Integrated: 8361712: Improve ShenandoahAsserts printing

[Thomas Stuefe]

On Thu, 10 Jul 2025 08:02:05 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> Small changes to ShenandoahAsserts and friends to improve fault-tolerance and usefulness:
> 
> - In assert_correct, we now check the forwarding pointer first before extracting the narrowKlass from it. The forwarding pointer may be invalid. To do this, the forwarding-pointer-checking-block was moved up to before Klass* checks. We also use SafeFetch-based checks for some added safety.
> - In assert_correct, we test the narrowKlass before decoding it. Otherwise, if it's 0, we'd assert, if garbage, the resulting Klass* would be garbage.
> - In print_failure, we now check pointers for readability before handing them to print_obj
> - In print_obj, we now print the extracted narrowKlass, too (useful on Lilliput with its terse ids)
> - In print_obj, we print a small hex dump of the header (32bytes). With little cost, we get a glance at the raw header and the first members.
> - In print_obj, we print to a single stringStream now and use auto indentation. Avoids having to deal with indentation and newlines manually and having to pay for four stringStreams.
> - In assert_correct, I raised the print level for some of the klass-related assertions to safe_oop since the oop, since the hardened version of print_obj now avoids tripping over invalid Klass*
> 
> Tests: I manually tested the patch with several manually broken oops of various flavours. 
> 
> Example output, invalid forwardee:
> 
> 
> Referenced from:
>   no interior location recorded (probably a plain heap scan, or detached oop)
> 
> Object:
>   0x00000007cd800000 - nk 1798104 klass 0x00000000921b6fd8 [Ljava.lang.Object;
> 
>         allocated after mark start
>         after update watermark
>         marked strong
>         marked weak
>     not in collection set
>     mark:  marked(0xdeaddeaddeaddeaf)
>     region: | 3812|R  |Y|BTE    7cd800000,    7cd920bf8,    7cdc00000|TAMS    7cd800000|UWM    7cd800000|U  1154K|T     0B|G     0B|S  1154K|L     0B|CP   0
> 
>   0x00000007cd800000:   deaddeaddeaddeaf 00000a96001b6fd8 f9b005cff9b0054d f9b006d3f9b00651   .........o......M.......Q.......
>   0x00000007cd800020:   f9b00797f9b00755 f9b00992f9b0087d f9b009a0f9b00999 f9b009a7f9b0098e   U.......}.......................
> 
> Forwardee:
>   0xdeaddeaddeaddeac - safe print, no details

This pull request has now been integrated.

Changeset: 75ff7e15
Author:    Thomas Stuefe <stuefe at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/75ff7e15fe0d22149e5b8c5ccf3b702d8dc9b3fa
Stats:     164 lines in 9 files changed: 122 ins; 12 del; 30 mod

8361712: Improve ShenandoahAsserts printing

Reviewed-by: rkennke, asmehra

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PR: https://git.openjdk.org/jdk/pull/26237