RFR: 8257020: [JVMCI] enable a JVMCICompiler to specify which GCs it supports [v2]

[Vladimir Kozlov]

On Wed, 25 Nov 2020 21:37:13 GMT, Doug Simon <dnsimon at openjdk.org> wrote:

>> Actually here is fundamental question. Why not build GraalVM JDK without GCs which Graal does not support? It is all configurable.
>
>> Actually here is fundamental question. Why not build GraalVM JDK without GCs which Graal does not support? It is all configurable.
> 
> GraalVM could indeed do that but there are other OpenJDK community members who want to be able to use Graal on stock JDK binaries. Even for GraalVM, we want to deviate as little as possible from how the JDK underlying GraalVM is built.

Yes, it is reasonable case. We already had such situation with CMS before. You and they understand which GC can be used with Graal. I agree with removal of `JVMCIGlobals::check_jvmci_supported_gc()` and not do any C++ checks during VM startup.

Now about testing and changes. I agree with @stefank that you should not modify `GCConfig::is_gc_supported()` because it is about Hotspot VM support (`Interpreter` only, for example).  I suggest to modify `isGcSupportedByGraal()` in VMProps.java by adding new `WB_IsGCSupportedByGraal()` WB api to call JVMCI runtime. Then `vmGC()` in VMProps.java will work as it is. And you can change `Graal` with `JVMCICompiler` in methods names if you want.

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PR: https://git.openjdk.java.net/jdk/pull/1423