RFR: 8283574: Use Klass::_id for type checks in the C++ code

[Thomas Schatzl]

On Wed, 23 Mar 2022 14:42:06 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> We have at least three ways to check the type of a given Klass*:
> 1) Using the Klass::_id field
> 2) Using the layout helper
> 3) Using the InstanceKlass::_kind field
> 
> The Klass::_id field was something that was added when we rewrote the oop_oop_iterate dispatch mechanism, but the other mechanisms where left in place.
> 
> The current Loom code uses both (2) and (3) every time a the code checks if an object is of type InstanceStackChunkKlass. In the Loom repository I intend to reduce that check to be a single test against the (1) field. To keep the code unified, and simpler, I changed all C++ Klass type checks to use (1).
> 
> I propose that we upstream this change to the mainline, to slightly reduce the Loom diff.

Lgtm.

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Marked as reviewed by tschatzl (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/7922