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

[Jiangli Zhou]

Hi Paul,

Thanks for the details and review. The bit field padding due to 
alignment is more of an issue when there are mixed types.

Thanks,
Jiangli

Paul Hohensee wrote:
> Actually, the C/C++ standards do guarantee dense bit packing in 
> bitfields,
> because they were originally intended to be used to access device 
> registers.
>
> The type of the bitfield determines how much space the underlying 
> integral
> type occupies.  E.g.,
>
> unsigned char bit1:1, bit2:1, bit3:1;
>
> occupies the "first" 3 bits of a one-byte class data member, while
>
> unsigned int bit1:1, bit2:1, bit3:1;
>
> occupies the "first" 3 bits of a 4-byte class data member.
>
> Note that "first" means that bitfields are allocated starting from the 
> lowest addressed
> byte in the bitfield type.  That means that on big-endian machines, 
> the first allocated
> bit in an unsigned int bitfield will be the msb, while on 
> little-endian machines
> it'll be the lsb.  If you want a bitfield that's endian-independent, 
> you have to
> use the masking technique in the webrev: e.g., if you want to pass a 
> bitfield to Java
> code, which has no concept of endianness.
>
> Paul
>
> On 11/10/11 2:32 PM, Jiangli Zhou wrote:
>> 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
>>