Oop.iterateFields() should support CompressedKlassPointers again

[云达(Yunda)]

Hi all,

When I used CLHSDB just now I met this error:
hsdb> inspect 0x00000000ee255080
instance of "java/io/InputStream" @ 0x00000000ee255080 @ 0x00000000ee255080 (size = 24)
Exception in thread "main" java.lang.InternalError: unimplemented
         at sun.jvm.hotspot.oops.Oop.iterateFields(Oop.java:151)
         at sun.jvm.hotspot.oops.Instance.iterateFields(Instance.java:66)
         at sun.jvm.hotspot.oops.Oop.iterate(Oop.java:143)
         at sun.jvm.hotspot.ui.tree.OopTreeNodeAdapter.getChildCount(OopTreeNodeAdapter.java:65)
         at sun.jvm.hotspot.CommandProcessor$Command.printNode(CommandProcessor.java:231)
         at sun.jvm.hotspot.CommandProcessor$24.doit(CommandProcessor.java:1008)
         at sun.jvm.hotspot.CommandProcessor.executeCommand(CommandProcessor.java:1897)
         at sun.jvm.hotspot.CommandProcessor.executeCommand(CommandProcessor.java:1867)
         at sun.jvm.hotspot.CommandProcessor.run(CommandProcessor.java:1747)
         at sun.jvm.hotspot.CLHSDB.run(CLHSDB.java:91)
         at sun.jvm.hotspot.CLHSDB.main(CLHSDB.java:35)

I found it’s caused by the code of sun.jvm.hotspot.oops.Oop.iterateFields():
   void iterateFields(OopVisitor visitor, boolean doVMFields) {
     if (doVMFields) {
       visitor.doCInt(mark, true);
       if (VM.getVM().isCompressedKlassPointersEnabled()) {
         throw new InternalError("unimplemented");
       } else {
         visitor.doMetadata(klass, true);
       }
     }
   }

When compressed oops( which is by default) are used an InternalError of “unimplemented” will be throwed. But actually it can be implemented easily by just one line of code:
visitor.doMetadata(compressedKlass, true);

I checked the hotspot-rt repo and I found it was implemented this way before changeset 3601, the main implementation of NPG. Since 3601 changed a whole lot of stuff, the ‘compressedKlass’ field couldn’t get the right value, as in line 51:
// compressedKlass  = new CIntField(type.getCIntegerField("_metadata._compressed_klass"), 0);
So the code in iterateFields() was changed to throwing an InternalError accordingly.

But the ‘compressedKlass’ field can get the right value now. So I think it’s time to change the code back and here’s the diff against the latest hotspot-rt:
diff -r 2f881161d085 agent/src/share/classes/sun/jvm/hotspot/oops/Oop.java
--- a/agent/src/share/classes/sun/jvm/hotspot/oops/Oop.java    Mon Feb 25 18:25:24 2013 +0800
+++ b/agent/src/share/classes/sun/jvm/hotspot/oops/Oop.java Mon Feb 25 18:47:08 2013 +0800
@@ -148,7 +148,7 @@
     if (doVMFields) {
       visitor.doCInt(mark, true);
       if (VM.getVM().isCompressedKlassPointersEnabled()) {
-        throw new InternalError("unimplemented");
+        visitor.doMetadata(compressedKlass, true);
       } else {
         visitor.doMetadata(klass, true);
       }


Regards,
Yunda


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