[15] RFR(S): 8239142: C2's UseUniqueSubclasses optimization is broken for array accesses

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8239142
http://cr.openjdk.java.net/~thartmann/8239142/webrev.00/

While investigating a performance issue that Sergey reported with Valhalla, I've noticed that C2's
UseUniqueSubclasses optimization for array accesses is broken since JDK 11 b15. When accessing an
array of an abstract klass type that has only one subtype, C2 registers a dependency and narrows the
type of the array elements to that one subtype:
http://hg.openjdk.java.net/jdk/jdk/file/690fc7e5a90f/src/hotspot/share/opto/parse2.cpp#l126

The fix for JDK-8202377 [1] added code that overwrites the elemtype and therefore disables this
optimization: http://hg.openjdk.java.net/jdk/jdk/rev/ffa644980dff#l27.26

My fix simply removes the lines that overwrite the elemtype. I've manually verified that the
generated code is now equal to before JDK 11 b15.

Thanks,
Tobias

[1] https://bugs.openjdk.java.net/browse/JDK-8202377