RFR: 8269476: Skip nmethod entry barrier if there is no oops in the jit code [v2]

[Zhengyu Gu]

On Mon, 28 Jun 2021 12:34:20 GMT, 王超 <github.com+25214855+casparcwang at openjdk.org> wrote:

>> I think there is a simpler way to accomplish this by playing with disarmed_value, e.g. setting nmethod's disarmed_value to certain pattern to indicate it is always disarmed.
>
>> I think there is a simpler way to accomplish this by playing with disarmed_value, e.g. setting nmethod's disarmed_value to certain pattern to indicate it is always disarmed.
> 
> `__ cmpl(disarmed_addr, 0);` will only set ZF=1 when `dest = source`. By playing with disarmed_value, do you mean also change the cmpl instruction to other instruction?

You may need an additional instruction, e.g. uses MSB of disarmed_value to indicate always disarmed and test the bit before `cmpl`. 

Also, you need a new method to set the bit during nmethod registration.

nmethod entry barrier tests is_armed() early, so may benefit more.

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