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

[Aleksey Shipilev]

On Wed, 26 Jan 2022 10:57:46 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)

I'll defer to Andrew to examine AArch64 code, but I have two quick comments.

src/hotspot/cpu/aarch64/stubGenerator_aarch64.cpp line 596:

> 594:     // we have lock pointer bits in the upper 32bits. We can't get a false
> 595:     // negative.
> 596:     __ ldrw(r0, Address(r0, oopDesc::nklass_offset_in_bytes()));  // get klass

Something should assert that `oopDesc::nklass_offset_in_bytes()` does not break the alignment. Probably assert that `oopDesc::nklass_offset_in_bytes()` is a multiple of 4?

src/hotspot/share/oops/oop.hpp line 311:

> 309:   static int mark_offset_in_bytes()      { return offset_of(oopDesc, _mark); }
> 310: #ifdef _LP64
> 311:   static int nklass_offset_in_bytes()    { return mark_offset_in_bytes() + markWord::klass_shift / 8; }

Ew. Need to assert that `markWord::klass_shift` is actually a multiple of 8.

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