[riscv-port] RFR: 8277391: riscv: implement tls function MacroAssembler::get_thread() by using tls.gd [v2]

[Fei Yang]

On Mon, 22 Nov 2021 02:46:03 GMT, zhengxiaolinX <duke at openjdk.java.net> wrote:

>> Hi team,
>> 
>> This patch fixes the unimplemented assembly version of `__ get_thread()`, bringing some performance improvement by directly using the x4 register to refer to the variable `Thread::_thr_current`. Tested tests under hotspot/ and jdk/. [Original patch](https://github.com/riscv-collab/riscv-openjdk/pull/4)
>> 
>> Thanks,
>> Xiaolin
>
> zhengxiaolinX has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:
> 
>   Implement tls function MacroAssembler::get_thread() using tls.gd

After some reconsideration, I think we can go one step further: simply keep the changes in file src/hotspot/share/utilities/globalDefinitions_gcc.hpp and remote all other changes in this PR.

Then USE_LIBRARY_BASED_TLS_ONLY macro won't be defined and Thread::current() will return '_ZN6Thread12_thr_currentE' in that case. This will be the same in functionality as the newly add assembly function '_ZN10JavaThread25riscv64_get_thread_helperEv'.

I have checked with Yadong, the reason for defining USE_LIBRARY_BASED_TLS_ONLY macro is to workaround some host compiler bug when accessing TLS variables in the very early days. This should not affect us now.

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PR: https://git.openjdk.java.net/riscv-port/pull/7