RFR: 8308903: Print detailed info for Java objects in -Xlog:cds+map

[Ioi Lam]

On Wed, 12 Jul 2023 05:51:58 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> This PR adds detailed printing of oop information with `-Xlog:cds+map+oop=trace`, or simply `-Xlog:cds+map*=trace`. The information is useful for debugging contents of the CDS archived heap objects.
>> 
>> The output looks like `oopDesc::print_on(tty)`, but we need to print the pointers using the locations of the objects at runtime. The examples below show how a `String` references its `value` array.
>> 
>> Example with `-XX:-UseCompressedOops`:
>> 
>> 
>> 0x00000000100001f0: @@ Object java.lang.String
>> 0x00000000100001f0: 0000006ff6ab8d01 88d47c5b00010290 0000000000010000 0000000010000210
>>  - klass: 'java/lang/String' 0x0000000800010290
>>  - ---- fields (total size 4 words):
>>  - private 'hash' 'I' @12 -1999340453 (0x88d47c5b)
>>  - private final 'coder' 'B' @16 0 (0x00)
>>  - private 'hashIsZero' 'Z' @17 false (0x00)
>>  - injected 'flags' 'B' @18 1 (0x01)
>>  - private final 'value' '[B' @24 0x0000000010000210 [B length: 6
>> 0x0000000010000210: @@ Object [B length: 6
>> 0x0000000010000210: 000000693b708001 00000006000024c8 0000574f5252414e
>>  - klass: {type array byte} 0x00000008000024c8
>>  - 0: 4e N
>>  - 1: 41 A
>>  - 2: 52 R
>>  - 3: 52 R
>>  - 4: 4f O
>>  - 5: 57 W
>> 
>> 
>> Example with `-XX:+UseCompressedOops`. Note that the narrorOop is also printed:
>> 
>> 
>> 0x00000007ffc001b8: @@ Object (0xfff80037) java.lang.String
>> 0x00000007ffc001b8: f6ab8d01 0000006f 00010290 88d47c5b 00010000 fff8003a
>>  - klass: 'java/lang/String' 0x0000000800010290
>>  - ---- fields (total size 3 words):
>>  - private 'hash' 'I' @12 -1999340453 (0x88d47c5b)
>>  - private final 'coder' 'B' @16 0 (0x00)
>>  - private 'hashIsZero' 'Z' @17 false (0x00)
>>  - injected 'flags' 'B' @18 1 (0x01)
>>  - private final 'value' '[B' @20 0x00000007ffc001d0 (0xfff8003a) [B length: 6
>> 0x00000007ffc001d0: @@ Object (0xfff8003a) [B length: 6
>> 0x00000007ffc001d0: 3b708001 00000069 000024c8 00000006 5252414e 0000574f
>>  - klass: {type array byte} 0x00000008000024c8
>>  - 0: 4e N
>>  - 1: 41 A
>>  - 2: 52 R
>>  - 3: 52 R
>>  - 4: 4f O
>>  - 5: 57 W
>
>> The output looks like oopDesc::print_on(tty), but we need to print the pointers using the locations of the objects at runtime. 
> 
> I don't understand. Are you not experimenting with a new allocation mode that would basically make the runtime address unpredictable? Or is this still for the old way of things where the heap archive is mapped as-is ?
> 
> Other than that, this looks very useful.

Thanks @tstuefe @calvinccheung @matias9927 for the review.

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PR Comment: https://git.openjdk.org/jdk/pull/14841#issuecomment-1648354099