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

[Thomas Stuefe]

On Wed, 12 Jul 2023 00:39:30 GMT, Ioi Lam <iklam 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

Looks useful. Found nothing to note but a small style nit. But a more extensive test would be nice, e.g. matching the output of this log against a known object layout.

src/hotspot/share/cds/archiveBuilder.cpp line 1152:

> 1150:         }
> 1151:       } else {
> 1152:         st.print_cr(" - ---- fields (total size " SIZE_FORMAT " words):", source_oop->size());

Nit, proposal: "fields (xx words):" (no leading dashes, no "total size") for more density

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

PR Review: https://git.openjdk.org/jdk/pull/14841#pullrequestreview-1531297755
PR Review Comment: https://git.openjdk.org/jdk/pull/14841#discussion_r1264342465