Export mutable wasm globals as JS objects. NFC

ChangeLog.md

@@ -20,6 +20,10 @@ See docs/process.md for more on how version tagging works.
 
 4.0.17 (in development)
 -----------------------
+- Mutable Wasm globals can now be exported from native code.  Currently these
+  cannot be declared in C/C++ but can be defined and exported in assembly code.
+  This currently only works for mutable globals since immutables are already
+  (and continue to be) exported as plain JS numbers. (#25530)
 - Minimum Firefox version was bumped up to Firefox 68 ESR, since older Firefox
   versions are not able to run the parallel browser harness: (#25493)
   - Firefox: v65 -> v68

src/lib/libdylink.js

@@ -230,24 +230,33 @@ var LibraryDylink = {
       }
 #endif
 
-      GOT[symName] ||= new WebAssembly.Global({'value': '{{{ POINTER_WASM_TYPE }}}', 'mutable': true});
-      if (replace || GOT[symName].value == 0) {
+
+      var existingEntry = GOT[symName] && GOT[symName].value != 0;
+      if (replace || !existingEntry) {
 #if DYLINK_DEBUG == 2
         dbg(`updateGOT: before: ${symName} : ${GOT[symName].value}`);
 #endif
+        var newValue;
         if (typeof value == 'function') {
-          GOT[symName].value = {{{ to64('addFunction(value)') }}};
+          newValue = {{{ to64('addFunction(value)') }}};
 #if DYLINK_DEBUG == 2
           dbg(`updateGOT: FUNC: ${symName} : ${GOT[symName].value}`);
 #endif
         } else if (typeof value == {{{ POINTER_JS_TYPE }}}) {
-          GOT[symName].value = value;
+          newValue = value;
         } else {
-          err(`unhandled export type for '${symName}': ${typeof value}`);
+          // The GOT can only contain addresses (i.e data addresses or function
+          // addresses so we currently ignore other types export here.
+#if DYLINK_DEBUG
+          dbg(`updateGOT: ignoring ${symName} due to its type: ${typeof value}`);
+#endif
+          continue;
         }
 #if DYLINK_DEBUG == 2
-        dbg(`updateGOT:  after: ${symName} : ${GOT[symName].value} (${value})`);
+        dbg(`updateGOT:  after: ${symName} : ${newValue} (${value})`);
 #endif
+        GOT[symName] ||= new WebAssembly.Global({'value': '{{{ POINTER_WASM_TYPE }}}', 'mutable': true});
+        GOT[symName].value = newValue;
       }
 #if DYLINK_DEBUG
       else if (GOT[symName].value != value) {
@@ -260,29 +269,43 @@ var LibraryDylink = {
 #endif
   },
 
+  $isImmutableGlobal__internal: true,
+  $isImmutableGlobal: (val) => {
+    if (val instanceof WebAssembly.Global) {
+      try {
+        val.value = val.value;
+      } catch {
+        return true;
+      }
+    }
+    return false;
+  },
+
   // Applies relocations to exported things.
   $relocateExports__internal: true,
-  $relocateExports__deps: ['$updateGOT'],
+  $relocateExports__deps: ['$updateGOT', '$isImmutableGlobal'],
   $relocateExports__docs: '/** @param {boolean=} replace */',
   $relocateExports: (exports, memoryBase, replace) => {
-    var relocated = {};
-
-    for (var e in exports) {
-      var value = exports[e];
+    function relocateExport(name, value) {
 #if SPLIT_MODULE
       // Do not modify exports synthesized by wasm-split
-      if (e.startsWith('%')) {
-        relocated[e] = value
-        continue;
+      if (name.startsWith('%')) {
+        return value;
       }
 #endif
-      // Detect wasm global exports. These represent data addresses
+      // Detect immuable wasm global exports. These represent data addresses
       // which are relative to `memoryBase`
-      if (value instanceof WebAssembly.Global) {
-        value = value.value;
-        value += {{{ to64('memoryBase') }}};
+      if (isImmutableGlobal(value)) {
+        return value.value + {{{ to64('memoryBase') }}};
       }
-      relocated[e] = value;
+
+      // Return unmodified value (no relocation required).
+      return value;
+    }
+
+    var relocated = {};
+    for (var e in exports) {
+      relocated[e] = relocateExport(e, exports[e])
     }
     updateGOT(relocated, replace);
     return relocated;

src/settings_internal.js

@@ -16,8 +16,8 @@
 // underscore.
 var WASM_EXPORTS = [];
 
-// Similar to above but only includes the global/data symbols.
-var WASM_GLOBAL_EXPORTS = [];
+// Similar to above but only includes the data symbols (address exports).
+var DATA_EXPORTS = [];
 
 // An array of all symbols exported from all the side modules specified on the
 // command line.

test/codesize/test_codesize_hello_dylink.json

@@ -1,10 +1,10 @@
 {
-  "a.out.js": 26912,
-  "a.out.js.gz": 11470,
+  "a.out.js": 26919,
+  "a.out.js.gz": 11469,
   "a.out.nodebug.wasm": 18567,
   "a.out.nodebug.wasm.gz": 9199,
-  "total": 45479,
-  "total_gz": 20669,
+  "total": 45486,
+  "total_gz": 20668,
   "sent": [
     "__heap_base",
     "__indirect_function_table",

test/codesize/test_codesize_hello_dylink_all.json

@@ -1,7 +1,7 @@
 {
-  "a.out.js": 245822,
+  "a.out.js": 245829,
   "a.out.nodebug.wasm": 597746,
-  "total": 843568,
+  "total": 843575,
   "sent": [
     "IMG_Init",
     "IMG_Load",

test/core/test_wasm_global.c

@@ -0,0 +1,43 @@
+#include <stdio.h>
+#include <emscripten/em_asm.h>
+#include <emscripten.h>
+
+__asm__(
+".section .data.my_global,\"\",@\n"
+".globl my_global\n"
+".globaltype my_global, i32\n"
+"my_global:\n"
+);
+
+int get_global() {
+  int val;
+  // Without volatile here this test fails in O1 and above.
+  __asm__ volatile ("global.get my_global\n"
+                    "local.set %0\n" : "=r" (val));
+  return val;
+}
+
+void set_global(int val) {
+  __asm__("local.get %0\n"
+          "global.set my_global\n" : : "r" (val));
+}
+
+int main() {
+  printf("in main: %d\n", get_global());
+  set_global(42);
+  printf("new value: %d\n", get_global());
+  EM_ASM({
+      // With the ESM integration, the Wasm global be exported as a regular
+      // number.  Otherwise it will be a WebAssembly.Global object.
+#ifdef ESM_INTEGRATION
+      assert(typeof _my_global == 'number', typeof _my_global);
+      out('from js:', _my_global);
+      _my_global += 1
+#else
+      assert(typeof _my_global == 'object', typeof _my_global);
+      out('from js:', _my_global.value);
+      _my_global.value += 1
+#endif
+  });
+  printf("done: %d\n", get_global());
+}

test/core/test_wasm_global.out

@@ -0,0 +1,4 @@
+in main: 0
+new value: 42
+from js: 42
+done: 43

test/test_core.py

@@ -9677,6 +9677,19 @@ def test_externref_emjs(self, dynlink):
       self.set_setting('MAIN_MODULE', 2)
     self.do_core_test('test_externref_emjs.c')
 
+  @parameterized({
+    '': [False],
+    'dylink': [True],
+  })
+  @no_esm_integration('https://github.com/emscripten-core/emscripten/issues/25543')
+  def test_wasm_global(self, dynlink):
+    if dynlink:
+      self.check_dylink()
+      self.set_setting('MAIN_MODULE', 2)
+    if self.get_setting('WASM_ESM_INTEGRATION'):
+      self.cflags.append('-DESM_INTEGRATION')
+    self.do_core_test('test_wasm_global.c', cflags=['-sEXPORTED_FUNCTIONS=_main,_my_global'])
+
   def test_syscall_intercept(self):
     self.do_core_test('test_syscall_intercept.c')
 

tools/building.py

@@ -810,7 +810,7 @@ def metadce(js_file, wasm_file, debug_info, last):
     exports = settings.WASM_EXPORTS
   else:
     # Ignore exported wasm globals.  Those get inlined directly into the JS code.
-    exports = sorted(set(settings.WASM_EXPORTS) - set(settings.WASM_GLOBAL_EXPORTS))
+    exports = sorted(set(settings.WASM_EXPORTS) - set(settings.DATA_EXPORTS))
 
   extra_info = '{ "exports": [' + ','.join(f'["{asmjs_mangle(x)}", "{x}"]' for x in exports) + ']}'
 

tools/emscripten.py

@@ -111,7 +111,7 @@ def update_settings_glue(wasm_file, metadata, base_metadata):
     settings.WASM_EXPORTS = base_metadata.all_exports
   else:
     settings.WASM_EXPORTS = metadata.all_exports
-  settings.WASM_GLOBAL_EXPORTS = list(metadata.global_exports.keys())
+  settings.DATA_EXPORTS = list(metadata.data_exports.keys())
   settings.HAVE_EM_ASM = bool(settings.MAIN_MODULE or len(metadata.em_asm_consts) != 0)
 
   # start with the MVP features, and add any detected features.
@@ -269,9 +269,9 @@ def trim_asm_const_body(body):
   return body
 
 
-def create_global_exports(global_exports):
+def create_data_exports(data_exports):
   lines = []
-  for k, v in global_exports.items():
+  for k, v in data_exports.items():
     if shared.is_internal_global(k):
       continue
 
@@ -408,11 +408,11 @@ def emscript(in_wasm, out_wasm, outfile_js, js_syms, finalize=True, base_metadat
     other_exports = base_metadata.other_exports
     # We want the real values from the final metadata but we only want to
     # include names from the base_metadata.  See phase_link() in link.py.
-    global_exports = {k: v for k, v in metadata.global_exports.items() if k in base_metadata.global_exports}
+    data_exports = {k: v for k, v in metadata.data_exports.items() if k in base_metadata.data_exports}
   else:
     function_exports = metadata.function_exports
     other_exports = metadata.other_exports
-    global_exports = metadata.global_exports
+    data_exports = metadata.data_exports
 
   if settings.ASYNCIFY == 1:
     function_exports['asyncify_start_unwind'] = webassembly.FuncType([webassembly.Type.I32], [])
@@ -421,7 +421,7 @@ def emscript(in_wasm, out_wasm, outfile_js, js_syms, finalize=True, base_metadat
     function_exports['asyncify_stop_rewind'] = webassembly.FuncType([], [])
 
   parts = [pre]
-  parts += create_module(metadata, function_exports, global_exports, other_exports,
+  parts += create_module(metadata, function_exports, data_exports, other_exports,
                          forwarded_json['librarySymbols'], forwarded_json['nativeAliases'])
   parts.append(post)
   settings.ALIASES = list(forwarded_json['nativeAliases'].keys())
@@ -1008,10 +1008,10 @@ def create_receiving(function_exports, other_exports, library_symbols, aliases):
   return '\n'.join(receiving)
 
 
-def create_module(metadata, function_exports, global_exports, other_exports, library_symbols, aliases):
+def create_module(metadata, function_exports, data_exports, other_exports, library_symbols, aliases):
   module = []
   module.append(create_receiving(function_exports, other_exports, library_symbols, aliases))
-  module.append(create_global_exports(global_exports))
+  module.append(create_data_exports(data_exports))
 
   sending = create_sending(metadata, library_symbols)
   if settings.WASM_ESM_INTEGRATION:

tools/extract_metadata.py

@@ -240,13 +240,16 @@ def get_main_reads_params(module, export_map):
   return True
 
 
-def get_global_exports(module, exports):
-  global_exports = {}
+def get_data_exports(module, exports):
+  data_exports = {}
   for export in exports:
     if export.kind == webassembly.ExternType.GLOBAL:
       g = module.get_global(export.index)
-      global_exports[export.name] = str(get_global_value(g))
-  return global_exports
+      # Data symbols (addresses) are exported as immutable Wasm globals.
+      # mutable globals are handled via get_other_exports
+      if not g.mutable:
+        data_exports[export.name] = str(get_global_value(g))
+  return data_exports
 
 
 def get_function_exports(module):
@@ -260,8 +263,14 @@ def get_function_exports(module):
 def get_other_exports(module):
   rtn = []
   for e in module.get_exports():
-    if e.kind not in (webassembly.ExternType.FUNC, webassembly.ExternType.GLOBAL):
-      rtn.append(e.name)
+    if e.kind == webassembly.ExternType.FUNC:
+      continue
+    if e.kind == webassembly.ExternType.GLOBAL:
+      g = module.get_global(e.index)
+      # Immutable globals are handled specially.  See get_data_exports
+      if not g.mutable:
+        continue
+    rtn.append(e.name)
   return rtn
 
 
@@ -311,7 +320,7 @@ class Metadata:
   features: List[str]
   invoke_funcs: List[str]
   main_reads_params: bool
-  global_exports: Dict[str, str]
+  data_exports: Dict[str, str]
   function_exports: Dict[str, webassembly.FuncType]
   other_exports: List[str]
   all_exports: List[str]
@@ -348,7 +357,7 @@ def extract_metadata(filename):
     metadata.em_js_funcs = em_js_funcs
     metadata.features = features
     metadata.main_reads_params = get_main_reads_params(module, export_map)
-    metadata.global_exports = get_global_exports(module, exports)
+    metadata.data_exports = get_data_exports(module, exports)
 
     read_module_imports(module, metadata)