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  1. JDK
  2. JDK-8221540

ZGC: Reduce width of zForwardingEntry::from_index field

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Details

    • Enhancement
    • Resolution: Fixed
    • P4
    • 13
    • 13
    • hotspot
    • gc
    • b15

    Description

      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.

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