hg: valhalla/valhalla: [lworld] Introduce a new annotation java.lang.ValueBased to distinguish migrating value based classes from pure value types and implement leniet semantics for them; Also flip the default for ACC_Flattenable (original) (raw)

Srikanth srikanth.adayapalam at oracle.com
Mon Apr 2 12:49:17 UTC 2018

On Monday 02 April 2018 05:51 PM, srikanth.adayapalam at oracle.com wrote:

Changeset: 32ff82e6b770 Author: sadayapalam Date: 2018-04-02 17:50 +0530 URL: http://hg.openjdk.java.net/valhalla/valhalla/rev/32ff82e6b770

[lworld] Introduce a new annotation java.lang.ValueBased to distinguish migrating value based classes from pure value types and implement leniet semantics for them; Also flip the default for ACCFlattenable

Notes:

    - Introduces a new annotation java.lang.ValueBased with which a __ByValue class may be decorated to indicate that it is a value based class evolving to become a value type.     - A __ByValue class without this annotation is considered a "pure" value type.     - ACC_FLATTENABLE defaults are flipped. A field whose type is a pure value type will automatically be flagged ACC_FLATTENABLE in its containing instance.     - For value based classes evolving to value types, the default flattenable status is NOT flattenable. Request flattening with the field modifier __Flattenable.     - A field whose type is a pure value type can opt out of flattening by using the new field modifier __NotFlattened     - Javac will redundantly accept __Flattenable with pure value typed fields and __NotFlattened with VBC typed fields.     - @ValueBased instances can be assigned null; null can be cast to them - this would result in a warning at compile time and not an error. (the unrealistic code pattern "null instanceof VBC" would also trigger a warning instead of error)     - Javac continues to implement a strict mode for pure value types (also see below)     - The lenient mode for @ValueBased classes is only for null assignment and (the unrealistic) casting of nulls to @ValueBased types. All identity related violations still result in an error. So does comparison of @ValueBased type instances. (Rationale: Karen originally asked for null related violations to be tolerated on the grounds that the documentation for value based classes does not expressly forbid these. However since == and != are expressly called out as inadvisable operations on value based classes, I have continued to treat them as errors - but I can be convinced to relax this)      - This change set does not identify and annotate the classes in JDK that are value based with @ValueBased. Only introduces the annotation and the new semantics for it.     - When an Object instance or a interface instance is cast to a value type, javac will insert a null check IFF the target of the cast is NOT @ValueBased.     - When compiling at a source level that is NOT __ByValue aware, if javac encounters a class file that has the ACC_VALUE bit set, the class will be demoted to @ValueBased even though it may not have been annotated with @ValueBased. This will result in null violation warnings to be emitted as appropriate.      - As of this change set the @ValueBased annotation is supposed to always be paired with __ByValue modifier (not checked). A new lint mode in consideration for JDK11 will implement sanity checks for @ValueBased classes that are candidates for evolution at a future date and so are not yet ready to be modified with __ByValue.     - Javac does not inject null checks with any opcode other than checkcast - see earlier mail raising some questions about the necessity of this with other opcodes.     - Expect some surprises in tests due to the flipping of defaults for flattening.

Karen, please let me know if this adequately covers your proposal.

Thanks! Srikanth

+ src/java.base/share/classes/java/lang/ValueBased.java ! src/java.compiler/share/classes/javax/lang/model/element/Modifier.java ! src/jdk.compiler/share/classes/com/sun/tools/javac/code/Flags.java ! src/jdk.compiler/share/classes/com/sun/tools/javac/code/Symtab.java ! src/jdk.compiler/share/classes/com/sun/tools/javac/code/Types.java ! src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Attr.java ! src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java ! src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Lower.java ! src/jdk.compiler/share/classes/com/sun/tools/javac/jvm/ClassReader.java ! src/jdk.compiler/share/classes/com/sun/tools/javac/parser/JavacParser.java ! src/jdk.compiler/share/classes/com/sun/tools/javac/parser/Tokens.java ! src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties ! src/jdk.jshell/share/classes/jdk/jshell/CompletenessAnalyzer.java + test/langtools/tools/javac/diags/examples/BadValueBasedAnno.java + test/langtools/tools/javac/diags/examples/SuspiciousNullMix.java + test/langtools/tools/javac/valhalla/lworld-values/BadValueBased.java + test/langtools/tools/javac/valhalla/lworld-values/BadValueBased.out ! test/langtools/tools/javac/valhalla/lworld-values/CastNullCheckTest.java + test/langtools/tools/javac/valhalla/lworld-values/CheckDefaultFlattenable.java ! test/langtools/tools/javac/valhalla/lworld-values/CheckNullCastable.out + test/langtools/tools/javac/valhalla/lworld-values/DemoteToValueBasedTest.java + test/langtools/tools/javac/valhalla/lworld-values/DemoteToValueBasedTest.out + test/langtools/tools/javac/valhalla/lworld-values/DemoteToValueBasedTest10.out ! test/langtools/tools/javac/valhalla/lworld-values/FlattenableFlagTest.java ! test/langtools/tools/javac/valhalla/lworld-values/Point.java + test/langtools/tools/javac/valhalla/lworld-values/ValueBasedWarningsTest.java + test/langtools/tools/javac/valhalla/lworld-values/ValueBasedWarningsTest.out

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