feature.SIGN_EXT, Feature.MUTABLE_GLOBALS}
+
+min_browser_versions = {
+  Feature.NON_TRAPPING_FPTOINT: {
+    'chrome': 75,
+    'firefox': 65,
+    'safari': 150000,
+  },
+  Feature.SIGN_EXT: {
+    'chrome': 74,
+    'firefox': 62,
+    'safari': 140100,
+  },
+  Feature.BULK_MEMORY: {
+    'chrome': 75,
+    'firefox': 79,
+    'safari': 150000,
+  },
+  Feature.MUTABLE_GLOBALS: {
+    'chrome': 74,
+    'firefox': 61,
+    'safari': 120000,
+  },
+  Feature.JS_BIGINT_INTEGRATION: {
+    'chrome': 67,
+    'firefox': 68,
+    'safari': 150000,
+  },
+}
+
+
+def caniuse(feature):
+  min_versions = min_browser_versions[feature]
+  if settings.MIN_CHROME_VERSION < min_versions['chrome']:
+    return False
+  # For edge we just use the same version requirements as chrome since,
+  # at least for modern versions of edge, they share version numbers.
+  if settings.MIN_EDGE_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 don't support any non-MVP features
+  if settings.MIN_IE_VERSION != 0x7FFFFFFF:
+    return False
+  return True
+
+
+def enable_feature(feature):
+  """Updates default settings for browser versions such that the given
+  feature is available everywhere.
+  """
+  for name, min_version in min_browser_versions[feature].items():
+    name = f'MIN_{name.upper()}_VERSION'
+    if settings[name] < min_version and name not in user_settings:
+      setattr(settings, name, min_version)
+
+
+# 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.
+def apply_min_browser_versions():
+  if settings.WASM_BIGINT:
+    enable_feature(Feature.JS_BIGINT_INTEGRATION)