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  1. JDK
  2. JDK-8264908

Investigate adding BOT range check in G1BlockOffsetTablePart::block_at_or_preceding

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Details

    • Enhancement
    • Status: Resolved
    • P4
    • Resolution: Fixed
    • 17
    • 18
    • hotspot
    • gc
    • b07

    Description

      BlockOffsetTable (BOT) is used to speedup the lookup of object starting address given a arbitrary pointer into the allocated part of a region, [_bottom, _top). BOT maintains a range limit (`_next_offset_index`) as allocation goes, corresponding to `_top`. In theory, these two limits (`_next_offset_index` and `_top`) should be in sync. Therefore, we never ask for cards `>= _next_offset_index`.

      `G1BlockOffsetTablePart::block_at_or_preceding`, used during the lookup, is more forgiving (hence the suffix `or_preceding`); it actually handles cards `>= _next_offset_index` by walking from the last previous known card.

      ```
        // We must make sure that the offset table entry we use is valid. If
        // "addr" is past the end, start at the last known one and go forward.
      ```

      Such "error handling" code could be slow. Therefore, it's better to tighten the pre-condition, adding the assertion below, and fix broken callers, if any.

      `assert(_bot->index_for(addr) < _next_offset_index);`

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              iwalulya Ivan Walulya
              ayang Albert Yang
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                Updated:
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