[master] RFR: Load narrowKlass from header, AArch64 assembler implementation [v2]

[Andrew Haley]

On Thu, 27 Jan 2022 13:51:42 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> This implements MacroAssembler::load_klass() to load the (narrow)Klass* from object header. Just like the x86_64 implementation, it checks whether it can take the fast path (object unlocked -> load from upper 32bits of header), or else calls the runtime to get a stable header and load from that.
>> 
>> It adds a runtime call stub, which will also be used in the C2 implementation. It also adds nklass_offset_in_bytes() which will also be used in C2 impl. The part in generate_verify_oop() is a little nasty, I added a comment that explains what's going on.
>> 
>> Testing:
>>  - [x] tier1 (aarch64)
>>  - [x] tier2 (aarch64)
>>  - [x] tier3 (aarch64)
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add asserts

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 3733:

> 3731:   // We don't need to preserve r0 here, but we need to preserve rscratch1 and rescratch2,
> 3732:   // because some users of load_klass() use them around the call.
> 3733:   push(RegSet::of(rscratch1, rscratch2), sp);

This is all very weird, and looks confusing. I think it might be safer to force the callers of `load_klass()` to provide a temp.

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 3737:

> 3735:   assert(StubRoutines::load_nklass() != NULL, "Must have stub");
> 3736:   mov(rscratch1, StubRoutines::load_nklass());
> 3737:   blr(rscratch1);

Suggestion:

  far_call(RuntimeAddress(StubRoutines::load_nklass()), rscratch1);

src/hotspot/cpu/aarch64/stubGenerator_aarch64.cpp line 6619:

> 6617:     __ set_last_Java_frame(sp, rfp, lr, rscratch1);
> 6618:     __ enter();
> 6619:     __ push_call_clobbered_registers_except(RegSet::of(r0));

Suggestion:

    __ push_call_clobbered_registers_except(r0);

... everywhere.

src/hotspot/cpu/aarch64/stubGenerator_aarch64.cpp line 6621:

> 6619:     __ push_call_clobbered_registers_except(RegSet::of(r0));
> 6620:     __ call_VM_leaf(CAST_FROM_FN_PTR(address, oopDesc::load_nklass_runtime), 1);
> 6621:     __ pop_call_clobbered_registers_except(RegSet::of(r0));

Suggestion:

    __ pop_call_clobbered_registers_except(r0);

src/hotspot/share/oops/oop.hpp line 312:

> 310: #ifdef _LP64
> 311:   static int nklass_offset_in_bytes()    {
> 312:     assert(markWord::klass_shift % 8 == 0, "klass_shift must be multiple of 8");

Suggestion:

    STATIC_ASSERT(markWord::klass_shift % 8 == 0, "klass_shift must be multiple of 8");

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PR: https://git.openjdk.java.net/lilliput/pull/36