Implement in-place rehashing of HashTable.

The algorithm puts elements into correct positions in  multiple iterations.
On the first iteration it tries to put elements at entries specified by
their first hash probe. On the second iteration -- by the second
hash probe, and so on. Overall it does O(k*n) memory accesses, where
k is the maximum number of probes required for an element and n is the
capacity of the hash table. The expectation is that k will be small.

R=mstarzinger@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23658031

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16678 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

diff --git a/src/objects.cc b/src/objects.cc
index 43e3f53..362b363 100644 (file)
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -13806,6 +13806,74 @@ MaybeObject* HashTable<Shape, Key>::Rehash(HashTable* new_table, Key key) {
 
 
 template<typename Shape, typename Key>
+uint32_t HashTable<Shape, Key>::EntryForProbe(Key key,
+                                              Object* k,
+                                              int probe,
+                                              uint32_t expected) {
+  uint32_t hash = HashTable<Shape, Key>::HashForObject(key, k);
+  uint32_t capacity = Capacity();
+  uint32_t entry = FirstProbe(hash, capacity);
+  for (int i = 1; i < probe; i++) {
+    if (entry == expected) return expected;
+    entry = NextProbe(entry, i, capacity);
+  }
+  return entry;
+}
+
+
+template<typename Shape, typename Key>
+void HashTable<Shape, Key>::Swap(uint32_t entry1,
+                                 uint32_t entry2,
+                                 WriteBarrierMode mode) {
+  int index1 = EntryToIndex(entry1);
+  int index2 = EntryToIndex(entry2);
+  Object* temp[Shape::kEntrySize];
+  for (int j = 0; j < Shape::kEntrySize; j++) {
+    temp[j] = get(index1 + j);
+  }
+  for (int j = 0; j < Shape::kEntrySize; j++) {
+    set(index1 + j, get(index2 + j), mode);
+  }
+  for (int j = 0; j < Shape::kEntrySize; j++) {
+    set(index2 + j, temp[j], mode);
+  }
+}
+
+
+template<typename Shape, typename Key>
+void HashTable<Shape, Key>::Rehash(Key key) {
+  DisallowHeapAllocation no_gc;
+  WriteBarrierMode mode = GetWriteBarrierMode(no_gc);
+  uint32_t capacity = Capacity();
+  bool done = false;
+  for (int probe = 1; !done; probe++) {
+    // All elements at entries given by one of the first _probe_ probes
+    // are placed correctly. Other elements might need to be moved.
+    done = true;
+    for (uint32_t current = 0; current < capacity; current++) {
+      Object* current_key = get(EntryToIndex(current));
+      if (IsKey(current_key)) {
+        uint32_t target = EntryForProbe(key, current_key, probe, current);
+        if (current == target) continue;
+        Object* target_key = get(EntryToIndex(target));
+        if (!IsKey(target_key) ||
+            EntryForProbe(key, target_key, probe, target) != target) {
+          // Put the current element into the correct position.
+          Swap(current, target, mode);
+          // The other element will be processed on the next iteration.
+          current--;
+        } else {
+          // The place for the current element is occupied. Leave the element
+          // for the next probe.
+          done = false;
+        }
+      }
+    }
+  }
+}
+
+
+template<typename Shape, typename Key>
 MaybeObject* HashTable<Shape, Key>::EnsureCapacity(int n, Key key) {
   int capacity = Capacity();
   int nof = NumberOfElements() + n;

diff --git a/src/objects.h b/src/objects.h
index af0a891..fff2717 100644 (file)
--- a/src/objects.h
+++ b/src/objects.h
@@ -3473,6 +3473,9 @@ class HashTable: public FixedArray {
   inline int FindEntry(Key key);
   int FindEntry(Isolate* isolate, Key key);
 
+  // Rehashes the table in-place.
+  void Rehash(Key key);
+
  protected:
   // Find the entry at which to insert element with the given key that
   // has the given hash value.
@@ -3519,6 +3522,13 @@ class HashTable: public FixedArray {
     return (last + number) & (size - 1);
   }
 
+  // Returns _expected_ if one of entries given by the first _probe_ probes is
+  // equal to  _expected_. Otherwise, returns the entry given by the probe
+  // number _probe_.
+  uint32_t EntryForProbe(Key key, Object* k, int probe, uint32_t expected);
+
+  void Swap(uint32_t entry1, uint32_t entry2, WriteBarrierMode mode);
+
   // Rehashes this hash-table into the new table.
   MUST_USE_RESULT MaybeObject* Rehash(HashTable* new_table, Key key);
 

diff --git a/test/cctest/test-dictionary.cc b/test/cctest/test-dictionary.cc
index 21c20bd..b9e8b1e 100644 (file)
--- a/test/cctest/test-dictionary.cc
+++ b/test/cctest/test-dictionary.cc
@@ -101,6 +101,57 @@ TEST(ObjectHashTable) {
 }
 
 
+class ObjectHashTableTest: public ObjectHashTable {
+ public:
+  void insert(int entry, int key, int value) {
+    set(EntryToIndex(entry), Smi::FromInt(key));
+    set(EntryToIndex(entry) + 1, Smi::FromInt(value));
+  }
+
+  int lookup(int key) {
+    return Smi::cast(Lookup(Smi::FromInt(key)))->value();
+  }
+
+  int capacity() {
+    return Capacity();
+  }
+};
+
+
+TEST(HashTableRehash) {
+  LocalContext context;
+  Isolate* isolate = Isolate::Current();
+  Factory* factory = isolate->factory();
+  v8::HandleScope scope(context->GetIsolate());
+  // Test almost filled table.
+  {
+    Handle<ObjectHashTable> table = factory->NewObjectHashTable(100);
+    ObjectHashTableTest* t = reinterpret_cast<ObjectHashTableTest*>(*table);
+    int capacity = t->capacity();
+    for (int i = 0; i < capacity - 1; i++) {
+      t->insert(i, i * i, i);
+    }
+    t->Rehash(Smi::FromInt(0));
+    for (int i = 0; i < capacity - 1; i++) {
+      CHECK_EQ(i, t->lookup(i * i));
+    }
+  }
+  // Test half-filled table.
+  {
+    Handle<ObjectHashTable> table = factory->NewObjectHashTable(100);
+    ObjectHashTableTest* t = reinterpret_cast<ObjectHashTableTest*>(*table);
+    int capacity = t->capacity();
+    for (int i = 0; i < capacity / 2; i++) {
+      t->insert(i, i * i, i);
+    }
+    t->Rehash(Smi::FromInt(0));
+    for (int i = 0; i < capacity / 2; i++) {
+      CHECK_EQ(i, t->lookup(i * i));
+    }
+  }
+}
+
+
 #ifdef DEBUG
 TEST(ObjectHashSetCausesGC) {
   i::FLAG_stress_compaction = false;