RFR: JDK-8287061: Support for rematerializing scalar replaced objects participating in allocation merges [v18]

[Cesar Soares Lucas]

On Fri, 23 Jun 2023 21:24:20 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> @iwanowww - I'm confused by what a "Diagnostic" flag is. According to [this documentation](https://wiki.openjdk.org/display/HotSpot/Hotspot+Command-line+Flags%3A+Kinds%2C+Lifecycle+and+the+CSR+Process) "Diagnostic flags are not meant for VM tuning or for product modes. They are to be used for VM quality assurance or field diagnosis of VM bugs [...]" I believe the patch I'm proposing is a VM tuning optimization, so should it really be a diagnostic flag? Besides, I think we'll try _at a later moment_ to make this a product flag. Do you think an experimental flag is more appropriate? Thank you.
>
> You can look at it in the following way: since the flag is set to true by default, the feature is unconditionally available in product binaries. The only reason to explicitly specify the flag is to turn the optimization off and it may be needed to diagnose VM crashes or performance regressions. 
> 
> As an afterthrought, maybe C2 should check a compiler directive (and not a global flag) to be able to control the optimization up to per-method granularity.

Thank you @iwanowww for clarifying. Now I understand this better.

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PR Review Comment: https://git.openjdk.org/jdk/pull/12897#discussion_r1244190081