RFR: 8354323: Safeguard SwitchBootstraps.typeSwitch when used outside the compiler

[Chen Liang]

On Wed, 7 May 2025 11:57:02 GMT, Aggelos Biboudis <abimpoudis at openjdk.org> wrote:

> While the compiler does not allow invalid queries to flow into `SwitchBootstraps:typeSwitch`, a library user could do that and `typeSwitch` does not prevent such usage pattern errors resulting in erroneous evaluation.
> 
> For example this is not valid Java (and protected) by javac:
> 
> 
> byte b = 1;
> switch (b) {
>     case String s -> System.out.println("How did we get here? byte is " + s.getClass());
> }
> 
> 
> but this is a valid call (and not protected):
> 
> 
> CallSite shortSwitch = SwitchBootstraps.typeSwitch(
>     MethodHandles.lookup(), 
>     "", 
>     MethodType.methodType(int.class, short.class, int.class),  // models (short, int) -> int
>     String.class);
> 
> 
> The `SwitchBootstraps.typeSwitch` returns wrong result since the code was reasoning erroneously that this pair was unconditionally exact. 
> 
> This PR proposes to add the safety check in unconditional exactness which will return false in erroneous pairs and then the actual check will be delegated to `instanceof`. For the case of erroneous pairs with primitive `boolean`s there is a check in the beginning of the type switch skeleton.

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java line 553:

> 551:                         continue;
> 552:                     }
> 553:                     else if (unconditionalExactnessCheck(selectorType, classLabel)) {

Can we merge this into `isNotValidPair(...) || unconditionalExactnessCheck(...)` then do nothing? The next label is already in theory immediately bound to the instruction after goto.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25090#discussion_r2078046340