RFR: CODETOOLS-7902931: JOL: Rework magic field offset accessor [v3]

[Aleksey Shipilev]

> See the bug report for discussion.
> 
> Basic sanity test:
> 
> 
> $ cat MyRecord.java
> record MyRecord(int x, int y) {}
> 
> 
> Current JDK 17 EA, out-of-box:
> 
> 
> $ java -jar jol-cli/target/jol-cli.jar internals -cp . MyRecord
> ...
> 
> java.lang.RuntimeException: Cannot get the field offset, try with -Djol.magicFieldOffset=true
> 	at org.openjdk.jol.vm.HotspotUnsafe.fieldOffset(HotspotUnsafe.java:455)
> 	at org.openjdk.jol.layouters.CurrentLayouter.layout(CurrentLayouter.java:70)
> 	at org.openjdk.jol.info.ClassLayout.parseInstance(ClassLayout.java:102)
> 	at org.openjdk.jol.info.ClassLayout.parseInstance(ClassLayout.java:85)
> 	at org.openjdk.jol.operations.ObjectInternals.runWith(ObjectInternals.java:51)
> 	at org.openjdk.jol.operations.ClasspathedOperation.run(ClasspathedOperation.java:76)
> 	at org.openjdk.jol.Main.main(Main.java:58)
> Caused by: java.lang.UnsupportedOperationException: can't get field offset on a record class: private final int MyRecord.x
> 	at jdk.unsupported/sun.misc.Unsafe.objectFieldOffset(Unsafe.java:648)
> 	at org.openjdk.jol.vm.HotspotUnsafe.fieldOffset(HotspotUnsafe.java:448)
> 	... 6 more
> 
> 
> Current JDK 17 EA, with `-Djol.magicFieldOffset=true`:
> 
> 
> $ java -Djol.magicFieldOffset=true -jar jol-cli/target/jol-cli.jar internals -cp . MyRecord
> ...
> 
> MyRecord object internals:
> OFF  SZ   TYPE DESCRIPTION               VALUE
>   0   8        (object header: mark)     0x0000000000000001 (non-biasable; age: 0)
>   8   4        (object header: class)    0x00218180
>  12   4    int MyRecord.x                0
>  16   4    int MyRecord.y                0
>  20   4        (object alignment gap)    
> Instance size: 24 bytes
> Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Even more comments

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

Changes:
  - all: https://git.openjdk.java.net/jol/pull/16/files
  - new: https://git.openjdk.java.net/jol/pull/16/files/e4f8bce2..062d106f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jol&pr=16&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jol&pr=16&range=01-02

  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jol/pull/16.diff
  Fetch: git fetch https://git.openjdk.java.net/jol pull/16/head:pull/16

PR: https://git.openjdk.java.net/jol/pull/16