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

[Jiangli Zhou]

Hi Chris,

Thanks for the review. Comments below.

On 11/10/2011 03:11 AM, Christian Thalinger wrote:
> 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.

Besides the bit addressing issue, it cannot guarantee that the bit 
fields are always packed into the smallest size by all compilers.

Thanks,
Jiangli

> -- 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