Add base::StaticAtomicSequenceNumber.

base/atomic_sequence_num.h

Index: base/atomic_sequence_num.h
diff --git a/base/atomic_sequence_num.h b/base/atomic_sequence_num.h
index 11805a0ae501b37207342dd33fae76604569baae..e813c9bac8bcdfb59a2cd43ccf4fe9e31b090ef3 100644
--- a/base/atomic_sequence_num.h
+++ b/base/atomic_sequence_num.h
@@ -11,18 +11,48 @@
 
 namespace base {
 
-class AtomicSequenceNumber {
+class AtomicSequenceNumber;
+
+// Static (POD) AtomicSequenceNumber that MUST be used in global scope (or
+// non-function scope) ONLY. This implementation does not generate any static
+// initializer.  Note that it does not implement any constructor which means
+// that its fields are not initialized except when it is stored in the global
+// data section (.data in ELF). If you want to allocate an atomic sequence
+// number on the stack (or heap), please use the AtomicSequenceNumber class
+// declared below.
+class StaticAtomicSequenceNumber {
  public:
-  AtomicSequenceNumber() : seq_(0) { }
-  explicit AtomicSequenceNumber(base::LinkerInitialized x) { /* seq_ is 0 */ }
-
-  int GetNext() {
+  inline int GetNext() {
     return static_cast<int>(
         base::subtle::NoBarrier_AtomicIncrement(&seq_, 1) - 1);
   }
 
  private:
+  friend class AtomicSequenceNumber;
+
+  inline void Reset() {
+    base::subtle::Release_Store(&seq_, 0);
+  }
+
   base::subtle::Atomic32 seq_;
+};
+
+// AtomicSequenceNumber that can be stored and used safely (i.e. its fields are
+// always initialized as opposed to StaticAtomicSequenceNumber declared above).
+// Please use StaticAtomicSequenceNumber if you want to declare an atomic
+// sequence number in the global scope.
+class AtomicSequenceNumber {
+ public:
+  AtomicSequenceNumber() {
+    seq_.Reset();
+  }
+
+  inline int GetNext() {
+    return seq_.GetNext();
+  }
+
+ private:
+  StaticAtomicSequenceNumber seq_;
   DISALLOW_COPY_AND_ASSIGN(AtomicSequenceNumber);
 };