Modernize SkMutex and SkSemaphore.

include/private/SkSemaphore.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSemaphore_DEFINED
 #define SkSemaphore_DEFINED
 
+#include "../private/SkOnce.h"
 #include "SkTypes.h"
-#include "../private/SkAtomics.h"
-#include "../private/SkOncePtr.h"
+#include <atomic>
 
-struct SkBaseSemaphore {
+class SkBaseSemaphore {
+public:
+    constexpr SkBaseSemaphore(int count = 0)
+        : fCount(count), fOSSemaphore(nullptr) {}
 
-    // Increment the counter by 1.
-    // This is a specialization for supporting SkMutex.
-    void signal() {
-        // Since this fetches the value before the add, 0 indicates that this thread is running and
-        // no threads are waiting, -1 and below means that threads are waiting, but only signal 1
-        // thread to run.
-        if (sk_atomic_fetch_add(&fCount, 1, sk_memory_order_release) < 0) {
-           this->osSignal(1);
-        }
-    }
-
-    // Increment the counter N times.
-    // Generally it's better to call signal(N) instead of signal() N times.
-    void signal(int N);
+    // Increment the counter n times.
+    // Generally it's better to call signal(n) instead of signal() n times.
+    void signal(int n = 1);
 
     // Decrement the counter by 1,
     // then if the counter is <= 0, sleep this thread until the counter is > 0.
-    void wait() {
-        // Since this fetches the value before the subtract, zero and below means that there are no
-        // resources left, so the thread needs to wait.
-        if (sk_atomic_fetch_sub(&fCount, 1, sk_memory_order_acquire) <= 0) {
-            this->osWait();
-        }
-    }
+    void wait();
 
-    struct OSSemaphore;
+    // SkBaseSemaphore has no destructor.  Call this to clean it up.
+    void cleanup();
 
-    void osSignal(int n);
-    void osWait();
-    void deleteSemaphore();
-
+private:
     // This implementation follows the general strategy of
     //     'A Lightweight Semaphore with Partial Spinning'
     // found here
     //     http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
     // That article (and entire blog) are very much worth reading.
     //
     // We wrap an OS-provided semaphore with a user-space atomic counter that
     // lets us avoid interacting with the OS semaphore unless strictly required:
     // moving the count from >0 to <=0 or vice-versa, i.e. sleeping or waking threads.
-    int                        fCount;
-    SkBaseOncePtr<OSSemaphore> fOSSemaphore;
+    struct OSSemaphore;
+
+    void osSignal(int n);
+    void osWait();
+
+    std::atomic<int> fCount;
+    SkOnce           fOSSemaphoreOnce;
+    OSSemaphore*     fOSSemaphore;
 };
 
-/**
- * SkSemaphore is a fast mostly-user-space semaphore.
- *
- * A semaphore is logically an atomic integer with a few special properties:
- *   - The integer always starts at 0.
- *   - You can only increment or decrement it, never read or write it.
- *   - Increment is spelled 'signal()'; decrement is spelled 'wait()'.
- *   - If a call to wait() decrements the counter to <= 0,
- *     the calling thread sleeps until another thread signal()s it back above 0.
- */
-class SkSemaphore : SkNoncopyable {
+class SkSemaphore : public SkBaseSemaphore {
 public:
-    // Initializes the counter to 0.
-    // (Though all current implementations could start from an arbitrary value.)
-    SkSemaphore();
-    ~SkSemaphore();
-
-    void wait();
-
-    void signal(int n = 1);
-
-private:
-    SkBaseSemaphore fBaseSemaphore;
+    using SkBaseSemaphore::SkBaseSemaphore;
+    ~SkSemaphore() { this->cleanup(); }
 };
 
+inline void SkBaseSemaphore::signal(int n) {
+    int prev = fCount.fetch_add(n, std::memory_order_release);
+
+    // We only want to call the OS semaphore when our logical count crosses
+    // from <= 0 to >0 (when we need to wake sleeping threads).
+    //
+    // This is easiest to think about with specific examples of prev and n.
+    // If n == 5 and prev == -3, there are 3 threads sleeping and we signal
+    // SkTMin(-(-3), 5) == 3 times on the OS semaphore, leaving the count at 2.
+    //
+    // If prev >= 0, no threads are waiting, SkTMin(-prev, n) is always <= 0,
+    // so we don't call the OS semaphore, leaving the count at (prev + n).
+    int toSignal = SkTMin(-prev, n);
+    if (toSignal > 0) {
+        this->osSignal(toSignal);
+    }
+}
+
+inline void SkBaseSemaphore::wait() {
+    // Since this fetches the value before the subtract, zero and below means that there are no
+    // resources left, so the thread needs to wait.
+    if (fCount.fetch_sub(1, std::memory_order_acquire) <= 0) {
+        this->osWait();
+    }
+}
+
 #endif//SkSemaphore_DEFINED