[platform] Implement a worker pool

src/libplatform/task-queue.h

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-#ifndef V8_HYDROGEN_MINUS_ZERO_H_
-#define V8_HYDROGEN_MINUS_ZERO_H_
+#ifndef V8_LIBPLATFORM_TASK_QUEUE_H_
+#define V8_LIBPLATFORM_TASK_QUEUE_H_
 
-#include "hydrogen.h"
+#include <queue>
+
+// TODO(jochen): We should have our own version of globals.h.
+#include "../globals.h"
+#include "../platform/mutex.h"
+#include "../platform/semaphore.h"
 
 namespace v8 {
+
+class Task;
+
 namespace internal {
 
+class TaskQueue {
+ public:
+  TaskQueue();
+  ~TaskQueue();
 
-class HComputeMinusZeroChecksPhase : public HPhase {
- public:
-  explicit HComputeMinusZeroChecksPhase(HGraph* graph)
-      : HPhase("H_Compute minus zero checks", graph),
-        visited_(graph->GetMaximumValueID(), zone()) { }
+  // Appends a task to the queue. The queue takes ownership of |task|.
+  void Append(Task* task);
 
-  void Run();
+  // Returns the next task to process. Blocks if no task is available. Returns
+  // NULL if the queue is terminated.
+  Task* GetNext();
+
+  // Terminate the queue.
+  void Terminate();
 
  private:
-  void PropagateMinusZeroChecks(HValue* value);
+  Mutex lock_;
+  Semaphore process_queue_semaphore_;
+  std::queue<Task*> task_queue_;
+  bool terminated_;
 
-  BitVector visited_;
-
-  DISALLOW_COPY_AND_ASSIGN(HComputeMinusZeroChecksPhase);
+  DISALLOW_COPY_AND_ASSIGN(TaskQueue);
 };
 
-
 } }  // namespace v8::internal
 
-#endif  // V8_HYDROGEN_MINUS_ZERO_H_
+
+#endif  // V8_LIBPLATFORM_TASK_QUEUE_H_