JDK-8302202: Incorrect desugaring of null-allowed nested patterns

[lahodaj]

Considering code like:

    private String test1b(Object i) {
        return switch (i) {
            case R1(Object o) when o == null -> "R1(null)";
            case R1(Object o) -> "R1(!null)";
            default -> "default";
        };
    }

javac will try to factor-out the common prefix, i.e. R1, and produce something along these lines:

        switch (i) {
            case R1 $r:
                Object c = $r.c();
                switch (c) {
                    case Object o when o == null: yield "R1(null)";
                    case Object o: yield "R1(!null)";
                }
            default -> "default";
        };

The problem with this code is that both cases in the nested switch must match null, but the bootstrap protocol only allows one case to match null (the SwitchBootstraps.typeSwitch method will return -1 for null). So this translation is broken, and will not match the first case if the component is null.

There are multiple ways to solve this problem, but the proposal here is change the way we accumulate the cases from which we factor out the common prefix, by stopping the accumulation after the first nullable case.

Another related issue is that when factoring out the common prefix, if the last case in the nested switch is has an unconditional pattern, but also has a guard, we need to generate a default to continue properly in the outer switch. Currently, the default is not generated when there's an unconditional pattern in the case, despite having a guard, and hence the case as a whole is not unconditional.

---

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Issue

• JDK-8302202: Incorrect desugaring of null-allowed nested patterns

Reviewers

• Vicente Romero (@vicente-romero-oracle - Reviewer)

Reviewing

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[mlbridge]

Webrevs

• 00: Full (4d1c8158)

[vicente-romero-oracle]

looks sensible

[openjdk]

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

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

8302202: Incorrect desugaring of null-allowed nested patterns

Reviewed-by: vromero

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At the time when this comment was updated there had been 89 new commits pushed to the master branch:

• 574b48c: 8302678: validate_source fails after JDK-8293313
• 90e0922: 8293313: NMT: Rework MallocLimit
• f558a6c: 8298276: JFR: Update NMT events to make use of common periodic timestamp feature
• a58fa6e: 8302514: Misleading error generated when empty class file encountered
• 3cc459b: 8302595: use-after-free related to GraphKit::clone_map
• 2e3cea0: 8302594: use-after-free in Node::destruct
• 9b911b4: 8301959: Compile command in compiler.loopopts.TestRemoveEmptyCountedLoop does not work
• c29a136: 8291735: methods_do() always run at exit
• 687a461: 8302464: Parallel: Remove unreachable code in callers of numa_has_static_binding
• 84c058b: 8300549: JFileChooser Approve button tooltip is null in Aqua L&F in CUSTOM_DIALOG mode
• ... and 79 more: https://git.openjdk.org/jdk/compare/723433dbebc6b542dab445a89b7437149730b858...master

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[biboudis]

After running some more examples, the patch looks good to me too 💯

[lahodaj]

/integrate

[openjdk]

Going to push as commit dc55a7f.
Since your change was applied there have been 100 commits pushed to the master branch:

• c4ffe4b: 8301494: Replace NULL with nullptr in cpu/arm
• 4f1cffd: 8302674: Parallel: Remove unused methods in MutableNUMASpace
• c91cd28: 8301481: Replace NULL with nullptr in os/windows
• 47ca577: 8301491: C2: java.lang.StringUTF16::indexOfChar intrinsic called with negative character argument
• 49eb68b: 8296158: Refactor the verification of CDS region checksum
• 655a712: 8301444: Port fdlibm hyperbolic transcendental functions to Java
• b242eef: 8280419: Remove dead code related to VerifyThread and verify_thread()
• 4ce493f: 8302225: SunJCE Provider doesn't validate key sizes when using 'constrained' transforms for AES/KW and AES/KWP
• a39cf2e: 8301753: AppendFile/WriteFile has differences between make 3.81 and 4+
• de80dd9: 8296010: AssertionError in annotationTargetType
• ... and 90 more: https://git.openjdk.org/jdk/compare/723433dbebc6b542dab445a89b7437149730b858...master

Your commit was automatically rebased without conflicts.

[openjdk]

@lahodaj Pushed as commit dc55a7f.

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