RFR: 8221540: ZGC: Reduce width of zForwardingEntry::from_index field
per.liden at oracle.com
Thu Mar 28 12:12:11 UTC 2019
On 3/28/19 12:20 PM, Stefan Karlsson wrote:
> Looks good.
> 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
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