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

[Aggelos Biboudis]

On Sun, 11 May 2025 21:45:46 GMT, Luca Kellermann <duke 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 782:
> 
>> 780:         else if (selectorType.equals(targetType) ||
>> 781:                 (targetType.isPrimitive() && selectorType.isPrimitive() &&
>> 782:                     ((selectorType.equals(byte.class) && !targetType.equals(char.class)) ||
> 
> Will `unconditionalExactnessMatch(byte.class, boolean.class` return `true`? I think it shouldn't, even if `isNotValidPair` is called before.

`unconditionalExactnessMatch` needs to return an answer whether a pair is unconditional assuming it is applicable. Fusing them would need to elaborate the type of the returned answer.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25090#discussion_r2086329089