RFR: CODETOOLS-7902931: JOL: Rework magic field offset accessor [v4]

[Zheka Kozlov]

On Tue, 11 May 2021 06:38:26 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> 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:
> 
>   Simplify the verification loop

jol-core/src/main/java/org/openjdk/jol/vm/HotspotUnsafe.java line 58:

> 56: 
> 57:     private static final boolean MAGIC_FIELD_OFFSET =
> 58:             Boolean.parseBoolean(System.getProperty(MAGIC_FIELD_OFFSET_OPTION, "false"));

Can be replaced with `Boolean.getBoolean(MAGIC_FIELD_OFFSET_OPTION)`

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

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