RFR: 8361363: ShenandoahAsserts::print_obj() does not work for forwarded objects and UseCompactObjectHeaders

[Thomas Stuefe]

On Thu, 3 Jul 2025 14:22:32 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> `ShenandoahAsserts::print_obj()` does not work for COH if the object is forwarded, since the mark word is overwritten with a forwarding pointer.
> 
> Depending on the size of the non-null bits of the forwardee (whether it spills into the nKlass bits), nKlass is either zero or garbage. So we get either an "assert(!is_null(v)) failed: narrow klass value can never be zero", or a SIGSEGV. 
> 
> Fix is trivial: don't use obj->klass() directly but get it from ShenandoahForwarding.

Ping @shipilev, maybe? Or @rkennke? Its really trivial.

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PR Comment: https://git.openjdk.org/jdk/pull/26117#issuecomment-3047877896