Add optional max elements limit for Map/Set mirror iterator preview.

src/runtime/runtime-collections.cc

Index: src/runtime/runtime-collections.cc
diff --git a/src/runtime/runtime-collections.cc b/src/runtime/runtime-collections.cc
index 45ac41c6209d702b28f7788c33fd89d19266944f..de1619e62f964406edd7321fe6f47d8fcfd0b630 100644
--- a/src/runtime/runtime-collections.cc
+++ b/src/runtime/runtime-collections.cc
@@ -227,23 +227,29 @@ RUNTIME_FUNCTION(Runtime_MapIteratorClone) {
 
 RUNTIME_FUNCTION(Runtime_GetWeakMapEntries) {
   HandleScope scope(isolate);
-  DCHECK(args.length() == 1);
+  DCHECK(args.length() == 2);
   CONVERT_ARG_HANDLE_CHECKED(JSWeakCollection, holder, 0);
+  CONVERT_NUMBER_CHECKED(int, max_entries, Int32, args[1]);
+  RUNTIME_ASSERT(max_entries >= 0);
+
   Handle<ObjectHashTable> table(ObjectHashTable::cast(holder->table()));
+  if (max_entries == 0 || max_entries > table->NumberOfElements()) {
+    max_entries = table->NumberOfElements();
+  }
   Handle<FixedArray> entries =
-      isolate->factory()->NewFixedArray(table->NumberOfElements() * 2);
+      isolate->factory()->NewFixedArray(max_entries * 2);
   {
     DisallowHeapAllocation no_gc;
-    int number_of_non_hole_elements = 0;
-    for (int i = 0; i < table->Capacity(); i++) {
+    int count = 0;
+    for (int i = 0; count / 2 < max_entries && i < table->Capacity(); i++) {
       Handle<Object> key(table->KeyAt(i), isolate);
       if (table->IsKey(*key)) {
-        entries->set(number_of_non_hole_elements++, *key);
+        entries->set(count++, *key);
         Object* value = table->Lookup(key);
-        entries->set(number_of_non_hole_elements++, value);
+        entries->set(count++, value);
       }
     }
-    DCHECK_EQ(table->NumberOfElements() * 2, number_of_non_hole_elements);
+    DCHECK_EQ(max_entries * 2, count);
   }
   return *isolate->factory()->NewJSArrayWithElements(entries);
 }
@@ -346,21 +352,24 @@ RUNTIME_FUNCTION(Runtime_WeakCollectionSet) {
 
 RUNTIME_FUNCTION(Runtime_GetWeakSetValues) {
   HandleScope scope(isolate);
-  DCHECK(args.length() == 1);
+  DCHECK(args.length() == 2);
   CONVERT_ARG_HANDLE_CHECKED(JSWeakCollection, holder, 0);
+  CONVERT_NUMBER_CHECKED(int, max_values, Int32, args[1]);
+  RUNTIME_ASSERT(max_values >= 0);
+
   Handle<ObjectHashTable> table(ObjectHashTable::cast(holder->table()));
-  Handle<FixedArray> values =
-      isolate->factory()->NewFixedArray(table->NumberOfElements());
+  if (max_values == 0 || max_values > table->NumberOfElements()) {
+    max_values = table->NumberOfElements();
+  }
+  Handle<FixedArray> values = isolate->factory()->NewFixedArray(max_values);
   {
     DisallowHeapAllocation no_gc;
-    int number_of_non_hole_elements = 0;
-    for (int i = 0; i < table->Capacity(); i++) {
+    int count = 0;
+    for (int i = 0; count < max_values && i < table->Capacity(); i++) {
       Handle<Object> key(table->KeyAt(i), isolate);
-      if (table->IsKey(*key)) {
-        values->set(number_of_non_hole_elements++, *key);
-      }
+      if (table->IsKey(*key)) values->set(count++, *key);
     }
-    DCHECK_EQ(table->NumberOfElements(), number_of_non_hole_elements);
+    DCHECK_EQ(max_values, count);
   }
   return *isolate->factory()->NewJSArrayWithElements(values);
 }