8273328: Compiler implementation for Pattern Matching for switch (Second Preview) #6209

lahodaj · 2021-11-02T10:51:58Z

The specification for pattern matching for switch (preview)[1] has been updated with two changes:
-any type of pattern (including guarded patterns) dominates constant cases. Dominance across pattern/non-constant cases is unchanged. (See the change in Attr.java.)
-for sealed hierarchies, it may happen some of the subtypes are impossible (not castable) to the selector type. Like, for example:

sealed interface I<T> {}
final class A implements I<String> {}
final class B<T> implements I<T> {}
...
I<Integer> i = ...;
switch (i) {
     case A a -> {} //case not allowed as I<Integer> is not castable to A
     case B b -> {}
}

But, the exhaustiveness checks in JDK 17 required all the permitted subclasses, including A, are covered by the switch, which forced a default clause in place of case A a for cases like this. The updated specification excludes such impossible permitted subclasses from exhaustiveness checks, so the default is no longer needed and this:

I<Integer> i = ...;
switch (i) {
     case B b -> {}
}

compiles OK.
(See the change in Flow.java.)

[1] http://cr.openjdk.java.net/~gbierman/jep420/jep420-20211020/specs/patterns-switch-jls.html

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JDK-8273328: Compiler implementation for Pattern Matching for switch (Second Preview)

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Vicente Romero (@vicente-romero-oracle - Reviewer)
Maurizio Cimadamore (@mcimadamore - Reviewer)

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bridgekeeper · 2021-11-02T10:53:36Z

👋 Welcome back jlahoda! 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 · 2021-11-02T10:55:59Z

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mlbridge · 2021-11-02T10:59:35Z

Webrevs

00: Full (bdfc894f)

vicente-romero-oracle

looks good to me

openjdk · 2021-11-02T19:34:14Z

@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:

8273328: Compiler implementation for Pattern Matching for switch (Second Preview)

Reviewed-by: vromero, mcimadamore

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At the time when this comment was updated there had been 237 new commits pushed to the master branch:

9160743: 8276058: Some swing test fails on specific CI macos system
8193800: 8274179: AArch64: Support SVE operations with encodable immediates
b8453eb: 8275007: Java fails to start with null charset if LC_ALL is set to certain locales
231fb61: 8276970: Default charset for PrintWriter that wraps PrintStream
29e552c: 8272358: Some tests may fail when executed with other locales than the US
ce4471f: 8277346: ProblemList 7 serviceability/sa tests on macosx-x64
45a60db: 8277045: G1: Remove unnecessary set_concurrency call in G1ConcurrentMark::weak_refs_work
6bb0462: 8277224: sun.security.pkcs.PKCS9Attributes.toString() throws NPE
d8c0280: 8277316: ciReplay: dump_replay_data is not thread-safe
007ad7c: 8277303: Terminology mismatch between JLS17-3.9 and SE17's javax.lang.model.SourceVersion method specs
... and 227 more: https://git.openjdk.java.net/jdk/compare/5bb1992b8408a0d196b1afa308bc00d007458dbd...master

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mcimadamore

Looks good. Left one comment about the logic in Flow.

mcimadamore · 2021-11-04T12:59:44Z

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java

@@ -784,7 +784,7 @@ private void transitiveCovers(Set<Symbol> covered) {
            }
        }

-        private boolean isTransitivelyCovered(Symbol sealed, Set<Symbol> covered) {
+        private boolean isTransitivelyCovered(Type seltype, Symbol sealed, Set<Symbol> covered) {
            DeferredCompletionFailureHandler.Handler prevHandler =


Why do we need to guard against completion failures here in Flow?

Consider a case like this:

sealed interface I permits A, B {} //A and B defined ... I i = null; switch (i) { case A -> {} }

This will produce an error, because the switch is not exhaustive. Now, consider, in addition, that B is missing. It feels a bit harsh to produce an error about the missing B permitted subclass (possibly producing that error instead of the exhaustiveness error), as it seems plausible permitted subclasses may be missing during compilation.

Uhm - I get what you say. But isn't sealing meant to be used within a single maintenance domain? If B is missing, how the heck did we get to Flow in the first place (since for sealed classes we check that permitted subclasses extend the sealed class being defined)

Uhm - I get what you say. But isn't sealing meant to be used within a single maintenance domain? If B is missing, how the heck did we get to Flow in the first place (since for sealed classes we check that permitted subclasses extend the sealed class being defined)

Ok - maybe you are considering the case where I is used in a compilation unit which did not defined it. And which cannot (for some reason) see B. Seems cornery, but what I'm mostly worried about is that it's not (I think) uniform with the rest of the javac code dealing with sealed classes. For instance, look at the cast conversion rules in Types::areDisjoint - in that case there's no protection against missing types.

I think the compiler code should be consistent in how we reason about sealed permitted types?

Right, the case is where the sealed class/interface is used from some other project. It is somewhat cornery, but might happen even for us - consider an sealed API interface in the JDK, which has permitted classes that are in an implementation package. The --release data normally do not include non-API packages, so when looking at the API interface, we may encounter a class that does not exist.

I didn't realize there is the same issue with areDisjoint, but you are right it suffers from the problem and that it should be consistent. I'll take a look at that, possibly separately, if that's OK.

Yep - fine to postpone!

lahodaj · 2021-11-05T08:30:43Z

/csr needed

openjdk · 2021-11-05T08:32:01Z

@lahodaj this pull request will not be integrated until the CSR request JDK-8275408 for issue JDK-8273328 has been approved.

lahodaj · 2021-11-24T10:07:01Z

/integrate

openjdk · 2021-11-24T10:08:00Z

Going to push as commit d085c2b.
Since your change was applied there have been 322 commits pushed to the master branch:

6d73460: 8277399: ZGC: Move worker thread logging out of gc+phase=debug
712b875: 8277397: ZGC: Add JFR event for temporary latency measurements
7b2d823: 8277503: compiler/onSpinWait/TestOnSpinWaitAArch64DefaultFlags.java failed with "OnSpinWaitInst with the expected value 'isb' not found."
7cb56a2: 8265796: vmTestbase/nsk/jdi/ObjectReference/referringObjects/referringObjects002/referringObjects002.java fails when running with JEP 416
24e586a: 8276764: Enable deterministic file content ordering for Jar and Jmod
ea85e01: 8271623: Omit enclosing instance fields from inner classes that don't use it
0320672: 8277451: java.lang.reflect.Field::set on static field with invalid argument type should throw IAE
e8acac2: 8277350: runtime/jni/checked/TestPrimitiveArrayCriticalWithBadParam.java times out
8a44e09: 8268725: jshell does not support the --enable-native-access option
7b67a49: 8261847: performance of java.lang.Record::toString should be improved
... and 312 more: https://git.openjdk.java.net/jdk/compare/5bb1992b8408a0d196b1afa308bc00d007458dbd...master

Your commit was automatically rebased without conflicts.

openjdk · 2021-11-24T10:08:22Z

@lahodaj Pushed as commit d085c2b.

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

Jan Lahoda added 3 commits Oct 14, 2021

Updating to reflect the new spec.

277c318

Merge branch 'master' into JDK-8273328

91a40f4

Fixing tests.

bdfc894

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8273328: Compiler implementation for Pattern Matching for switch (Second Preview) #6209

8273328: Compiler implementation for Pattern Matching for switch (Second Preview) #6209

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8273328: Compiler implementation for Pattern Matching for switch (Second Preview) #6209

8273328: Compiler implementation for Pattern Matching for switch (Second Preview) #6209

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lahodaj commented Nov 2, 2021 • edited by openjdk[bot] bot

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