[jdk17] RFR: 8268766: Desugaring of pattern matching enum switch should be improved [v4]

[Maurizio Cimadamore]

On Tue, 22 Jun 2021 19:41:55 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

>> Currently, an enum switch with patterns is desugared in a very non-standard, and potentially slow, way. It would be better to use the standard `typeSwitch` bootstrap to classify the enum constants. The bootstrap needs to accept enum constants as labels in order to allow this. A complication is that if an enum constant is missing, that is not an incompatible change for the switch, and the switch should simply work as if the case for the missing constant didn't exist. So, the proposed solution is to have a new bootstrap `enumConstant` that converts the enum constant name to the enum constant, returning `null`, if the constant does not exist. It delegates to `ConstantBootstraps.enumConstant` to do the actual conversion. And `typeSwitch` accepts `null`s as padding.
>> 
>> How does this look?
>
> Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Updating javadoc, as suggested.

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java line 175:

> 173:      * Bootstrap method for linking an {@code invokedynamic} call site that
> 174:      * implements a {@code switch} on a target of an enum type. The static
> 175:      * arguments are used to encode the case labels associated to the switch

`String` and `Class` should appear in code blocks perhaps, or link tags? Also, I think this text could be improved by splitting the sentence by using a bullet list:


The static arguments are used to encode the case labels associated to the switch
construct, where each label can be encoded in two ways:
* as a String value, which represents the name of the enum constant associated with the label
* as a Class value, which represents the enum type associated with a type test pattern

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java line 218:

> 216:      * {@code Class} of the target enum type.
> 217:      * @throws Throwable if there is any error linking the call site
> 218:      * @jvms 4.4.6 The CONSTANT_NameAndType_info Structure

Not sure what we do for other bootstraps, but I wonder if the throws clause here should be more specific - for instance the LambdaMetafactory tells when an exception is gonna be triggered. Here it seems like there are a couple of cases which can be called out, where we know that linkage is going to fail.

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PR: https://git.openjdk.java.net/jdk17/pull/81