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

[David Holmes]

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. 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.

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
>