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

[Aleksey Shipilev]

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.

You know what, this is fine as it is.

The whole business with `ShenandoahForwarding::klass` is super-duper-awkward, and verifier already touches the forwardees using that method in places where it really should not. It needs a more comprehensive fix for `+UCOH`.

It is a mess, but not your mess. So integrate this one, we will figure out `ShenandoahForwarding::klass` after this.

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Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26117#pullrequestreview-2997654169