RFR: 8284161: Implementation of Virtual Threads (Preview) [v12]

[Alan Bateman]

On Thu, 5 May 2022 17:43:58 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> I am sorry to be a buzzkill here, but this integration would break lots of platforms even when Loom functionality is not enabled/used. For example, running `java -version` on RISC-V runs into many issues: `TemplateInterpreterGenerator::generate_Continuation_doYield_entry` runs into `Unimplemented()`, `UnsafeCopyMemory` asserts about UCM table size, `NativeDeoptInstruction::is_deopt` would run into `Unimplemented()` while being called from signal handler.
> 
> This effectively blocks development on those platforms, and it seems to be too much breakage for a preview feature. Are we really breaking these platforms? Do we have plans in place with maintainers of those platforms to fix all this in JDK 19 timeframe? I'd suggest holding off the integration until such a plan and agreement is in place.

We mailed porters-dev in Sep 2021 to give a heads up that this feature would require porting work so it shouldn't be a surprise. We have been open to including other ports with the initial integration but it was never a goal to have all the ports done in advance of that.

Most of the new code in the VM is only used when running with --enable-preview. You'll see several places that test Continuations::enabled(). It should be possible to get these port running without -enable-preview without much effort. The feature has to be implemented to pass all the tests of course.

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PR: https://git.openjdk.java.net/jdk/pull/8166