RFR (L): 8022853: add ability to Unsafe to load and store uncompressed object and Klass references in a compressed environment

[Christian Thalinger]

On Aug 21, 2013, at 10:55 AM, Coleen Phillimore <coleen.phillimore at oracle.com> wrote:

> 
> Hi,
> 
> I've reviewed this change and I don't know how it can work.   You cannot put uncompressed oops in an object when the setting is UseCompressedOops and have garbage collection work.

This is not the main use case for these methods.  Sometimes oops are not compressed even when we run with compressed oops as for the Klass::_java_mirror case.  There may be other fields like this but I don't know about them.

>   Similarly you can't compress arbitrary metadata because only Klass* types can be compressed.   This doesn't seem like it would work at all.

Currently get/putMetadata only supports StorageMode.UNCOMPRESSED_KLASS and StorageMode.COMPRESSED_KLASS.  We might add other storage modes in the future if we decide to compress other things.

>   Also, we lay out objects in classFileParser assuming the setting of UseCompressedOops/ClassPointers and allocates space based on that.
> 
> I don't understand the motivation of this change.   It appears incorrect.

The motivation is to enable Java code which is tightly coupled to the VM to read and possibly write such fields.  Right now it is not possible.

> 
> From reading the bug, it seems that you want to get to mirror from InstanceKlass from Java.   This will break when we move the mirror.

As said above, since it's tightly coupled to the VM we will notice very soon when something changes and will change the Java code accordingly.

-- Chris

> 
> Coleen
> 
> On 8/19/2013 5:28 PM, Christian Thalinger wrote:
>> http://cr.openjdk.java.net/~twisti/8022853/webrev/
>> http://cr.openjdk.java.net/~twisti/8022853-jdk/webrev/
>> 
>> 8022853: add ability to Unsafe to load and store uncompressed object and Klass references in a compressed environment
>> Reviewed-by:
>> 
>> In some native data structures of the VM oops are stored uncompressed when compressed oops are turned on, for example Klass::_java_mirror. Java code which is tightly coupled with the VM should have the ability to read and write such fields.
>> 
>