RFR: 8266074: Vtable-based CHA implementation

[Igor Ignatyev]

On Fri, 30 Apr 2021 22:10:02 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> I'm fine with both approaches. 
>> 
>> Explicitly setting the flag looked to me more robust and clearer communicating the intent. But if you prefer `@requires`, I'll use it.
>
> Let hear @iignatev opinion.

from my point of view, `@requires` is clearer and also eliminates "wasted" execution (if someone tries to run this test w/ `-XX:-UseVtableBasedCHA`), so I'd prefer if we use it.

I have a more generic comment about `UseVtableBasedCHA`. I understand the desire to introduce a flag to switch back to the old implementation, but I'm somewhat concern that it adds a new dimension into configuration space that won't be covered by our existing tests (w/ the test which exercises interesting parts of the related code is inapplicable) and isn't part of our regular test configurations. Can we make it an experimental flag (w/ vtable-based CHA still being enabled by default)? this way, the quality bar for the old implementation will be somewhat lower, yet the end-users will still be able to return to the old implementation if it, for some reason, works better in their use-cases.

-- Igor

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PR: https://git.openjdk.java.net/jdk/pull/3727