RFR: 8257020: [JVMCI] enable a JVMCICompiler to specify which GCs it supports [v3]

[Doug Simon]

On Wed, 25 Nov 2020 23:13:21 GMT, Vladimir Kozlov <kvn 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.
>> 
>> 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.

Ok, I get the point about `GCConfig::is_gc_supported()` now. It's answering the question "is the given GC built into the VM" as opposed to "can the given GC work in the current VM runtime configuration".

Based on your suggestions, I've removed the JVMCI logic from `GCConfig`, added `WB_isGCSupportedByJVMCI` (it's better to keep things JVMCI compiler neutral at this layer) and modified `VMProps` to use it (https://github.com/openjdk/jdk/pull/1423/commits/379f48f794f2f59e1b4ffd837025f9ea999d06ff).

I've also implemented the Graal side of this interface so that it can be demonstrated to work in the JDK (https://github.com/openjdk/jdk/pull/1423/commits/384287b748acfbb3ebea7d9b5eb1cbbe106d912d):
> sh configure --disable-precompiled-headers --enable-jvm-feature-graal
> make images

# With Graal
> jtreg -v -noreport -jdk:build/macosx-x86_64-server-release/images/jdk -vmoptions:"-XX:+UnlockExperimentalVMOptions -XX:+UseJVMCICompiler" test/hotspot/jtreg/gc/epsilon/TestAlignment.java
Test results: no tests selected
Results written to /Users/dnsimon/jdk-jdk/open/JTwork

# Without Graal
> jtreg -v -noreport -jdk:build/macosx-x86_64-server-release/images/jdk test/hotspot/jtreg/gc/epsilon/TestAlignment.java
runner starting test: gc/epsilon/TestAlignment.java
runner finished test: gc/epsilon/TestAlignment.java
Passed. Execution successful
Test results: passed: 1
Results written to /Users/dnsimon/jdk-jdk/open/JTwork

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