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8361712: Improve ShenandoahAsserts printing #26237

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@tstuefe tstuefe commented Jul 10, 2025 •
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Small changes to ShenandoahAsserts and friends to improve fault-tolerance and usefulnes:

  • 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 its 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 a number of 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

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  • JDK-8361712: Improve ShenandoahAsserts printing (Enhancement - P4)

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$ git fetch https://git.openjdk.org/jdk.git pull/26237/head:pull/26237
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$ git pull https://git.openjdk.org/jdk.git pull/26237/head

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$ git pr checkout 26237

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

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bridgekeeper bot commented Jul 10, 2025

👋 Welcome back stuefe! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

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openjdk bot commented Jul 10, 2025 •
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@tstuefe This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8361712: Improve ShenandoahAsserts printing

Reviewed-by: rkennke

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At the time when this comment was updated there had been 71 new commits pushed to the master branch:

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openjdk bot commented Jul 10, 2025

@tstuefe The following labels will be automatically applied to this pull request:

  • hotspot
  • shenandoah

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing lists. If you would like to change these labels, use the /label pull request command.

@openjdk openjdk bot added hotspot hotspot-dev@openjdk.org shenandoah shenandoah-dev@openjdk.org labels Jul 10, 2025
@tstuefe tstuefe changed the title assert_correct JDK-8361712: Improve ShenandoahAsserts printing Jul 10, 2025
@openjdk openjdk bot changed the title JDK-8361712: Improve ShenandoahAsserts printing 8361712: Improve ShenandoahAsserts printing Jul 10, 2025
@tstuefe tstuefe closed this Jul 11, 2025
@tstuefe tstuefe reopened this Jul 11, 2025
@tstuefe tstuefe force-pushed the harden-shenandoah-asserts branch from 63b1f4c to aab9d9c Compare July 11, 2025 14:42
@tstuefe tstuefe marked this pull request as ready for review July 12, 2025 06:39
@openjdk openjdk bot added the rfr Pull request is ready for review label Jul 12, 2025
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rkennke
rkennke reviewed Jul 17, 2025
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This looks really useful! I have only two comments/questions.

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tstuefe commented Jul 23, 2025

@rkennke Did I address all your concerns? Anyone else willing to look at this, @earthling-amzn maybe?

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rkennke commented Jul 23, 2025

@rkennke Did I address all your concerns? Anyone else willing to look at this, @earthling-amzn maybe?

Yes, looks all good to me.

@openjdk openjdk bot added the ready Pull request is ready to be integrated label Jul 23, 2025
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