RFR(m): 8220351: Cross-modifying code

[Robbin Ehn]

> What is this hunk for?
> 
> @@ -2558,12 +2558,9 @@
>     //     VM thread changes sync state to synchronizing and suspends threads for GC.
>     //     Thread A is resumed to finish this native method, but doesn't block here since it
>     //     didn't see any synchronization is progress, and escapes.
> -  __ movl(Address(r15_thread, JavaThread::thread_state_offset()), _thread_in_native_trans);
> -
> -  // Force this write out before the read below
> -  __ membar(Assembler::Membar_mask_bits(
> -              Assembler::LoadLoad | Assembler::LoadStore |
> -              Assembler::StoreLoad | Assembler::StoreStore));
> +
> +  __ lock();
> +  __ addl(Address(r15_thread, JavaThread::thread_state_offset()), 1 /* _thread_in_native_trans */);
> 
>     Label after_transition;
> 

Yes, a bit unrelated.
I thought:
movl thread_state, native_trans
lock addl rsp-x, 0 /* dummy */

Seemed a bit silly when we could do:
lock addl thread_state, 1

If nothing else saves an instruction. (assuming cache-lines are equally contended)

Shall I remove that change ?

Thanks, Robbin