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

[Vladimir Kozlov]

On Wed, 25 Nov 2020 21:31:44 GMT, Doug Simon <dnsimon at openjdk.org> wrote:

>> I think you merged 2 things into this PR which made me confused. The title says that Graal should specify which GC it supports (new JVMCI API). But your description is about fixing Graal testing with different GCs. Yes, they are connected but I think they should be reviewed separately.
>> 
>> The reason currently we test Graal supported GC in C++ code during VM startup is because we want to avoid executing application in Interpreter and bailout later when Graal is loaded to compile a hot method. Note, Graal in JDK is loaded on first compilation request.
>> To get answer from Graal about GC you would have to load and initialize it very early which will affect startup (even with libgraal).
>> 
>> To add new JVMCI API just to resolve testing issues is overkill for me.
>
> Now I'm confused ;-) The title and description of this PR don't mention Graal at all. While Graal is obviously the concrete JVMCI compiler motivating this change, the problem relates to any Java based JVMCI compiler. This is why an API change is needed.
> 
> I understand the motivation for wanting to test which GCs are supported by JVMCI in C++ code during VM startup. But I don't think that can justify giving a wrong answer. What happens when support for ZGC is added to Graal? The VM will incorrectly exit during startup based on the current code.
> 
> With JVMCI, the problem of delayed VM exit upon misconfiguration already exists. For example, an incorrectly specified or unrecognized `-Dgraal.*` option will only be detected when compiling the first method with Graal (i.e. when Graal is lazily initialized). Maybe I'm missing something, but why is this a real problem? Does it matter whether the VM exits after 1ms of execution or 500ms?

Actually here is fundamental question. Why not build GraalVM JDK without GCs which Graal does not support? It is all configurable.

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