On Fri, 5 Aug 2022 16:12:08 GMT, Jan Lahoda <jlah...@openjdk.org> wrote:
> The pattern matching switches are using a bootstrap method
> `SwitchBootstrap.typeSwitch` to implement the jumps in the switch. Basically,
> for a switch like:
>
> switch (obj) {
> case String s when s.isEmpty() -> {}
> case String s -> {}
> case CharSequence cs -> {}
> ...
> }
>
>
> this method will produce a MethodHandle that will be analyze the provided
> selector value (`obj` in the example), and will return the case index to
> which the switch should jump. This method also accepts a (re)start index for
> the search, which is used to handle guards. For example, if the `s.isEmpty()`
> guard in the above sample returns false, the matching is restarted on the
> next case.
>
> The current implementation is fairly slow, it basically goes through the
> labels in a loop. The proposal here is to replace that with a MethodHandle
> structure like this:
>
> obj == null ? -1
> : switch (restartIndex) {
> case 0 -> obj instanceof String ? 0 : obj instanceof
> CharSequence ? 2 : ... ;
> case 1 -> obj instanceof String ? 1 : obj instanceof
> CharSequence ? 2 : ... ;
> case 2 -> obj instanceof CharSequence ? 2 : ... ;
> ...
> default -> <labels-count>;
> }
>
>
> This appear to run faster than the current implementation, using testcase
> similar to the one used for https://github.com/openjdk/jdk/pull/9746 , these
> are the results
>
> PatternsOptimizationTest.testLegacyIndyLongSwitch thrpt 25 1515989.562
> ± 32047.918 ops/s
> PatternsOptimizationTest.testHandleIndyLongSwitch thrpt 25 2630707.585
> ± 37202.210 ops/s
>
> PatternsOptimizationTest.testLegacyIndyShortSwitch thrpt 25 6789310.900
> ± 61921.636 ops/s
> PatternsOptimizationTest.testHandleIndyShortSwitch thrpt 25 10771729.464
> ± 69607.467 ops/s
>
>
> The "LegacyIndy" is the current implementation, "HandleIndy" is the one
> proposed here. The translation in javac used is the one from #9746 in all
> cases.
> Would it be possible to add a special case where the labels are all the same
> type? Like all labels are Class.class or Object.class? While testing out the
> pattern matching, I've found myself doing the following
>
> ```java
> switch (o) {
> case Class<?> c when c == Object.class -> {}
> case Class<?> c when c == Integer.class -> {}
> case Class<?> c when c == String.class -> {}
> case Class<?> c when c == Double.class -> {}
> // ...
> }
> ```
I think that there may eventually be some improvements, although the current
round of patches is not intended to improve this pattern, sorry. Also, I would
not expect better behavior that a sequence of ifs.
-------------
PR: https://git.openjdk.org/jdk/pull/9779
