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8360175: C2 crash: assert(edge_from_to(prior_use,n)) failed: before block local scheduling #26157

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8360175: C2 crash: assert(edge_from_to(prior_use,n)) failed: before block local scheduling #26157

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@mhaessig mhaessig commented Jul 7, 2025 •
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The triggered assert is part of the schedule verification code that runs just before machine code is emitted. The debug output showed that a leaPCompressedOopOffset node was causing the assert, which suggested the peephole optimization introduced in #25471 as the cause. The failure proved quite difficult to reproduce. It failed more often on Windows and required -XX:+UseKNLSetting (forces code generation for Intel's Knights Landing platform), which forces -XX:+OptoScheduling.

The root-cause is a subtle bug in the rewiring of the base edge of leaP* nodes in the remove_redundant_lea peephole. When the peephole removed a decodeHeapOop_not_null including a spill, it did not set the base edge of the leaP* node to the same node as the address edge, which is the intent of the peephole, but to the parent node of the spill. That is not catastrophic in most cases, but might reference another register slot, which causes this assert. Concretely, we see the following graph

    MemToRegSpillCopy
     |             |
     |    MemToRegSpillCopy
     |             |    
DefiniinoSpillCopy |
     |             |
     |  decodeHeapOop_not_null
     |             |
   leaPCompressedHeapOop

gets rewired to

     MemToRegSpillCopy
       |            |    
DefinitionSpillCopy |
       |            |
   leaPCompressedHeapOop

instead of

  MemToRegSpillCopy
         |
 DefinitionSpillCopy
        / \     
leaPCompressedHeapOop

This PR fixes this by always setting the base edge of the leaP* node to the same node as the address edge. Unfortunately, I was not able to construct a regression test because of the difficulty of reproducing the bug.

Testing

  • Github Actions
  • tier1,tier2 plus internal testing on all Oracle supported platforms
  • tier3,tier4,tier5 plus internal testing on Linux and Windows x64
  • Runthese8H on windows-x64-debug (test that reliably produced the failure addressed in this PR)

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  • Change must not contain extraneous whitespace
  • Commit message must refer to an issue

Issue

  • JDK-8360175: C2 crash: assert(edge_from_to(prior_use,n)) failed: before block local scheduling (Bug - P3)

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@mhaessig mhaessig marked this pull request as draft July 7, 2025 09:47
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👋 Welcome back mhaessig! 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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@mhaessig 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:

8360175: C2 crash:  assert(edge_from_to(prior_use,n)) failed: before block local scheduling

Reviewed-by: kvn, chagedorn

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

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@openjdk openjdk bot changed the title 8360175: C2 crash: assert(edge_from_to(prior_use,n)) failed: before block local scheduling 8360175: C2 crash: assert(edge_from_to(prior_use,n)) failed: before block local scheduling Jul 7, 2025
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@mhaessig The following label will be automatically applied to this pull request:

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

@openjdk openjdk bot added the hotspot-compiler hotspot-compiler-dev@openjdk.org label Jul 7, 2025
@mhaessig mhaessig marked this pull request as ready for review July 7, 2025 10:25
@openjdk openjdk bot added the rfr Pull request is ready for review label Jul 7, 2025
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Webrevs

vnkozlov
vnkozlov approved these changes Jul 8, 2025
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Seems fine.

@openjdk openjdk bot added the ready Pull request is ready to be integrated label Jul 8, 2025
chhagedorn
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mhaessig commented Jul 9, 2025

Thank you for your reviews, @vnkozlov and @chhagedorn!

/integrate

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openjdk bot commented Jul 9, 2025

Going to push as commit db4b4a5.
Since your change was applied there have been 46 commits pushed to the master branch:

Your commit was automatically rebased without conflicts.

@openjdk openjdk bot added the integrated Pull request has been integrated label Jul 9, 2025
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@openjdk openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Jul 9, 2025
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@mhaessig Pushed as commit db4b4a5.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

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@chhagedorn chhagedorn chhagedorn approved these changes

@vnkozlov vnkozlov vnkozlov approved these changes

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