UseCompressedOops UseCompressedClassPointers in jdk-13 and jdk-15

问题

Accidentally I have stumbled into a change in jdk-15 that I was not aware of. Suppose I have a very simple question: what is the size of an array of 3 intergers? For this, I use JOL. The code is fairly trivial:

import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.vm.VM;

public class Array {
    public static void main(String [] args){
       int [] array = new int[3];
       System.out.println(ClassLayout.parseInstance(array).toPrintable());
    }
}

I run this with jdk-13:

  java -Djdk.attach.allowAttachSelf -Djol.tryWithSudo=true -cp jol-cli.jar  Array.java

I get the output:

    [I object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           01 00 00 00 (00000001 00000000 00000000 00000000) (1)
      4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4        (object header)                           18 0e 07 00 (00011000 00001110 00000111 00000000) (462360)
     12     4        (object header)                           03 00 00 00 (00000011 00000000 00000000 00000000) (3)
     16    12    int [I.<elements>                             N/A
     28     4        (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

This is pretty much obvious:

     12 bytes --> Object headers
     4  bytes --> size of array
     12 bytes --> elements of array themselves
     4  bytes --> padding to align by 8 bytes
     ----
     32 bytes total

Running this example with jdk-15 yields the same output, same 32 bytes. Expected...

---

For the second part, I want to disable a JVM optimization: -XX:-UseCompressedOops. I run this with jdk-13:

java -Djdk.attach.allowAttachSelf -Djol.tryWithSudo=true -cp jol-cli.jar -XX:-UseCompressedOops  Array.java


    [I object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           11 00 00 00 (00010001 00000000 00000000 00000000) (17)
      4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4        (object header)                           40 0c f0 33 (01000000 00001100 11110000 00110011) (871369792)
     12     4        (object header)                           02 00 00 00 (00000010 00000000 00000000 00000000) (2)
     16     4        (object header)                           03 00 00 00 (00000011 00000000 00000000 00000000) (3)
     20     4        (alignment/padding gap)
     24    12    int [I.<elements>                             N/A
     36     4        (loss due to the next object alignment)
Instance size: 40 bytes
Space losses: 4 bytes internal + 4 bytes external = 8 bytes total

Well, sort of expected too:

     16 bytes --> object headers (I did -XX:-UseCompressedOops after all)
     4 bytes  --> array size
     4 bytes  --> alignment for array headers (AFAIK this is only done for arrays)
     12 bytes --> array elements themselves
     4 bytes  --> 4 bytes padding
     ----
     40 bytes total

Now let's run the same example with jdk-15:

[I object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           01 00 00 00 (00000001 00000000 00000000 00000000) (1)
      4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4        (object header)                           0e 09 00 00 (00001110 00001001 00000000 00000000) (2318)
     12     4        (object header)                           03 00 00 00 (00000011 00000000 00000000 00000000) (3)
     16    12    int [I.<elements>                             N/A
     28     4        (loss due to the next object alignment)
Instance size: 32 bytes

Why is this 32 bytes now? How come not 40, just like with jdk-13?

回答1:

In both jdk-13 and jdk-15, both of these options are on by default:

java -XX:+PrintFlagsFinal -version | grep Compressed

 bool UseCompressedClassPointers  = true                                 
 bool UseCompressedOops           = true

When -XX:-UseCompressedOops is disabled, it means that UseCompressedClassPointers is disabled also. That is why when UseCompressedOops is turned off, the header size goes up by 4 bytes, because UseCompressedOops turns off UseCompressedClassPointers. At least this is how it is in jdk-13:

  java -XX:+PrintFlagsFinal -XX:-UseCompressedOops -version | grep Compressed

    bool UseCompressedClassPointers = false                                  
    bool UseCompressedOops          = false

Things have changed in jdk-15:

    bool UseCompressedClassPointers = true                                 
    bool UseCompressedOops          = false

So disabling UseCompressedOops does not mean that UseCompressedClassPointers is disabled also, so it stays at 4 bytes.

Though, I answered this myself, it would be nice if someone finds the relevant bug/change for this? I have not been successful in that, so far.

来源：https://stackoverflow.com/questions/62879157/usecompressedoops-usecompressedclasspointers-in-jdk-13-and-jdk-15