Implement IdentityMap<V>, a robust, GC-safe object-identity HashMap.

src/heap/identity-map.cc

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/v8.h"
+
+#include "src/heap/heap.h"
+#include "src/heap/identity-map.h"
+
+namespace v8 {
+namespace internal {
+
+static const int kInitialIdentityMapSize = 32;

[Erik Corry, 2015/04/28 09:32:34]

I just reduced the similar constant for element dictionaries from 32 to 4 at no
cost in performance.  Let's not burn memory if we can avoid it.

[titzer, 2015/04/28 11:23:48]

Done.

+static const int kResizeFactor = 4;

[Erik Corry, 2015/04/28 09:32:34]

Add a comment that the correctness of the table depends on this being >= 4

Alternatively change from a consecutive-collisions based growing heuristic to a
more conventional fullness-based heuristic.  This would prevent the growth from
being dependent on a source of randomness (ASLR).

[titzer, 2015/04/28 11:23:48]

As written, it doesn't. It will just keep resizing and trying to insert until
the longest collision chain is less than the limit. It so happens that 4x
guarantees that it will never iterate more than once.

+
+IdentityMapBase::~IdentityMapBase() {
+  if (keys_) {
+    Heap::OptionalRelocationLock lock(heap_, concurrent_);
+    heap_->UnregisterStrongRoots(keys_);
+  }
+}
+
+
+IdentityMapBase::RawEntry IdentityMapBase::Lookup(Handle<Object> key) {
+  AllowHandleDereference for_lookup;
+  int index = LookupIndex(*key);
+  return index >= 0 ? &values_[index] : nullptr;
+}
+
+
+IdentityMapBase::RawEntry IdentityMapBase::Insert(Handle<Object> key) {
+  AllowHandleDereference for_lookup;
+  int index = InsertIndex(*key);
+  DCHECK_GE(index, 0);
+  return &values_[index];
+}
+
+
+int IdentityMapBase::Hash(Object* address) {
+  uintptr_t raw_address = reinterpret_cast<uintptr_t>(address);
+  CHECK_NE(0, raw_address);  // Cannot store Smi 0 as a key here, sorry.
+  // Xor some of the upper bits, since the lower 2 or 3 are usually aligned.
+  return static_cast<int>((raw_address >> 11) ^ raw_address);
+}
+
+
+int IdentityMapBase::LookupIndex(Object* address) {
+  int start = Hash(address) & mask_;
+  for (int index = start; index < size_; index++) {
+    if (keys_[index] == address) return index;  // Found.
+    if (keys_[index] == nullptr) return -1;     // Not found.
+  }
+  for (int index = 0; index < start; index++) {
+    if (keys_[index] == address) return index;  // Found.
+    if (keys_[index] == nullptr) return -1;     // Not found.
+  }
+  return -1;
+}
+
+
+int IdentityMapBase::InsertIndex(Object* address) {
+  while (true) {
+    int start = Hash(address) & mask_;
+    int limit = size_ / 2;
+    // Search up to {limit} entries.
+    for (int index = start; --limit > 0; index = (index + 1) & mask_) {
+      if (keys_[index] == address) return index;  // Found.
+      if (keys_[index] == nullptr) {              // Free entry.
+        keys_[index] = address;
+        return index;
+      }
+    }
+    Resize();  // Should only have to resize once, since we grow 4x.
+  }
+  UNREACHABLE();
+  return -1;
+}
+
+
+// Searches this map for the given key using the object's address
+// as the identity, returning:
+//    found => a pointer to the storage location for the value
+//    not found => a pointer to a new storage location for the value
+IdentityMapBase::RawEntry IdentityMapBase::GetEntry(Handle<Object> key) {
+  Heap::OptionalRelocationLock lock(heap_, concurrent_);
+  RawEntry result;
+  if (size_ == 0) {
+    // Allocate the initial storage for keys and values.
+    size_ = kInitialIdentityMapSize;
+    mask_ = kInitialIdentityMapSize - 1;
+    gc_counter_ = heap_->gc_count();
+
+    keys_ = zone_->NewArray<Object*>(size_);
+    memset(keys_, 0, sizeof(Object*) * size_);
+    values_ = zone_->NewArray<void*>(size_);
+    memset(values_, 0, sizeof(void*) * size_);
+
+    heap_->RegisterStrongRoots(keys_, keys_ + size_);
+    result = Insert(key);
+  } else {
+    // Perform an optimistic lookup.
+    result = Lookup(key);
+    if (result == nullptr) {
+      // Miss; rehash if there was a GC, then insert.
+      if (gc_counter_ != heap_->gc_count()) Rehash();
+      result = Insert(key);
+    }
+  }
+  return result;
+}
+
+
+// Searches this map for the given key using the object's address
+// as the identity, returning:
+//    found => a pointer to the storage location for the value
+//    not found => {nullptr}
+IdentityMapBase::RawEntry IdentityMapBase::FindEntry(Handle<Object> key) {
+  if (size_ == 0) return nullptr;
+
+  Heap::OptionalRelocationLock lock(heap_, concurrent_);
+  RawEntry result = Lookup(key);
+  if (result == nullptr && gc_counter_ != heap_->gc_count()) {
+    Rehash();  // Rehash is expensive, so only do it in case of a miss.
+    result = Lookup(key);
+  }
+  return result;
+}
+
+
+void IdentityMapBase::Rehash() {
+  // Record the current GC counter.
+  gc_counter_ = heap_->gc_count();
+  // Assume that most objects won't be moved.
+  ZoneVector<std::pair<Object*, void*>> reinsert(zone_);
+  // Search the table looking for keys that wouldn't be found with their
+  // current hashcode and evacuate them.
+  int last_empty = -1;
+  for (int i = 0; i < size_; i++) {
+    if (keys_[i] == nullptr) {
+      last_empty = i;
+    } else {
+      int pos = Hash(keys_[i]) & mask_;
+      if (pos <= last_empty || pos > i) {
+        // Evacuate an entry that is in the wrong place.
+        reinsert.push_back(std::pair<Object*, void*>(keys_[i], values_[i]));
+        keys_[i] = nullptr;
+        values_[i] = nullptr;
+        last_empty = i;
+      }
+    }
+  }
+  // Reinsert all the key/value pairs that were in the wrong place.
+  for (auto pair : reinsert) {
+    int index = InsertIndex(pair.first);
+    DCHECK_GE(index, 0);
+    DCHECK_NULL(values_[index]);
+    values_[index] = pair.second;
+  }
+}
+
+
+void IdentityMapBase::Resize() {
+  // Grow the internal storage and reinsert all the key/value pairs.
+  int old_size = size_;
+  Object** old_keys = keys_;
+  void** old_values = values_;
+
+  size_ = size_ * kResizeFactor;
+  mask_ = size_ - 1;
+  gc_counter_ = heap_->gc_count();
+
+  CHECK_LE(size_, (1024 * 1024 * 16));  // that would be extreme...

[Erik Corry, 2015/04/28 09:32:34]

You really want to deliberately crash the release mode VM for this?
Also capital 't'.

[titzer, 2015/04/28 11:23:48]

Good question...then again, having 16 million entries in such a hashmap is going
to be a performance problem and might indicate a bug elsewhere.

+
+  keys_ = zone_->NewArray<Object*>(size_);
+  memset(keys_, 0, sizeof(Object*) * size_);
+  values_ = zone_->NewArray<void*>(size_);
+  memset(values_, 0, sizeof(void*) * size_);
+
+  for (int i = 0; i < old_size; i++) {
+    if (old_keys[i] == nullptr) continue;
+    int index = InsertIndex(old_keys[i]);
+    DCHECK_GE(index, 0);
+    values_[index] = old_values[i];
+  }
+
+  // Unregister old keys and register new keys.
+  heap_->UnregisterStrongRoots(old_keys);
+  heap_->RegisterStrongRoots(keys_, keys_ + size_);

[Erik Corry, 2015/04/28 09:32:34]

Where do we free the keys and values?

[titzer, 2015/04/28 11:23:48]

They're zone-allocated.

+}
+}
+}  // namespace v8::internal