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

[Stefan Karlsson]

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