Avoid copying all exports in applySignatureConversions

ChangeLog.md

@@ -26,6 +26,13 @@ See docs/process.md for more on how version tagging works.
 - The undocumented `FS.filesystems` object was removed. Replace things like
   `FS.filesystems['MEMFS']` with plain `MEMFS`. (#27940)
 - libpng port updated from 1.6.58 to 1.6.59. (#27956)
+- When exports are wrapped for pointer conversions (`MEMORY64`, or
+  `MAXIMUM_MEMORY` over 2GB), the `wasmExports` object no longer contains a
+  copy of every export.  Only the wrapped exports are its own properties and
+  the rest are inherited from the original `WebAssembly.Instance` exports.
+  This speeds up startup for programs with many exports (e.g. `MAIN_MODULE`).
+  Code that enumerates `wasmExports` should use `for...in` rather than
+  `Object.keys`/`Object.entries`. (#27996)
 
 6.0.12 - 10/08/26
 -----------------

src/lib/libcore.js

@@ -1765,7 +1765,11 @@ addToLibrary({
 #endif
   ],
   $assignWasmExports: (wasmExports) => {
-    for (var [name, exportedSymbol] of Object.entries(wasmExports)) {
+    // Use `for...in` rather than `Object.entries` here since, after
+    // `applySignatureConversions`, some exports are inherited rather than own
+    // properties.
+    for (var name in wasmExports) {
+      var exportedSymbol = wasmExports[name];
       name = asmjsMangle(name);
 #if DYNCALLS || !WASM_BIGINT
       if (name.startsWith('dynCall_')) {

test/codesize/test_codesize_minimal_64.json

@@ -1,10 +1,10 @@
 {
-  "a.out.js": 2367,
-  "a.out.js.gz": 1157,
+  "a.out.js": 2381,
+  "a.out.js.gz": 1169,
   "a.out.nodebug.wasm": 75,
   "a.out.nodebug.wasm.gz": 88,
-  "total": 2442,
-  "total_gz": 1245,
+  "total": 2456,
+  "total_gz": 1257,
   "sent": [],
   "imports": [],
   "exports": [

test/test_other.py

@@ -13986,6 +13986,47 @@ def test_wasmfs_before_preload(self):
   def test_hello_world_above_2gb(self):
     self.do_runf_out_file('hello_world.c', cflags=['-sGLOBAL_BASE=2GB', '-sINITIAL_MEMORY=3GB'])
 
+  @parameterized({
+    '': ([],),
+    # Minified export names
+    'O3': (['-O3'],),
+  })
+  def test_signature_conversions(self, args):
+    # Exports returning pointers get wrapped so that they return unsigned
+    # values when memory can be above 2GB.  Check that the wrapped exports are
+    # used, and that all other exports are still present on `wasmExports`
+    # (including when enumerated).
+    create_file('main.c', r'''
+      #include <stdint.h>
+      #include <emscripten.h>
+
+      EMSCRIPTEN_KEEPALIVE void* get_high_ptr() { return (void*)(uintptr_t)0x90000000u; }
+      EMSCRIPTEN_KEEPALIVE int add_one(int x) { return x + 1; }
+
+      int main() {
+        EM_ASM({
+          out('_get_high_ptr: ' + _get_high_ptr());
+          out('wasmExports.get_high_ptr: ' + wasmExports['get_high_ptr']());
+          out('_add_one: ' + _add_one(41));
+          out('wasmExports.add_one: ' + wasmExports['add_one'](41));
+          var found = 0;
+          for (var name in wasmExports) {
+            if (wasmExports[name] === wasmExports['get_high_ptr']) found |= 1;
+            if (wasmExports[name] === wasmExports['add_one']) found |= 2;
+          }
+          out('enumerated: ' + found);
+        });
+      }
+    ''')
+    expected = '''\
+_get_high_ptr: 2415919104
+wasmExports.get_high_ptr: 2415919104
+_add_one: 42
+wasmExports.add_one: 42
+enumerated: 3
+'''
+    self.do_runf('main.c', expected, cflags=['-sALLOW_MEMORY_GROWTH', '-sMAXIMUM_MEMORY=4GB', '-sSIGNATURE_CONVERSIONS=get_high_ptr:p'] + args)
+
   def test_unistd_strerror(self):
     self.do_runf_out_file('unistd/strerror.c')
 

tools/acorn-optimizer.mjs

@@ -464,11 +464,12 @@ function isExportSpecifierList(node) {
 function isExportUse(node) {
   // Match usages of symbols on the `wasmExports` object. e.g:
   //   wasmExports['X']
+  // `origExports` holds the unwrapped exports in applySignatureConversions.
   return (
     node.type === 'MemberExpression' &&
     node.object.type === 'Identifier' &&
     isLiteralString(node.property) &&
-    node.object.name === 'wasmExports'
+    (node.object.name === 'wasmExports' || node.object.name === 'origExports')
   );
 }
 

tools/emscripten.py

@@ -1165,8 +1165,6 @@ def create_pointer_conversion_wrappers(metadata):
 // that it is recognised by metadce and minify-import-export-names
 // passes.
 function applySignatureConversions(wasmExports) {
-  // First, make a copy of the incoming exports object
-  wasmExports = Object.assign({}, wasmExports);
 '''
 
   sigs_seen = set()
@@ -1195,9 +1193,17 @@ def create_pointer_conversion_wrappers(metadata):
           sigs_seen.add(sig)
         wrap_functions.append(symbol)
 
-  for f in wrap_functions:
-    sig = native_sigs[f]
-    wrappers += f"\n  wasmExports['{f}'] = makeWrapper_{sig}(wasmExports['{f}']);"
+  if wrap_functions:
+    # Rather than copying the entire exports object (which is slow when there
+    # are many exports) we put just the wrapped exports on a new object, which
+    # then inherits all the others from the original.  The wrappers can't be
+    # assigned to an object that already inherits from the original exports
+    # object, since that is frozen, so we set the prototype afterwards.
+    wrappers += '\n  var origExports = wasmExports;\n  wasmExports = {};'
+    for f in wrap_functions:
+      sig = native_sigs[f]
+      wrappers += f"\n  wasmExports['{f}'] = makeWrapper_{sig}(origExports['{f}']);"
+    wrappers += '\n  Object.setPrototypeOf(wasmExports, origExports);'
   wrappers += '\n  return wasmExports;\n}'
 
   return wrappers