RFR: 8266074: Vtable-based CHA implementation

[Igor Ignatev]

Hi David,

I meant both: in a generic sense, meaning we won't properly document, advertise it or claim it as supported; and changing its type to EXPERIMENTAL, so it will be somewhat harder for people to switch it.

-- Igor

On May 1, 2021, at 3:03 PM, David Holmes <david.holmes at oracle.com<mailto:david.holmes at oracle.com>> wrote:

On 2/05/2021 1:39 am, Igor Ignatyev wrote:
On Fri, 30 Apr 2021 22:10:02 GMT, Vladimir Kozlov <kvn at openjdk.org<mailto: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.

Did you mean "experimental" in a generic sense or actually change it from DIAGNOSTIC to EXPERIMENTAL? If the latter then I don't agree this is an experimental flag, it is diagnostic. But either way the testing requirements are the same if we expect to tell end users to try this flag if they hit an problem - the flag has to be known to be functional, so we will have to expand the test coverage.

Cheers,
David
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-- Igor
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PR: https://git.openjdk.java.net/jdk/pull/3727