RFR: 8239347: Refactor Symbol to make _length a standalone field again

[Claes Redestad]

Thanks, Ioi!

/Claes

On 2020-02-18 18:27, Ioi Lam wrote:
> Hi Claes,
> 
> This looks good. Thanks for finding and fixing this!
> 
> - Ioi
> 
> On 2/18/20 8:21 AM, Claes Redestad wrote:
>> Hi,
>>
>> JDK-8207359 merged Symbol::_refcount and _length into a single field. We
>> could refactor this to instead pack _refcount and _identity_hash,
>> leaving _length as a standalone field.
>>
>> _length is used in a lot more places, so not having to call a method to
>> extract the value using shifts results in a code size reduction and
>> potential speed-up. Except for a path only used when dumping CDS
>> archives, _identity_hash is only accessed in one place, and there in
>> conjunction with an access to length.
>>
>> Bug:    https://bugs.openjdk.java.net/browse/JDK-8239347
>> Webrev: http://cr.openjdk.java.net/~redestad/8239347/open.00/
>>
>> Testing: tier1-4, verified a small static footprint reduction (~4kb),
>>          verified accesses to _length reduced from a mov+shr to a
>>          movzwl
>>
>> /Claes
>