Prioritizing input and compositor tasks in the blink scheduler.

Source/platform/scheduler/AtomicRingbuffer.h

Index: Source/platform/scheduler/AtomicRingbuffer.h
diff --git a/Source/platform/scheduler/AtomicRingbuffer.h b/Source/platform/scheduler/AtomicRingbuffer.h
new file mode 100644
index 0000000000000000000000000000000000000000..e0c7e3d2d89677dd4e4613e09f48e531010cd73a
--- /dev/null
+++ b/Source/platform/scheduler/AtomicRingbuffer.h
@@ -0,0 +1,50 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef AtomicRingbuffer_h
+#define AtomicRingbuffer_h
+
+#include "wtf/Atomics.h"
+#include "wtf/Noncopyable.h"
+#include "wtf/Vector.h"
+
+namespace blink {
+
+template <typename T, int size> class AtomicRingBuffer {
+    WTF_MAKE_NONCOPYABLE(AtomicRingBuffer);
+public:
+    AtomicRingBuffer() : m_writeCounter(0), m_readIndex(1)

[Sami, 2014/08/06 10:19:52]

nit: Blink has a different style for initializers, see
http://www.chromium.org/blink/coding-style

[alexclarke, 2014/08/07 12:08:06]

Acknowledged.

+    {
+        m_values.resize(size);
+        for (int i = 0; i < size; i++)
+            m_values[i] = nullptr;
+    }
+
+    // May be called from any thread
+    void pushMaySpinlock(T* value)
+    {
+        int pos = atomicAdd(&m_writeCounter, 1) % size;
+        while (!atomicTestAndSwap((void* volatile*) &m_values[pos], 0, value)) {

[Sami, 2014/08/06 10:19:52]

nit: nullptr instead of 0?

[alexclarke, 2014/08/07 12:08:06]

Acknowledged.

+            // spin
+        }
+    }
+
+    // Returns null if there's nothing queued up. May only be called from the main thread.
+    T* pop()
+    {
+        T* value = (T*) atomicExchange((void* volatile*) &m_values[m_readIndex], nullptr);

[Sami, 2014/08/06 10:19:52]

Please use C++-style casts instead of C ones. Is the "*" after the void
redundant?

[alexclarke, 2014/08/07 12:08:06]

Acknowledged.

+        if (value)
+            m_readIndex = (m_readIndex + 1) % size;
+        return value;
+    }
+
+private:
+    volatile int m_writeCounter;

[Sami, 2014/08/06 10:19:52]

nit: m_writeIndex to match the other var?

[alexclarke, 2014/08/07 12:08:06]

Acknowledged.

+    int m_readIndex;
+    Vector<T*> m_values;
+};
+
+} // namespace blink
+
+#endif // AtomicRingbuffer_h