[base] Add Decrement and assignment operators to AtomicNumber

AtomicNumber should make dealing with atomic counters easier. This is not the
case with size_t, as we cannot properly use the Increment() method for negative
numbers.

With this CL we can use AtomicNumber<size_t> and have proper decrements.

R=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2215693002
Cr-Commit-Position: refs/heads/master@{#38407}

src/base/atomic-utils.h

@@ -19,12 +19,18 @@ class AtomicNumber {
   AtomicNumber() : value_(0) {}
   explicit AtomicNumber(T initial) : value_(initial) {}
 
-  // Returns the newly set value.
+  // Returns the value after incrementing.
   V8_INLINE T Increment(T increment) {
     return static_cast<T>(base::Barrier_AtomicIncrement(
         &value_, static_cast<base::AtomicWord>(increment)));
   }
 
+  // Returns the value after decrementing.
+  V8_INLINE T Decrement(T decrement) {
+    return static_cast<T>(base::Barrier_AtomicIncrement(
+        &value_, -static_cast<base::AtomicWord>(decrement)));
+  }
+
   V8_INLINE T Value() { return static_cast<T>(base::Acquire_Load(&value_)); }
 
   V8_INLINE void SetValue(T new_value) {
@@ -36,6 +42,9 @@ class AtomicNumber {
     return value;
   }
 
+  V8_INLINE T operator+=(T value) { return Increment(value); }
+  V8_INLINE T operator-=(T value) { return Decrement(value); }
+
  private:
   STATIC_ASSERT(sizeof(T) <= sizeof(base::AtomicWord));
 

test/unittests/base/atomic-utils-unittest.cc

@@ -58,6 +58,29 @@ TEST(AtomicNumber, Increment) {
   EXPECT_EQ(std::numeric_limits<size_t>::max(), c.Value());
 }
 
+TEST(AtomicNumber, Decrement) {
+  AtomicNumber<size_t> a(std::numeric_limits<size_t>::max());
+  a.Increment(1);
+  EXPECT_EQ(0, a.Value());
+  a.Decrement(1);
+  EXPECT_EQ(std::numeric_limits<size_t>::max(), a.Value());
+}
+
+TEST(AtomicNumber, OperatorAdditionAssignment) {
+  AtomicNumber<size_t> a(0u);
+  AtomicNumber<size_t> b(std::numeric_limits<size_t>::max());
+  a += b.Value();
+  EXPECT_EQ(a.Value(), b.Value());
+  EXPECT_EQ(b.Value(), std::numeric_limits<size_t>::max());
+}
+
+TEST(AtomicNumber, OperatorSubtractionAssignment) {
+  AtomicNumber<size_t> a(std::numeric_limits<size_t>::max());
+  AtomicNumber<size_t> b(std::numeric_limits<size_t>::max());
+  a -= b.Value();
+  EXPECT_EQ(a.Value(), 0u);
+  EXPECT_EQ(b.Value(), std::numeric_limits<size_t>::max());
+}
 
 namespace {