RFR: 8293170: Improve encoding of the debuginfo nmethod section

[Boris Ulasevich]

On Mon, 3 Oct 2022 20:46:07 GMT, Dean Long <dlong at openjdk.org> wrote:

>>> What is the performance impact of making several of the methods virtual?
>> 
>> Good question! My experiments show that in the worst case, the performance of the debug write thread is reduced by 424->113 MB/s with virtual functions. Compared to compile time, this is miserable: сompilation takes 1000ms per method, while generation of 300 bytes of scopes data with virtual function (worst case) takes 3ms. And I do not see any regression with benchmarks.
>
>> > What is the performance impact of making several of the methods virtual?
>> 
>> Good question! My experiments show that in the worst case, the performance of the debug write thread is reduced by 424->113 MB/s with virtual functions. Compared to compile time, this is miserable: сompilation takes 1000ms per method, while generation of 300 bytes of scopes data with virtual function (worst case) takes 3ms. And I do not see any regression with benchmarks.
> 
> I was wondering more about read performance.  I would expect that the debuginfo could be read many more times than it is written.  Also, from 424 to 113 seems like a very large slowdown.

@dean-long @vnkozlov 
I eliminated virtual methods, changed the implementation internals and added tests.
Let me once again ask you to review. Thanks

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PR: https://git.openjdk.org/jdk/pull/10025