3 files changed, 42 insertions, 18 deletions

diff --git a/absl/synchronization/blocking_counter.cc b/absl/synchronization/blocking_counter.cc
index 3cea7aed..d2f82da3 100644
--- a/absl/synchronization/blocking_counter.cc
+++ b/absl/synchronization/blocking_counter.cc
@@ -14,41 +14,51 @@
 
 #include "absl/synchronization/blocking_counter.h"
 
+#include <atomic>
+
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
-// Return whether int *arg is zero.
-static bool IsZero(void *arg) {
-  return 0 == *reinterpret_cast<int *>(arg);
+namespace {
+
+// Return whether int *arg is true.
+bool IsDone(void *arg) { return *reinterpret_cast<bool *>(arg); }
+
+}  // namespace
+
+BlockingCounter::BlockingCounter(int initial_count)
+    : count_(initial_count),
+      num_waiting_(0),
+      done_{initial_count == 0 ? true : false} {
+  ABSL_RAW_CHECK(initial_count >= 0, "BlockingCounter initial_count negative");
 }
 
 bool BlockingCounter::DecrementCount() {
-  MutexLock l(&lock_);
-  count_--;
-  if (count_ < 0) {
-    ABSL_RAW_LOG(
-        FATAL,
-        "BlockingCounter::DecrementCount() called too many times.  count=%d",
-        count_);
+  int count = count_.fetch_sub(1, std::memory_order_acq_rel) - 1;
+  ABSL_RAW_CHECK(count >= 0,
+                 "BlockingCounter::DecrementCount() called too many times");
+  if (count == 0) {
+    MutexLock l(&lock_);
+    done_ = true;
+    return true;
   }
-  return count_ == 0;
+  return false;
 }
 
 void BlockingCounter::Wait() {
   MutexLock l(&this->lock_);
-  ABSL_RAW_CHECK(count_ >= 0, "BlockingCounter underflow");
 
   // only one thread may call Wait(). To support more than one thread,
   // implement a counter num_to_exit, like in the Barrier class.
   ABSL_RAW_CHECK(num_waiting_ == 0, "multiple threads called Wait()");
   num_waiting_++;
 
-  this->lock_.Await(Condition(IsZero, &this->count_));
+  this->lock_.Await(Condition(IsDone, &this->done_));
 
-  // At this point, We know that all threads executing DecrementCount have
-  // released the lock, and so will not touch this object again.
+  // At this point, we know that all threads executing DecrementCount
+  // will not touch this object again.
   // Therefore, the thread calling this method is free to delete the object
   // after we return from this method.
 }
diff --git a/absl/synchronization/blocking_counter.h b/absl/synchronization/blocking_counter.h
index 1f53f9f2..1908fdb1 100644
--- a/absl/synchronization/blocking_counter.h
+++ b/absl/synchronization/blocking_counter.h
@@ -20,6 +20,8 @@
 #ifndef ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
 #define ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
 
+#include <atomic>
+
 #include "absl/base/thread_annotations.h"
 #include "absl/synchronization/mutex.h"
 
@@ -60,8 +62,7 @@ ABSL_NAMESPACE_BEGIN
 //
 class BlockingCounter {
  public:
-  explicit BlockingCounter(int initial_count)
-      : count_(initial_count), num_waiting_(0) {}
+  explicit BlockingCounter(int initial_count);
 
   BlockingCounter(const BlockingCounter&) = delete;
   BlockingCounter& operator=(const BlockingCounter&) = delete;
@@ -89,8 +90,9 @@ class BlockingCounter {
 
  private:
   Mutex lock_;
-  int count_ ABSL_GUARDED_BY(lock_);
+  std::atomic<int> count_;
   int num_waiting_ ABSL_GUARDED_BY(lock_);
+  bool done_ ABSL_GUARDED_BY(lock_);
 };
 
 ABSL_NAMESPACE_END
diff --git a/absl/synchronization/blocking_counter_test.cc b/absl/synchronization/blocking_counter_test.cc
index 2926224a..06885f57 100644
--- a/absl/synchronization/blocking_counter_test.cc
+++ b/absl/synchronization/blocking_counter_test.cc
@@ -63,6 +63,18 @@ TEST(BlockingCounterTest, BasicFunctionality) {
   }
 }
 
+TEST(BlockingCounterTest, WaitZeroInitialCount) {
+  BlockingCounter counter(0);
+  counter.Wait();
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(BlockingCounterTest, WaitNegativeInitialCount) {
+  EXPECT_DEATH(BlockingCounter counter(-1),
+               "BlockingCounter initial_count negative");
+}
+#endif
+
 }  // namespace
 ABSL_NAMESPACE_END
 }  // namespace absl