Reviewer of https://github.com/openjdk/jdk/pull/20395 raises the question:

if CAS failed, no store actually happened, and we don't need the post-store barrier, right? I.e.:

if (ShenandoahCardBarrier && (result == compare_value)) {
  ...
}

I don't expect this to matter in this barrier set, but I think C2 side would benefit from this. (I remember doing this in C2 CAS intrinsic expansion...)