RFR: 8259020: null-check of g1 write_ref_field_pre_entry is not necessary

[Xin Liu]

On Fri, 1 Jan 2021 18:07:51 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> orig is not null because G1BarrierSetC2 won't invoke write_ref_field_pre_entry 
>> if pre_val is NULL.
>
> Marked as reviewed by ayang (Author).

Regression tests is clear. 
https://github.com/navyxliu/jdk/actions/runs/454177250

I can confirm it's safe for hotspot/c2 and I haven't found evidence that c1 generates call of write_ref_field_pre_entry.
on the other side, it is exported to external via JVMCIRuntime::write_barrier_pre.  I am not familiar with jvmci. I guess it's used by graal compiler, but I don't understand it much.  Can a graal expert tell me if it's safe on your site?

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PR: https://git.openjdk.java.net/jdk/pull/1913