RFR: 8269476: Skip nmethod entry barrier if there is no oops in the jit code [v4]

[王超]

On Tue, 29 Jun 2021 11:35:52 GMT, 王超 <github.com+25214855+casparcwang at openjdk.org> wrote:

>> Lots of c1 and c2 jit methods do not contain any oop, so the nmethod entry barrier can be skipped.
>> 
>> 1, c1 jit code will patch oops or Klass into the nmethod, so the entry barrier cannot directly be eliminated, current implementation uses a jump instruction to replace the jcc instruction. If the jit code is patched to contain oops, the entry barrier is patched back to the jcc instruction.
>> 
>> 2, only the jit code of core library methods do not contain any oops.
>> 
>> 3, currently only support zgc
>
> 王超 has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix assert error

In summary, the benefit is too small, and it adds maintenance burden to future loom and generational zgc design, so just close this pr.

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PR: https://git.openjdk.java.net/jdk/pull/4610