RFR: 8221540: ZGC: Reduce width of zForwardingEntry::from_index field

[Per Liden]

Thanks Stefan!

/Per

On 3/28/19 12:20 PM, Stefan Karlsson wrote:
> Looks good.
> 
> StefanK
> 
> On 2019-03-27 12:33, Per Liden wrote:
>> The zForwardingEntry::from_index field only needs 18 bits, as it only 
>> needs to be able to represent the largest object index, which is:
>>
>> ZPageSizeSmall / MinObjectAlignment == 2M / 8 == 262144 == 1 << 18.
>>
>> Shrinking the from_index allows us to grow the to_offset, which in 
>> turn allows us to increase the max heap size in the future. However, 
>> with these changes, we can no longer assume that (uint64_t)-1 is an 
>> empty entry. So we need to introduce a new flag to indicate if an 
>> entry is populated or not.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8221540
>> Webrev: http://cr.openjdk.java.net/~pliden/8221540/webrev.0
>>
>> /Per