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8257594: C2 compiled checkcast of non-null object triggers endless deoptimization/recompilation cycle #1566

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8257594: C2 compiled checkcast of non-null object triggers endless deoptimization/recompilation cycle #1566

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TobiHartmann
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@TobiHartmann TobiHartmann commented Dec 2, 2020 •
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When C2 determines at compile time that there is no subtype relation between the objects involved in a checkcast, it inserts a null assert because the checkcast can only pass if the value is null. This will trigger deoptimization and recompilation if the object is non-null. However, we can do better here if we already know that the object is non-null (i.e. the check always fails) or if we hit too_many_traps_or_recompiles (which suggests that the object is often null) to avoid an endless deoptimization/recompilation cycle until we hit the PerMethodRecompilationCutoff and starve at lower tiers.

In the former case we can simply throw right away and in the latter case we should fall back to a full check in case there is an exception handler in compiled code that handles the failing (non-null) case. That way we avoid deoptimization.

I've spotted this with our test framework for inline types in project Valhalla, while investigating an intermittent failure where several test methods suddenly became non-compilable at tier 4.

Thanks,
Tobias

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  • JDK-8257594: C2 compiled checkcast of non-null object triggers endless deoptimization/recompilation cycle

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$ git fetch https://git.openjdk.java.net/jdk pull/1566/head:pull/1566
$ git checkout pull/1566

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bridgekeeper bot commented Dec 2, 2020

👋 Welcome back thartmann! 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.

@openjdk openjdk bot added the rfr Pull request is ready for review label Dec 2, 2020
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openjdk bot commented Dec 2, 2020

@TobiHartmann The following label will be automatically applied to this pull request:

  • hotspot-compiler

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 Dec 2, 2020
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mlbridge bot commented Dec 2, 2020

Webrevs

rwestrel
rwestrel approved these changes Dec 3, 2020
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Looks good to me.

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openjdk bot commented Dec 3, 2020 •
edited

@TobiHartmann 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:

8257594: C2 compiled checkcast of non-null object triggers endless deoptimization/recompilation cycle

Reviewed-by: roland, vlivanov

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

At the time when this comment was updated there had been 20 new commits pushed to the master branch:

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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@openjdk openjdk bot added the ready Pull request is ready to be integrated label Dec 3, 2020
iwanowww
iwanowww approved these changes Dec 3, 2020
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Looks good.

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Thanks for the reviews @rwestrel and @iwanowww !

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/integrate

@openjdk openjdk bot closed this Dec 3, 2020
@openjdk openjdk bot added integrated Pull request has been integrated and removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Dec 3, 2020
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openjdk bot commented Dec 3, 2020

@TobiHartmann Since your change was applied there have been 21 commits pushed to the master branch:

Your commit was automatically rebased without conflicts.

Pushed as commit 129c377.

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

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@iwanowww iwanowww iwanowww approved these changes

@rwestrel rwestrel rwestrel approved these changes

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