[in-place weak refs] Make WeakArrayList::RemoveOne more efficient

Previously, removing an element in the middle made it consume space
forever. This fixes that, without changing the complexity of removal /
addition. The trade-off is that RemoveOne will shuffle indices (which should be
OK for the current users).

BUG=v8:7308

Change-Id: I0373e30f2d9d1ffb93a78d383d41b500dbbf3429
Reviewed-on: https://chromium-review.googlesource.com/1159371
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54875}

src/objects.cc

@@ -10612,13 +10612,14 @@ int WeakArrayList::CountLiveWeakReferences() const {
 bool WeakArrayList::RemoveOne(MaybeObjectHandle value) {
   if (length() == 0) return false;
   // Optimize for the most recently added element to be removed again.
-  for (int i = length() - 1; i >= 0; --i) {
+  int last_index = length() - 1;
+  for (int i = last_index; i >= 0; --i) {
     if (Get(i) == *value) {
-      // Users should make sure that there are no duplicates.
-      Set(i, HeapObjectReference::ClearedValue());
-      if (i == length() - 1) {
-        set_length(length() - 1);
-      }
+      // Move the last element into the this slot (or no-op, if this is the
+      // last slot).
+      Set(i, Get(last_index));
+      Set(last_index, HeapObjectReference::ClearedValue());
+      set_length(last_index);
       return true;
     }
   }

src/objects/fixed-array.h

@@ -370,7 +370,10 @@ class WeakArrayList : public HeapObject {
   // Returns the number of non-cleaned weak references in the array.
   int CountLiveWeakReferences() const;
 
-  // Returns whether an entry was found and removed.
+  // Returns whether an entry was found and removed. Will move the elements
+  // around in the array - this method can only be used in cases where the user
+  // doesn't care about the indices! Users should make sure there are no
+  // duplicates.
   bool RemoveOne(MaybeObjectHandle value);
 
   class Iterator {

test/cctest/heap/test-weak-references.cc

@@ -564,31 +564,24 @@ TEST(WeakArrayListRemove) {
 
   CHECK(array->RemoveOne(MaybeObjectHandle::Weak(elem1)));
 
-  CHECK_EQ(array->length(), 3);
+  CHECK_EQ(array->length(), 2);
   CHECK_EQ(array->Get(0), HeapObjectReference::Weak(*elem0));
-  CHECK(array->Get(1)->IsClearedWeakHeapObject());
-  CHECK_EQ(array->Get(2), HeapObjectReference::Weak(*elem2));
+  CHECK_EQ(array->Get(1), HeapObjectReference::Weak(*elem2));
 
   CHECK(!array->RemoveOne(MaybeObjectHandle::Weak(elem1)));
 
-  CHECK_EQ(array->length(), 3);
+  CHECK_EQ(array->length(), 2);
   CHECK_EQ(array->Get(0), HeapObjectReference::Weak(*elem0));
-  CHECK(array->Get(1)->IsClearedWeakHeapObject());
-  CHECK_EQ(array->Get(2), HeapObjectReference::Weak(*elem2));
+  CHECK_EQ(array->Get(1), HeapObjectReference::Weak(*elem2));
 
   CHECK(array->RemoveOne(MaybeObjectHandle::Weak(elem0)));
 
-  CHECK_EQ(array->length(), 3);
-  CHECK(array->Get(0)->IsClearedWeakHeapObject());
-  CHECK(array->Get(1)->IsClearedWeakHeapObject());
-  CHECK_EQ(array->Get(2), HeapObjectReference::Weak(*elem2));
+  CHECK_EQ(array->length(), 1);
+  CHECK_EQ(array->Get(0), HeapObjectReference::Weak(*elem2));
 
   CHECK(array->RemoveOne(MaybeObjectHandle::Weak(elem2)));
 
-  // Removing the last element decreases the array length.
-  CHECK_EQ(array->length(), 2);
-  CHECK(array->Get(0)->IsClearedWeakHeapObject());
-  CHECK(array->Get(1)->IsClearedWeakHeapObject());
+  CHECK_EQ(array->length(), 0);
 }
 
 TEST(Regress7768) {