RFR: 8270098: ZGC: ZBarrierSetC2::clone_at_expansion fails with "Guard against surprises" assert

[Tobias Hartmann]

On Mon, 9 Aug 2021 09:44:17 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> The fix for [JDK-8268125](https://bugs.openjdk.java.net/browse/JDK-8268125) ([PR 53](https://git.openjdk.java.net/jdk17/pull/53)) assumed that only dead code can have a clone array with an unknown array ptr type at expansion time. As `TestObjectArrayClone::testCloneObject` proves, that is not true because we can invoke `Object::clone` on any object, including arrays.
> 
> As a result, invoking `Object::clone` on an Object array leads to triggering the "Guard against surprises" assert in debug and to incorrectly using the `T_LONG` copy stub (without GC barriers) in product. I was not able to trigger a crash in product but a corresponding runtime check (see [patch](https://bugs.openjdk.java.net/secure/attachment/95954/runtime_check.patch)) triggered reliably.
> 
> The fix is to fall back to `ZBarrierSetRuntime::clone` if the array ptr type is unknown. I will backport this fix to JDK 17u.
> 
> Thanks,
> Tobias

Thanks for the review, Vladimir!

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PR: https://git.openjdk.java.net/jdk/pull/5048