Store HValue uses in a custom small list structure.

src/small-pointer-list.h

+// Copyright 2011 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef V8_SMALL_POINTER_LIST_H_
+#define V8_SMALL_POINTER_LIST_H_
+
+#include "checks.h"
+#include "v8globals.h"
+#include "zone.h"
+
+namespace v8 {
+namespace internal {
+
+// SmallPointerList is a list optimized for storing no or just a
+// single value. When more values are given it falls back to ZoneList.
+//
+// The interface tries to be as close to List from list.h as possible.
+template <typename T>
+class SmallPointerList {
+ public:
+  SmallPointerList() : data_(kEmptyTag) {}
+
+  bool is_empty() const { return length() == 0; }
+
+  int length() const {
+    if ((data_ & kTagMask) == kEmptyTag) return 0;
+    if ((data_ & kTagMask) == kSingletonTag) return 1;
+    return list()->length();
+  }
+
+  void Add(T* pointer) {
+    ASSERT(IsAligned(reinterpret_cast<intptr_t>(pointer), kPointerAlignment));
+    if ((data_ & kTagMask) == kEmptyTag) {
+      data_ = reinterpret_cast<intptr_t>(pointer) | kSingletonTag;
+      return;
+    }
+    if ((data_ & kTagMask) == kSingletonTag) {
+      PointerList* list = new PointerList(2);

[Kevin Millikin (Chromium), 2011/03/23 07:38:44]

We should count the lengths of use lists that are >= 2.  This backing store will
be resized as soon as we add a another use.  

If this is frequent, we should avoid the doubling and wasted 2-element list and
preallocate it a bit bigger.

[Vitaly Repeshko, 2011/03/23 14:42:32]

When compiling the V8 benchmark the number of values with 2 uses is
approximately equal to the number of values with strictly more than 2 uses. So I
think it makes sense to keep 2 here.

+      list->Add(single_value());
+      list->Add(pointer);
+      ASSERT(IsAligned(reinterpret_cast<intptr_t>(list), kPointerAlignment));
+      data_ = reinterpret_cast<intptr_t>(list) | kListTag;
+      return;
+    }
+    list()->Add(pointer);
+  }
+
+  // Note: returns T* and not T*& (unlike List from list.h).
+  // This makes the implementation simpler and more const correct.
+  T* at(int i) const {
+    ASSERT((data_ & kTagMask) != kEmptyTag);
+    if ((data_ & kTagMask) == kSingletonTag) {
+      ASSERT(i == 0);
+      return single_value();
+    }
+    return list()->at(i);
+  }
+
+  // See the note above.
+  T* operator[](int i) const { return at(i); }
+
+  void RemoveElement(T* pointer) {

[Kevin Millikin (Chromium), 2011/03/23 07:38:44]

Maybe a short comment that the intended semantics is to remove if present (like
List::RemoveElement).

[Vitaly Repeshko, 2011/03/23 14:42:32]

Done.

+    if ((data_ & kTagMask) == kEmptyTag) return;
+    if ((data_ & kTagMask) == kSingletonTag) {
+      if (pointer == single_value()) {
+        data_ = kEmptyTag;
+      }
+      return;
+    }
+    list()->RemoveElement(pointer);
+  }
+
+  T* RemoveLast() {
+    ASSERT((data_ & kTagMask) != kEmptyTag);
+    if ((data_ & kTagMask) == kSingletonTag) {
+      T* result = single_value();
+      data_ = kEmptyTag;
+      return result;
+    }
+    return list()->RemoveLast();
+  }
+
+  void Rewind(int pos) {
+    if ((data_ & kTagMask) == kEmptyTag) {
+      ASSERT(pos == 0);
+      return;
+    }
+    if ((data_ & kTagMask) == kSingletonTag) {
+      ASSERT(pos == 0 || pos == 1);
+      if (pos == 0) {
+        data_ = kEmptyTag;
+      }
+      return;
+    }
+    list()->Rewind(pos);
+  }
+
+  int CountOccurrences(T* pointer, int start, int end) const {
+    if ((data_ & kTagMask) == kEmptyTag) return 0;
+    if ((data_ & kTagMask) == kSingletonTag) {
+      if (start == 0 && end >= 0) {
+        return (single_value() == pointer) ? 1 : 0;
+      }
+      return 0;
+    }
+    return list()->CountOccurrences(pointer, start, end);
+  }
+
+ private:
+  typedef ZoneList<T*> PointerList;
+
+  static const intptr_t kEmptyTag = 1;
+  static const intptr_t kSingletonTag = 0;
+  static const intptr_t kListTag = 2;
+  static const intptr_t kTagMask = 3;
+  static const intptr_t kValueMask = ~kTagMask;
+
+  STATIC_ASSERT(kTagMask + 1 <= kPointerAlignment);
+
+  T* single_value() const {
+    ASSERT((data_ & kTagMask) == kSingletonTag);
+    STATIC_ASSERT(kSingletonTag == 0);
+    return reinterpret_cast<T*>(data_);
+  }
+
+  PointerList* list() const {
+    ASSERT((data_ & kTagMask) == kListTag);
+    return reinterpret_cast<PointerList*>(data_ & kValueMask);
+  }
+
+  intptr_t data_;
+
+  DISALLOW_COPY_AND_ASSIGN(SmallPointerList);
+};
+
+} }  // namespace v8::internal
+
+#endif  // V8_SMALL_POINTER_LIST_H_