RFR: 8285979: G1: G1SegmentedArraySegment::header_size() is incorrect since JDK-8283368

[Jie Fu]

Hi all,

`G1SegmentedArraySegment::header_size()` is incorrect if `DEFAULT_CACHE_LINE_SIZE <= 32`.
This bug can be easily reproduced by running `gc/g1` with VM configured and built with `--with-jvm-features=-compiler2`.

The reason is that there are paddings in the layout of `G1SegmentedArraySegment`.

  const uint _slot_size;                   // offset 0-byte
  const uint _num_slots;                   // offset 4-byte
  const MEMFLAGS _mem_flag;                // offset 8-byte
    // --- padding 4 bytes
  G1SegmentedArraySegment* volatile _next; // offset 16-byte
  uint volatile _next_allocate;            // offset 24
    // --- padding 4 bytes
  char* _bottom;                           // offset 32-byte


So if we calculate `header_size()` like this, it will return 32 bytes when `DEFAULT_CACHE_LINE_SIZE=32`, which should be at least 40 bytes actually.

static size_t header_size() { return align_up(offset_of(G1SegmentedArraySegment, _bottom) + sizeof(_bottom), DEFAULT_CACHE_LINE_SIZE); }


Two changes are made in this patch:
- Fix the implementation of `header_size()`.
- Fix the layout of `G1SegmentedArraySegment` to eliminate paddings.

Testing:
- tier1~3 on Linux/x64, no regression
- gc/g1 with `--with-jvm-features=-compiler2` on Linux/x64, all passed

Thanks.
Best regards,
Jie

-------------

Commit messages:
 - 8285979: G1: G1SegmentedArraySegment::header_size() is incorrect since JDK-8283368

Changes: https://git.openjdk.java.net/jdk/pull/8494/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8494&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285979
  Stats: 6 lines in 2 files changed: 2 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8494.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8494/head:pull/8494

PR: https://git.openjdk.java.net/jdk/pull/8494