Rename Common::FifoQueue to Common::SPSCQueue

Since all queues are FIFO data structures, the name wasn't informative
as to why you'd use it over a normal queue. I originally thought it had
something to do with the hardware graphics FIFO.

This renames it using the common acronym SPSC, which stands for
single-producer single-consumer, and is most commonly used to talk about
lock-free data structures, both of which this is.

Source/UnitTests/Common/SPSCQueueTest.cpp

@@ -0,0 +1,71 @@
+// Copyright 2014 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include <gtest/gtest.h>
+#include <thread>
+
+#include "Common/SPSCQueue.h"
+
+TEST(SPSCQueue, Simple)
+{
+  Common::SPSCQueue<u32> q;
+
+  EXPECT_EQ(0u, q.Size());
+  EXPECT_TRUE(q.Empty());
+
+  q.Push(1);
+  EXPECT_EQ(1u, q.Size());
+  EXPECT_FALSE(q.Empty());
+
+  u32 v;
+  q.Pop(v);
+  EXPECT_EQ(1u, v);
+  EXPECT_EQ(0u, q.Size());
+  EXPECT_TRUE(q.Empty());
+
+  // Test the FIFO order.
+  for (u32 i = 0; i < 1000; ++i)
+    q.Push(i);
+  EXPECT_EQ(1000u, q.Size());
+  for (u32 i = 0; i < 1000; ++i)
+  {
+    u32 v2;
+    q.Pop(v2);
+    EXPECT_EQ(i, v2);
+  }
+  EXPECT_TRUE(q.Empty());
+
+  for (u32 i = 0; i < 1000; ++i)
+    q.Push(i);
+  EXPECT_FALSE(q.Empty());
+  q.Clear();
+  EXPECT_TRUE(q.Empty());
+}
+
+TEST(SPSCQueue, MultiThreaded)
+{
+  Common::SPSCQueue<u32> q;
+
+  auto inserter = [&q]() {
+    for (u32 i = 0; i < 100000; ++i)
+      q.Push(i);
+  };
+
+  auto popper = [&q]() {
+    for (u32 i = 0; i < 100000; ++i)
+    {
+      while (q.Empty())
+        ;
+      u32 v;
+      q.Pop(v);
+      EXPECT_EQ(i, v);
+    }
+  };
+
+  std::thread popper_thread(popper);
+  std::thread inserter_thread(inserter);
+
+  popper_thread.join();
+  inserter_thread.join();
+}