Revert of SkThreadPool ~~> SkTaskGroup

src/utils/SkTaskGroup.cpp

-#include "SkTaskGroup.h"
-
-#include "SkCondVar.h"
-#include "SkTDArray.h"
-#include "SkThread.h"
-#include "SkThreadUtils.h"
-
-#if defined(SK_BUILD_FOR_WIN32)
-    static inline int num_cores() {
-        SYSTEM_INFO sysinfo;
-        GetSystemInfo(&sysinfo);
-        return sysinfo.dwNumberOfProcessors;
-    }
-#else
-    #include <unistd.h>
-    static inline int num_cores() {
-        return (int) sysconf(_SC_NPROCESSORS_ONLN);
-    }
-#endif
-
-namespace {
-
-class ThreadPool : SkNoncopyable {
-public:
-    static void Add(SkRunnable* task, int32_t* pending) {
-        SkASSERT(gGlobal);  // If this fails, see SkTaskGroup::Enabler.
-        gGlobal->add(task, pending);
-    }
-
-    static void Wait(int32_t* pending) {
-        SkASSERT(gGlobal);  // If this fails, see SkTaskGroup::Enabler.
-        while (sk_acquire_load(pending) > 0) {  // Pairs with sk_atomic_dec here or in Loop.
-            // Lend a hand until our SkTaskGroup of interest is done.
-            Work work;
-            {
-                AutoLock lock(&gGlobal->fReady);
-                if (gGlobal->fWork.isEmpty()) {
-                    // Someone has picked up all the work (including ours).  How nice of them!
-                    // (They may still be working on it, so we can't assert *pending == 0 here.)
-                    continue;
-                }
-                gGlobal->fWork.pop(&work);
-            }
-            // This Work isn't necessarily part of our SkTaskGroup of interest, but that's fine.
-            // We threads gotta stick together.  We're always making forward progress.
-            work.task->run();
-            sk_atomic_dec(work.pending);  // Release pairs with the sk_acquire_load() just above.
-        }
-    }
-
-private:
-    struct AutoLock {
-        AutoLock(SkCondVar* c) : fC(c) { fC->lock(); }
-        ~AutoLock() { fC->unlock(); }
-    private:
-        SkCondVar* fC;
-    };
-
-    struct Work {
-        SkRunnable* task;  // A task to ->run(),
-        int32_t* pending;  // then sk_atomic_dec(pending) afterwards.
-    };
-
-    explicit ThreadPool(int threads) : fDraining(false) {
-        if (threads == 0) {
-            threads = num_cores();
-        }
-        for (int i = 0; i < threads; i++) {
-            fThreads.push(SkNEW_ARGS(SkThread, (&ThreadPool::Loop, this)));
-            fThreads.top()->start();
-        }
-    }
-
-    ~ThreadPool() {
-        SkASSERT(fWork.isEmpty());  // All SkTaskGroups should be destroyed by now.
-        {
-            AutoLock lock(&fReady);
-            fDraining = true;
-            fReady.broadcast();
-        }
-        for (int i = 0; i < fThreads.count(); i++) {
-            fThreads[i]->join();
-        }
-        SkASSERT(fWork.isEmpty());  // Can't hurt to double check.
-        fThreads.deleteAll();
-    }
-
-    void add(SkRunnable* task, int32_t* pending) {
-        Work work = { task, pending };
-        sk_atomic_inc(pending);  // No barrier needed.
-        {
-            AutoLock lock(&fReady);
-            fWork.push(work);
-            fReady.signal();
-        }
-    }
-
-    static void Loop(void* arg) {
-        ThreadPool* pool = (ThreadPool*)arg;
-        Work work;
-        while (true) {
-            {
-                AutoLock lock(&pool->fReady);
-                while (pool->fWork.isEmpty()) {
-                    if (pool->fDraining) {
-                        return;
-                    }
-                    pool->fReady.wait();
-                }
-                pool->fWork.pop(&work);
-            }
-            work.task->run();
-            sk_atomic_dec(work.pending);  // Release pairs with sk_acquire_load() in Wait().
-        }
-    }
-
-    SkTDArray<Work>      fWork;
-    SkTDArray<SkThread*> fThreads;
-    SkCondVar            fReady;
-    bool                 fDraining;
-
-    static ThreadPool* gGlobal;
-    friend struct SkTaskGroup::Enabler;
-};
-ThreadPool* ThreadPool::gGlobal = NULL;
-
-}  // namespace
-
-SkTaskGroup::Enabler::Enabler(int threads) {
-    SkASSERT(ThreadPool::gGlobal == NULL);
-    ThreadPool::gGlobal = SkNEW_ARGS(ThreadPool, (threads));
-}
-
-SkTaskGroup::Enabler::~Enabler() {
-    SkASSERT(ThreadPool::gGlobal != NULL);
-    SkDELETE(ThreadPool::gGlobal);
-}
-
-SkTaskGroup::SkTaskGroup() : fPending(0) {}
-
-void SkTaskGroup::add(SkRunnable* task) { ThreadPool::Add(task, &fPending); }
-void SkTaskGroup::wait()                { ThreadPool::Wait(&fPending); }
-