With debug build, run with Valhalla disabled:

> make exploded-test TEST_VM_OPTS=" -XX:-EnableValhalla" TEST=compiler/arraycopy/TestEliminateArrayCopy.java

Results in:

# Internal Error (/localhome/davids/hotspot/git/valhalla/open/src/hotspot/share/opto/library_call.cpp:226), pid=2999585, tid=2999599
# assert((EnableValhalla && (FlatArrayElementMaxSize != 0))) failed: can never be flattened
#
# JRE version: Java(TM) SE Runtime Environment (16.0) (slowdebug build 16-lworld3ea+0-2020-08-24-0632439.davids...)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (slowdebug 16-lworld3ea+0-2020-08-24-0632439.davids..., mixed mode, tiered, compressed oops, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0xd47264] LibraryCallKit::generate_flatArray_guard(Node*, RegionNode*)+0x3e
V [libjvm.so+0xd47264] LibraryCallKit::generate_flatArray_guard(Node*, RegionNode*)+0x3e
V [libjvm.so+0xd39016] LibraryCallKit::inline_native_clone(bool)+0x5c8
V [libjvm.so+0xd28941] LibraryCallKit::try_to_inline(int)+0x18eb
V [libjvm.so+0xd26469] LibraryIntrinsic::generate(JVMState*)+0x175
V [libjvm.so+0x836284] Parse::do_call()+0xc24
V [libjvm.so+0xf76ab0] Parse::do_one_bytecode()+0x4586
V [libjvm.so+0xf60100] Parse::do_one_block()+0x4dc
V [libjvm.so+0xf5be9b] Parse::do_all_blocks()+0x3b5
V [libjvm.so+0xf5b9d7] Parse::Parse(JVMState*, ciMethod*, float)+0x13a7
V [libjvm.so+0x5f63f8] ParseGenerator::generate(JVMState*)+0xfc
V [libjvm.so+0x836284] Parse::do_call()+0xc24
V [libjvm.so+0xf76ab0] Parse::do_one_bytecode()+0x4586
V [libjvm.so+0xf60100] Parse::do_one_block()+0x4dc
V [libjvm.so+0xf5be9b] Parse::do_all_blocks()+0x3b5
V [libjvm.so+0xf5b9d7] Parse::Parse(JVMState*, ciMethod*, float)+0x13a7
V [libjvm.so+0x5f63f8] ParseGenerator::generate(JVMState*)+0xfc
V [libjvm.so+0x703f51] Compile::Compile(ciEnv*, ciMethod*, int, bool, bool, bool, bool, DirectiveSet*)+0xcbd

Merge PR: https://github.com/openjdk/valhalla/pull/162
Commit: https://github.com/openjdk/valhalla/commit/cd5c75f8ad82cc01620070bfd5f02df01b704825