[dart2wasm] More compact encoding of deferred load lists

Measured on size of e main module (baseline is we don't embed
it in application code):

* embedding before: +16.5% uncompressed / +9.1% compressed
* embedding with this CL: +4% uncompressed / +4.3% compressed

When embeddeding deferred load list information into the app
(as opposed to a separate json file) we now use a more compact
encoding.

Specifically: Instead of encoding it as an array of an array of
strings (which are module names), we encode it as an array of an
array of module ids and construct the module name from the id.

To make the array of module ids more compact we utilize the fact
that we can sort them and encode in delta encoding (i.e. instead
of absolute module ids, encode the diff between previous module
id in the list).

We put the encoded module id lists in a data section and create
`WasmArray<WasmI8>`s from them at startup. When we trigger a load
we then decode them into the list of module names.

There's more opportunity to optimize it, but it's good to do
this as a first step.

Change-Id: I293fb8879d992fc370786f6c9b258ccd27e1559b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508980
Reviewed-by: Srujan Gaddam <srujzs@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

sdk/lib/_internal/wasm/js_common/deferred_patch.dart

@@ -22,11 +22,19 @@ final Map<int, Future<void>> _loading = {};
 final Set<int> _loaded = {};
 
 /// Only used when loading modules directly, will get populated by the compiler.
-external ImmutableWasmArray<ImmutableWasmArray<WasmExternRef>> get _loadingMap;
+///
+/// Maps a loading id (aka deferred prefix) to the set of module ids that have
+/// to be loaded.
+external WasmArray<WasmArray<WasmI8>?> get _loadingMap;
 
 /// Maps load id to (import uri, import prefix).
 external ImmutableWasmArray<WasmExternRef> get _loadingMapNames;
 
+/// The prefix of all module names.
+///
+/// For `test_module<id>.wasm` it will be `test_module`.
+external WasmExternRef get _moduleNamePrefix;
+
 @pragma("wasm:import", "moduleLoadingHelper.loadDeferredModules")
 external WasmExternRef _loadDeferredModules(WasmExternRef moduleNames);
 
@@ -94,8 +102,10 @@ Future<void> loadLibraryFromLoadId(int loadId) {
           'Error loading load ID: ${_loadIdInJson(loadId)}\n$e',
         );
       }
-      return 'Error loading ${_prefixName(loadId)} of library '
-          '${_importUri(loadId)}\n$e';
+      throw DeferredLoadException(
+        'Error loading ${_prefixName(loadId)} of library '
+        '${_importUri(loadId)}\n$e',
+      );
     },
   );
 }
@@ -108,18 +118,47 @@ Future<void> _loadLibraryViaEmbedderLoadId(int loadId) {
 
 Future<void> _loadLibraryViaEmbedderModuleNames(int loadId) {
   assert(loadId < _loadingMap.length);
-
-  final ImmutableWasmArray<WasmExternRef> moduleNames = _loadingMap[loadId];
-  if (moduleNames.length == 0) {
+  final WasmArray<WasmI8>? encodedModuleIds = _loadingMap[loadId];
+  if (encodedModuleIds == null) {
     // No modules to load.
     return Future.value();
   }
-  final moduleNamesAsList = <JSString>[];
-  for (int i = 0; i < moduleNames.length; ++i) {
-    moduleNamesAsList.add(JSValue(moduleNames[i]) as JSString);
-  }
+  final moduleNamesAsList = _decodeEncodedModuleIds('test', encodedModuleIds);
+
   final promise =
       (_loadDeferredModules(moduleNamesAsList.toJS.toExternRef!).toJS
           as JSPromise);
   return promise.toDart;
 }
+
+/// Keep in sync with pkg/dart2wasm/lib/translator.dart:Translator._patchLoadingMapGetter`
+List<JSString> _decodeEncodedModuleIds(
+  String prefix,
+  WasmArray<WasmI8> encoded,
+) {
+  int offset = 0;
+
+  int nextULEB128() {
+    int result = 0;
+    int shift = 0;
+    while (true) {
+      final byte = encoded.readUnsigned(offset++);
+      result |= (byte & 0x7F) << shift;
+      shift += 7;
+      if ((byte & 0x80) == 0) break;
+    }
+    return result;
+  }
+
+  final length = nextULEB128();
+  final moduleIds = <JSString>[];
+  int previousModuleId = 0;
+  final prefix = JSStringImpl.fromRefUnchecked(_moduleNamePrefix);
+  for (int i = 0; i < length; ++i) {
+    int diff = nextULEB128();
+    final moduleId = previousModuleId + diff;
+    moduleIds.add('$prefix${moduleId.toString()}.wasm'.toJS);
+    previousModuleId = moduleId;
+  }
+  return moduleIds;
+}