Request for review: 7102776 Pack instanceKlass boolean fields into single u1 field

[Christian Thalinger]

On Nov 10, 2011, at 12:02 PM, David Holmes wrote:

> Hi Chris,
> 
> On 10/11/2011 7:26 PM, Christian Thalinger wrote:
>> What about:
>> 
>> -  bool            _is_marked_dependent:1;  // used for marking during flushing and deoptimization
>> -  bool            _rewritten:1;            // methods rewritten.
>> -  bool            _has_nonstatic_fields:1; // for sizing with UseCompressedOops
>> -  bool            _should_verify_class:1;  // allow caching of preverification
>> 
>> Wouldn't that be much easier?  And we already have code like that in e.g. nmethod.hpp.
> 
> I'm not familiar with this particular notation but I assume it is similar to using bitfields.

Correct.

> This had been discussed internally but unfortunately the SA code needs to access the "is-marked-dependent" bit and as there's no guarantee as to the order in which bitfields are packed, the SA would not know how to extract that bit.

Good point.  Bitfields won't work then.

-- Chris

> 
> David
> -----
> 
>> -- Chris
>> 
>> On Nov 9, 2011, at 6:47 PM, Jiangli Zhou wrote:
>> 
>>> Compact following 4 instanceKlass boolean fields into a signal u1 field. Each flag now uses 1-bit. The new field is placed after the _idnum_allocated_count field to utilize the unused 2-byte after _idnum_allocated_count. Compacting these fields saves 4-bytes for each loaded classes.
>>> 
>>>  bool _is_marked_dependent; // used for marking during flushing and
>>>  deoptimization
>>>  bool _rewritten; // methods rewritten.
>>>  bool _has_nonstatic_fields; // for sizing with UseCompressedOops
>>>  bool _should_verify_class; // allow caching of preverification
>>> 
>>> http://cr.openjdk.java.net/~bobv/7102776/webrev.00/
>>> 
>>> Tested with runThese on ubuntu. Ran JPRT.
>>> 
>>> Thanks,
>>> Jiangli
>>