Noticed this while looking at Late Barrier Expansion work.

Shenandoah C2 clone barrier is inserted before calling into arraycopy stub:

void ShenandoahBarrierSetC2::clone_at_expansion(PhaseMacroExpand* phase, ArrayCopyNode* ac) const {
...
    // Heap is unstable, call into clone barrier stub
    Node* call = phase->make_leaf_call(unstable_ctrl, mem,
                                       ShenandoahBarrierSetC2::clone_barrier_Type(),
                                       CAST_FROM_FN_PTR(address, ShenandoahRuntime::clone_barrier),
                                       "shenandoah_clone",
                                       TypeRawPtr::BOTTOM,
                                       src_base);
    call = phase->transform_later(call);
...
    // Wire up the actual arraycopy stub now
    ctrl = phase->transform_later(region);
    mem = phase->transform_later(mem_phi);

    const char* name = "arraycopy";
    call = phase->make_leaf_call(ctrl, mem,
                                 OptoRuntime::fast_arraycopy_Type(),
                                 phase->basictype2arraycopy(T_LONG, nullptr, nullptr, true, name, true),
                                 name, TypeRawPtr::BOTTOM,
                                 src, dest, length
                                 LP64_ONLY(COMMA phase->top()));
    call = phase->transform_later(call);

The following arraycopy call is doing T_LONG copy. But this dance looks unnecessary, because arraycopy stub *itself* calls into BarrierSetAssembler::arraycopy_prologue, which Shenandoah handles! See:

address StubGenerator::generate_conjoint_copy_avx3_masked(StubId stub_id, address* entry, address nooverlap_target) {
...
  BarrierSetAssembler *bs = BarrierSet::barrier_set()->barrier_set_assembler();
  bs->arraycopy_prologue(_masm, decorators, type, from, to, count);

There are also other ways GCs handle the clones, including having the full runtime clone routine that applies both relevant clone barriers and do the copy.

WIP: https://github.com/openjdk/jdk/compare/master...shipilev:JDK-8376749-shenandoah-no-c2-clone