8292285: C2: remove unreachable block after NeverBranch-to-Goto conversion

[robcasloz]

NeverBranch nodes introduce fake branches out of infinite loops to simplify intermediate optimizations that assume full reachability:

PhaseCFG::convert_NeverBranch_to_Goto() transforms such nodes into simple Goto nodes, removing the fake branch and thus making its destination block unreachable:

This changeset removes the unreachable destination block created by PhaseCFG::convert_NeverBranch_to_Goto() from the list of basic blocks:

Removing unreachable blocks created by PhaseCFG::convert_NeverBranch_to_Goto() simplifies late control-flow analysis (for example, recomputing dominators during the output phase as required for late barrier analysis in Generational ZGC) and slightly reduces the size of the generated code.

Testing

• hs-tier1-3 (windows-x64, linux-x64, linux-aarch64, and macosx-x64; release and debug mode). The changeset does not introduce new test cases because it is already exercised by at least two existing ones (compiler/inlining/StringConcatInfiniteLoop.java, compiler/loopopts/TestInfiniteLoopNotInnerMost.java).
• fuzzing (~30 min. on each platform).

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8292285: C2: remove unreachable block after NeverBranch-to-Goto conversion

Reviewers

• Tobias Hartmann (@TobiHartmann - Reviewer) ⚠️ Review applies to e633fbec
• Christian Hagedorn (@chhagedorn - Reviewer) ⚠️ Review applies to e633fbec
• Vladimir Kozlov (@vnkozlov - Reviewer)

Reviewing

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[bridgekeeper]

👋 Welcome back rcastanedalo! 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]

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[mlbridge]

Webrevs

• 01: Full - Incremental (966a1279)
• 00: Full (e633fbec)

[TobiHartmann]

Looks good to me.

[openjdk]

@robcasloz 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:

8292285: C2: remove unreachable block after NeverBranch-to-Goto conversion

Reviewed-by: thartmann, chagedorn, kvn

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 26 new commits pushed to the master branch:

• 3e12241: 8290775: Some doc errors in DerOutputStream.java
• ec53fba: 8292361: Build failure on PPC64 BE after JDK-8290840
• 4cc6cb9: 8290041: ModuleDescriptor.hashCode is inconsistent
• d1edda8: 8292338: aarch64: Use cbnz instruction in gen_continuation_enter when possible
• 21f4eb2: 6521141: DebugGraphics NPE @ setFont();
• 6e6ae59: 8292286: Convert PlaceholderTable to ResourceHashtable
• ea2c82e: 8291949: Unexpected extending of SupportedGroups
• b5707b0: 8292196: Reduce runtime of java.util.regex microbenchmarks
• b00eede: 8291511: Redefinition of EXIT_FAILURE in libw2k_lsa_auth
• 3a09077: 8291337: Reduce runtime of vm.lamdba microbenchmarks
• ... and 16 more: https://git.openjdk.org/jdk/compare/871b7dab143fd92f14724563d448126a537fa5d1...master

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.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

[robcasloz]

Thanks for reviewing, Tobias!

[chhagedorn]

Looks good to me, too!

[robcasloz]

Looks good to me, too!

Thanks, Christian!

[robcasloz]

Thanks for reviewing, Vladimir!

[robcasloz]

/integrate

[openjdk]

Going to push as commit a25e1dc.
Since your change was applied there have been 32 commits pushed to the master branch:

• 1ef4e48: 8273860: Javadoc Deprecated API list should not use italic font for description column
• 0cc66ae: 8285790: AArch64: Merge C2 NEON and SVE matching rules
• da477b1: 8292509: ProblemList java/lang/invoke/lambda/LogGeneratedClassesTest.java on windows
• 01b87ba: 8289616: Drop use of Thread.stop in AppContext
• e44e3f0: 8289106: Add model of class file versions to core reflection
• c3d3662: 8292313: 2 runtime/cds/appcds tests fail after JDK-8284313
• 3e12241: 8290775: Some doc errors in DerOutputStream.java
• ec53fba: 8292361: Build failure on PPC64 BE after JDK-8290840
• 4cc6cb9: 8290041: ModuleDescriptor.hashCode is inconsistent
• d1edda8: 8292338: aarch64: Use cbnz instruction in gen_continuation_enter when possible
• ... and 22 more: https://git.openjdk.org/jdk/compare/871b7dab143fd92f14724563d448126a537fa5d1...master

Your commit was automatically rebased without conflicts.

[openjdk]

@robcasloz Pushed as commit a25e1dc.

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