[dart2wasm] Avoid indirect call to key.hashCode in json map creations

After a recent change we started to eagerly calculate the hash code of
json object keys so when we create the string objects we initialize the
hash as well (as we know it will be needed to create the map).

This means that the actual creation of the hash map can do less work.
Right now it does an indirect call to `key.hashCode`, but since we know
those particular maps from the json decoder come only with string keys
and most of them will have their hash code already prepopulated we can
get avoid the indirect call to `key.hashCode` in most cases.

Change-Id: I38987281bd8b216832cac950ea74d0ca201ff651
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/409980
Reviewed-by: Ömer Ağacan <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: mkustermann

sdk/lib/_internal/wasm/lib/compact_hash.dart

@@ -11,6 +11,7 @@ import "dart:_internal"
         TypeTest,
         unsafeCast;
 import "dart:_list" show GrowableList;
+import "dart:_object_helper";
 import "dart:_wasm";
 
 import "dart:collection";
@@ -459,7 +460,7 @@ mixin _LinkedHashMapMixin<K, V> on _HashBase, _EqualsAndHashCode {
   /// This function is unsafe: it does not perform any type checking on
   /// keys and values assuming that caller has ensured that types are
   /// correct.
-  void _populateUnsafe(WasmArray<Object?> data, int usedData) {
+  void _populateUnsafeOnlyStringKeys(WasmArray<Object?> data, int usedData) {
     assert(usedData.isEven);
     int size = data.length;
     if (size == 0) {
@@ -486,7 +487,17 @@ mixin _LinkedHashMapMixin<K, V> on _HashBase, _EqualsAndHashCode {
     for (int i = 0; i < usedData; i += 2) {
       final key = unsafeCast<K>(data[i]);
       final value = unsafeCast<V>(data[i + 1]);
-      _set(key, value, _hashCode(key));
+
+      // Strings store their hash code in the object header's identity hash code
+      // field. So before doing a indirect call to obtain the hash code, we can
+      // check if it was already computed and if so just use it.
+      final int idHash = getIdentityHashField(unsafeCast<Object>(key));
+      if (idHash != 0) {
+        assert(idHash == key.hashCode);
+        _set(key, value, idHash);
+      } else {
+        _set(key, value, key.hashCode);
+      }
     }
   }
 
@@ -1254,7 +1265,9 @@ base class CompactLinkedCustomHashSet<E> extends _HashFieldBase
 }
 
 @pragma('wasm:prefer-inline')
-Map<K, V> createMapFromKeyValueListUnsafe<K, V>(
+Map<K, V> createMapFromStringKeyValueListUnsafe<K, V>(
   WasmArray<Object?> keyValuePairData,
   int usedData,
-) => DefaultMap<K, V>().._populateUnsafe(keyValuePairData, usedData);
+) =>
+    DefaultMap<K, V>()
+      .._populateUnsafeOnlyStringKeys(keyValuePairData, usedData);

sdk/lib/_internal/wasm/lib/convert_patch.dart

@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import "dart:_compact_hash" show createMapFromKeyValueListUnsafe;
+import "dart:_compact_hash" show createMapFromStringKeyValueListUnsafe;
 import "dart:_error_utils";
 import "dart:_internal"
     show patch, POWERS_OF_TEN, unsafeCast, pushWasmArray, popWasmArray;
@@ -210,7 +210,7 @@ class _JsonListener {
   }
 
   void endObject() {
-    value = createMapFromKeyValueListUnsafe<String, dynamic>(
+    value = createMapFromStringKeyValueListUnsafe<String, dynamic>(
       unsafeCast<WasmArray<Object?>>(currentContainer),
       currentContainerLength,
     );