private static final sun.misc.Unsafe UNSAFE;
    private static final long HEAD_OFFSET;

    /**
     * A special instance of {@link RunnableExecutorPair} that is used as a sentinel value for the
     * bottom of the stack.
     */
    private static final RunnableExecutorPair NULL_PAIR = new RunnableExecutorPair(null, null);

    static {
      try {
        UNSAFE = getUnsafe();

    private static final sun.misc.Unsafe UNSAFE;
    private static final long HEAD_OFFSET;

    /**
     * A special instance of {@link RunnableExecutorPair} that is used as a sentinel value for the
     * bottom of the stack.
     */
    private static final RunnableExecutorPair NULL_PAIR = new RunnableExecutorPair(null, null);

    static {
      try {
        UNSAFE = getUnsafe();