8350329: C2: Div looses dependency on condition that guarantees divisor not zero in counted loop after peeling

[rwestrel]

This is an issue similar to 8349139: the type of the iv phi of a
counted loop is narrowed down so a Div node doesn't need a control
input. The loop is then peeled. The Div in the loop body is
guaranteed to be non zero only if it is actually executed so the Div
is implicitly dependent on the zero trip guard. Then the loop looses
its backedge and the Div freely floats. The Div instruction is
scheduled above the zero trip guard and faults. Had the Div been
control dependent on the zero trip guard, it wouldn't have
executed. The fix, similar to 8349139 is to add a CastII on peeling
to make the dependency between what's in the loop body and relies on
the narrowed down type of the iv phi and the zero trip guard explicit.

---

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Issue

• JDK-8350329: C2: Div looses dependency on condition that guarantees divisor not zero in counted loop after peeling (Bug - P3)

Reviewers

• Christian Hagedorn (@chhagedorn - Reviewer) Review applies to eb3a13b3
• Tobias Hartmann (@TobiHartmann - Reviewer)

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[openjdk]

@rwestrel This change now passes all automated pre-integration checks.

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8350329: C2: Div looses dependency on condition that guarantees divisor not zero in counted loop after peeling

Reviewed-by: thartmann, chagedorn

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At the time when this comment was updated there had been 44 new commits pushed to the master branch:

• 079fccf: 8353888: Implement JEP 510: Key Derivation Function API
• 0d86757: 8355488: Add stress mode for C2 loop peeling
• a0a3060: 8353638: C2: deoptimization and re-execution cycle with StringBuilder
• ... and 41 more: https://git.openjdk.org/jdk/compare/d1032d71bfd50ebc4b328ce9b0cc81cbb80554b0...master

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[mlbridge]

Webrevs

• 01: Full - Incremental (f5069b5e)
• 00: Full (eb3a13b3)

[chhagedorn]

Looks good to me.

[chhagedorn]

You should use -XX:+UnlockDiagnosticVMOptions for StressGCM and maybe also add -XX:+IgnoreUnrecognizedVMOptions when run without C2 since UseLoopPredicate is a C2 only flag.

[rwestrel]

Thanks for reviewing this. Done in the new commit.

[TobiHartmann]

Looks good to me too.