别测空循环

今天有朋友提到一个叫 ReflectASM的库，为Java环境提供高性能的反射操作支持。它的实现方式是动态代码生成。
以前我的一篇日志里写过，Oracle/Sun JDK6的反射方法调用的实现当中重要的一环就是动态代码生成。

但是今天的主角不是ReflectASM，而是Oracle/Sun JDK里的sun.misc.Unsafe类，以及这个类上的getInt(Object, long)方法：

引用

/**
 * Fetches a value from a given Java variable.
 * More specifically, fetches a field or array element within the given
 * object <code>o</code> at the given offset, or (if <code>o</code> is
 * null) from the memory address whose numerical value is the given
 * offset.
 * <p>
 * The results are undefined unless one of the following cases is true:
 * <ul>
 * <li>The offset was obtained from {@link #objectFieldOffset} on
 * the {@link java.lang.reflect.Field} of some Java field and the object
 * referred to by <code>o</code> is of a class compatible with that
 * field's class.
 *
 * <li>The offset and object reference <code>o</code> (either null or
 * non-null) were both obtained via {@link #staticFieldOffset}
 * and {@link #staticFieldBase} (respectively) from the
 * reflective {@link Field} representation of some Java field.
 *
 * <li>The object referred to by <code>o</code> is an array, and the offset
 * is an integer of the form <code>B+N*S</code>, where <code>N</code> is
 * a valid index into the array, and <code>B</code> and <code>S</code> are
 * the values obtained by {@link #arrayBaseOffset} and {@link
 * #arrayIndexScale} (respectively) from the array's class.  The value
 * referred to is the <code>N</code><em>th</em> element of the array.
 *
 * </ul>
 * <p>
 * If one of the above cases is true, the call references a specific Java
 * variable (field or array element).  However, the results are undefined
 * if that variable is not in fact of the type returned by this method.
 * <p>
 * This method refers to a variable by means of two parameters, and so
 * it provides (in effect) a <em>double-register</em> addressing mode
 * for Java variables.  When the object reference is null, this method
 * uses its offset as an absolute address.  This is similar in operation
 * to methods such as {@link #getInt(long)}, which provide (in effect) a
 * <em>single-register</em> addressing mode for non-Java variables.
 * However, because Java variables may have a different layout in memory
 * from non-Java variables, programmers should not assume that these
 * two addressing modes are ever equivalent.  Also, programmers should
 * remember that offsets from the double-register addressing mode cannot
 * be portably confused with longs used in the single-register addressing
 * mode.
 *
 * @param o Java heap object in which the variable resides, if any, else
 *        null
 * @param offset indication of where the variable resides in a Java heap
 *        object, if any, else a memory address locating the variable
 *        statically
 * @return the value fetched from the indicated Java variable
 * @throws RuntimeException No defined exceptions are thrown, not even
 *         {@link NullPointerException}
 */
public native int getInt(Object o, long offset);

（注意：sun.misc.Unsafe是Oracle/Sun JDK里的私有实现，不是Java标准库的共有API，请权衡好利害关系后再使用。）

朋友利用Unsafe.getInt()来实现了快速的反射访问字段的功能，写了这么篇日志：
Java 反射调用的一种优化
在文章末尾，他提到直接用Unsafe.getInt()与用普通反射访问字段的性能对比，

aliveTimeID 写道

通过测试发现，效率是普通java.lang.reflect.Field.get(Object)的3倍，当然，性能这个东西，还是自己测试了放心。

于是问题就来了：在我的机器上也是快三倍么？为什么“快了三倍”？是什么跟什么比较快了三倍？

把原本的测试代码拿来：（有细微改动）

import java.io.Serializable;
import java.lang.reflect.Field;
import sun.misc.Unsafe;
/**
 * @author haitao.yao Dec 14, 2010
 */
public class ReflectionCompare {
  private static final int count = 10000000;
  /**
   * @param args
   */
  public static void main(String[] args) {
    long duration = testIntCommon();
    System.out.println("int common test for  " + count
        + " times, duration: " + duration);
    duration = testUnsafe();
    System.out.println("int unsafe test for  " + count
        + " times, duration: " + duration);
  }
  private static long testUnsafe() {
    long start = System.currentTimeMillis();
    sun.misc.Unsafe unsafe = getUnsafe();
    int temp = count;
    Field field = getIntField();
    long offset = unsafe.objectFieldOffset(field);
    while (temp-- > 0) {
      unsafe.getInt(new TestBean(), offset);
    }
    return System.currentTimeMillis() - start;
  }
  private static long testIntCommon() {
    long start = System.currentTimeMillis();
    int temp = count;
    getIntField().setAccessible(true);
    try {
      while (temp-- > 0) {
        TestBean bean = new TestBean();
        getIntField().get(bean);
      }
      return System.currentTimeMillis() - start;
    } catch (Exception e) {
      e.printStackTrace();
    }
    return -1;
  }
  private static final sun.misc.Unsafe unsafe;
  static {
    sun.misc.Unsafe value = null;
    try {
      Class<?> clazz = Class.forName("sun.misc.Unsafe");
      Field field = clazz.getDeclaredField("theUnsafe");
      field.setAccessible(true);
      value = (Unsafe) field.get(null);
    } catch (Exception e) {
      e.printStackTrace();
      throw new RuntimeException("error to get theUnsafe", e);
    }
    unsafe = value;
  }
  public static final sun.misc.Unsafe getUnsafe() {
    return unsafe;
  }
  private static final Field intField;
  private static final Field stringField;
  static {
    try {
      intField = TestBean.class.getDeclaredField("age");
      stringField = TestBean.class.getDeclaredField("name");
    } catch (Exception e) {
      e.printStackTrace();
      throw new IllegalStateException("failed to init testbean field", e);
    }
  }
  public static final Field getIntField() {
    return intField;
  }
  public static final Field getStringField() {
    return stringField;
  }
  
  /**
   * @author haitao.yao
   * Dec 14, 2010
   */
  static class TestBean implements Serializable{
    /**
     * 
     */
    private static final long serialVersionUID = -5994966479456252766L;
    
    private String name;
    private int age;
    /**
     * @return the name
     */
    public String getName() {
      return name;
    }
    /**
     * @param name the name to set
     */
    public void setName(String name) {
      this.name = name;
    }
    /**
     * @return the age
     */
    public int getAge() {
      return age;
    }
    /**
     * @param age the age to set
     */
    public void setAge(int age) {
      this.age = age;
    }
  }
}

在我的测试环境里跑：

$ java -version
java version "1.6.0_25"
Java(TM) SE Runtime Environment (build 1.6.0_25-b06)
Java HotSpot(TM) 64-Bit Server VM (build 20.0-b11, mixed mode)
$ java -cp . ReflectionCompare
int common test for  10000000 times, duration: 616
int unsafe test for  10000000 times, duration: 10
$ java -cp . ReflectionCompare
int common test for  10000000 times, duration: 425
int unsafe test for  10000000 times, duration: 274
$ java -cp . ReflectionCompare
int common test for  10000000 times, duration: 502
int unsafe test for  10000000 times, duration: 10
$ java -cp . ReflectionCompare
int common test for  10000000 times, duration: 659
int unsafe test for  10000000 times, duration: 9
$ java -cp . ReflectionCompare
int common test for  10000000 times, duration: 646
int unsafe test for  10000000 times, duration: 10

这时间看起来不像是“三倍”，而更像是哪里出岔子了

那么在揭开谜底前，我们来从侧面看看能不能有啥发现。

我们已经知道上面对比的双方是Unsafe.getInt()和Field.get()。Unsafe.getInt()是个native方法，就算是到头了。
实际上Unsafe.getInt()方法在HotSpot VM里是个intrinsic方法，有特别的优化处理。欲知详情可以参考HotSpot server compiler的LibraryCallKit::inline_unsafe_access()函数，它可以将原本的Unsafe.getInt()方法调用内联到调用点上，彻底削除JNI函数调用开销。
但想必这点优化没啥影响，大家是公平的？后面再继续说。

在Oracle/Sun JDK6里，Field.get()又是怎样实现的呢？
跟随Field、ReflectionFactory、UnsafeFieldAccessorFactory、UnsafeFieldAccessorImpl、UnsafeIntegerFieldAccessorImpl这些类的相关方法的实现，可以发现当我们要通过反射获取一个int类型的字段时，最终在UnsafeIntegerFieldAccessorImpl也是通过Unsafe.getInt()来干活的：

public int getInt(Object obj) throws IllegalArgumentException {
    ensureObj(obj);
    return unsafe.getInt(obj, fieldOffset);
}

也就是说，Oracle/Sun JDK6虽然用动态生成代码的方式来实现反射调用方法，但在反射访问字段上却选择了不动态生成代码而直接使用Unsafe这样的native后门。

那么，自己直接使用Unsafe.getInt()比Field.get()占优的地方就在于前者很直接而后者经过了若干层间接/包装，所以前者比后者快。这足以上面看到的测试时间的差异么？

现在让我们来看看事情的真相。
换用Oracle JDK 6 update 25 build 03 fastdebug版，通过-XX:+PrintOptoAssembly来观察JIT编译器生成出来的代码的样子：

$ ~/jdk/6u25b03_x64_debug/fastdebug/bin/java -cp . -XX:+PrintOptoAssembly ReflectionCompare

可以看到测试里的ReflectionCompare.testUnsafe()方法中的主要计时循环生成为这样的代码：

060   B6: #     B6 B7 <- B5 B6  Loop: B6-B6 inner stride: not constant  Freq: 999998
060   
060   
060     decl    RBP     # int
062     cmpl    RBP, #-1
065     jg,s   B6       # loop end  P=1.000000 C=12203.000000

换回等价的Java代码来表达，也就是：

while (temp-- > 0) {
}

但原本测试代码里写的是：

while (temp-- > 0) {
  unsafe.getInt(new TestBean(), offset);
}

而这正是本篇日志的标题所说的：别测空循环。
连被测的对象都被优化掉了的话，这种microbenchmark就彻底没意义了。
前面已经提到，Unsafe.getInt()是HotSpot VM的一个intrinsic方法，因而它的所有特性都是VM清楚了解的，包括有没有副作用、是否能安全的内联、内联后是否可以进一步优化直至完全削除，等。
所以说不要小看了某个native方法的优化对microbenchmark的影响。

用同样办法观察原本测试里的ReflectionCompare.testIntCommon()方法，能看到整个调用过程，包括unsafe.getInt()被内联后的逻辑，都还完好的存在于JIT编译生成的代码中。
在这种条件下比较两边的性能，自然是毫无参考价值可言。

当然，我们还是能得到一些收获。
这个测试里，两边最终干活的都是Unsafe.getInt()，但直接调就优化掉了，而通过Field.get()来调就没被优化掉。
这正好体现了通用型API的多层繁复的封装带来的问题——过多的封装引来了过多间接层，间接层多了之后编译器要优化就会很吃力。
想像一下，如果有人发现直接调用Unsafe.getInt()方法字段的速度很快，然后好心的想把它封装成通用的反射库提供给大家用，很可能发生的状况就是他又把间接层堆积了起来，结果最终用户用的时候就跟直接用反射差不多。

话说回来，一般程序员写Java程序的时候也无需为这种问题纠结就是了。这篇故事看了笑笑就好

评论

1 楼 lydawen 2011-11-02  

这么个‘小问题’就研究这么深入，真是有心了。如果循环中新的实例需要在循环个引用，如加入到数组，结果应该比较真实