8370880: [lworld] Split ciObjArrayKlass into ciRefArrayKlass and ciFlatArrayKlass

[merykitty]

Hi,

This PR splits ciObjArrayKlass into ciRefArrayKlass and ciFlatArrayKlass, aligns the hierarchy with the corresponding types of the VM.

I also had to fix GraphKit::cast_to_flat_array, currently it incorrectly assumes that the cast is to an exact type, splitting into cast_to_flat_array and cast_to_flat_array_exact seems to make it clearer.

Please kindly review, thanks a lot.

---

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Issue

• JDK-8370880: [lworld] Split ciObjArrayKlass into ciRefArrayKlass and ciFlatArrayKlass (Enhancement - P2)

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[bridgekeeper]

👋 Welcome back qamai! A progress list of the required criteria for merging this PR into lworld will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
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[mlbridge]

Webrevs

• 01: Full - Incremental (be614362)
• 00: Full (b401f2ff)

[merykitty]

There are still a lot of refactoring to be done regarding moving elements from ciArrayKlass to ciObjArrayKlass since now it is the new common ancestor of all arrays of objects.

[TobiHartmann]

Thank you for working on this, Quan-Anh! Testing looks good except for this:

compiler/valhalla/inlinetypes/TestArrayNullMarkers.java#AVF-nAVF
-XX:+IgnoreUnrecognizedVMOptions -XX:-TieredCompilation -XX:-DoEscapeAnalysis -XX:+AlwaysIncrementalInline

# A fatal error has been detected by the Java Runtime Environment:
#
#  Internal Error (/workspace/open/src/hotspot/share/ci/ciMetadata.hpp:105), pid=418142, tid=418159
#  assert(is_flat_array_klass()) failed: bad cast
#
# JRE version: Java(TM) SE Runtime Environment (26.0) (fastdebug build 26-jep401ea2-2025-11-17-0811237.tobias.hartmann.valhalla2)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 26-jep401ea2-2025-11-17-0811237.tobias.hartmann.valhalla2, mixed mode, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V  [libjvm.so+0x1ca876a]  TypeAryPtr::flat_layout_helper() const+0x7a

Current CompileTask:
C2:2887  380 % !b        compiler.valhalla.inlinetypes.TestArrayNullMarkers::main @ 698 (2967 bytes)

Stack: [0x00007f5639cfe000,0x00007f5639dfe000],  sp=0x00007f5639df8240,  free space=1000k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V  [libjvm.so+0x1ca876a]  TypeAryPtr::flat_layout_helper() const+0x7a  (ciMetadata.hpp:105)
V  [libjvm.so+0x102a8a9]  GraphKit::get_layout_helper(Node*, int&)+0x299  (graphKit.cpp:4100)
V  [libjvm.so+0x102cd43]  GraphKit::new_array(Node*, Node*, int, Node**, bool, Node*)+0x43  (graphKit.cpp:4345)
V  [libjvm.so+0x158b712]  LibraryCallKit::inline_newArray(bool, bool)+0x242  (library_call.cpp:4904)
V  [libjvm.so+0x15b2c6a]  LibraryIntrinsic::generate(JVMState*)+0x22a  (library_call.cpp:130)
V  [libjvm.so+0xdbc50f]  Parse::do_call()+0xebf  (doCall.cpp:771)
V  [libjvm.so+0x18fee88]  Parse::do_one_bytecode()+0x4c8  (parse2.cpp:3526)
V  [libjvm.so+0x18e6007]  Parse::do_one_block()+0x357  (parse1.cpp:1703)
V  [libjvm.so+0x18e7300]  Parse::do_all_blocks()+0x130  (parse1.cpp:760)
V  [libjvm.so+0x18eae81]  Parse::Parse(JVMState*, ciMethod*, float)+0xea1  (parse1.cpp:664)
V  [libjvm.so+0x9e1975]  ParseGenerator::generate(JVMState*)+0x135  (callGenerator.cpp:99)
V  [libjvm.so+0x9e6ba3]  CallGenerator::do_late_inline_helper()+0xa03  (callGenerator.cpp:751)
V  [libjvm.so+0xbdc968]  Compile::inline_incrementally_one()+0x1b8  (compile.cpp:2614)
V  [libjvm.so+0xbde0ab]  Compile::inline_incrementally(PhaseIterGVN&)+0x38b  (compile.cpp:2707)
V  [libjvm.so+0xbe0c9e]  Compile::Optimize()+0x47e  (compile.cpp:2843)
V  [libjvm.so+0xbe45c5]  Compile::Compile(ciEnv*, ciMethod*, int, Options, DirectiveSet*)+0x1da5  (compile.cpp:879)
V  [libjvm.so+0x9de490]  C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x4e0  (c2compiler.cpp:149)
V  [libjvm.so+0xbf3e20]  CompileBroker::invoke_compiler_on_method(CompileTask*)+0x780  (compileBroker.cpp:2345)
V  [libjvm.so+0xbf5680]  CompileBroker::compiler_thread_loop()+0x530  (compileBroker.cpp:1989)
V  [libjvm.so+0x118d15b]  JavaThread::thread_main_inner()+0x13b  (javaThread.cpp:772)
V  [libjvm.so+0x1c68ee6]  Thread::call_run()+0xb6  (thread.cpp:243)
V  [libjvm.so+0x1893bc8]  thread_native_entry(Thread*)+0x128  (os_linux.cpp:883)

[merykitty]

Thanks for your testing. Unfortunately, the issue cuts pretty deep. We have multiple potential inconsistencies when dealing with _klass such as when trying to obtain a flat TypeAryPtr from a non-flat TypeAryPtr. I think it can be addressed as follow-up issues. To fix the immediate crash, I choose to fix a particular issue, that is we can only compute the _klass if we have klass_is_exact.

[merykitty]

Too bad it seems pretty hard to walk on the thin line right now. I think it is better to tighten the type system around arrays first.

[TobiHartmann]

Right, I think so too, the latest version hits more issues :(