RFR: 8257020: [JVMCI] enable a JVMCICompiler to specify which GCs it supports [v7]

[Stefan Karlsson]

On Thu, 3 Dec 2020 11:35:08 GMT, Doug Simon <dnsimon at openjdk.org> wrote:

>> A number of jtreg tests require a specific GC. These tests should be ignored when EnableJVMCI is true and the JVMCI compiler does not support the required GC.
>> 
>> This PR adds `JVMCICompiler.isGCSupported` and makes use of it in `WhiteBox.isGCSupported`.
>> 
>> Prior to this PR, a test requiring a GC not yet supported by a JVMCI compiler fail as follows:
>> Error occurred during initialization of VM
>> JVMCI Compiler does not support selected GC: epsilon gc
>
> Doug Simon has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix @requires vm.jvmci

I've been talking to Doug the last few days and we've settled on this approach that both solves their testing needs, and at the same time satisfy my wish to *not* turn on EnableJVMCI for GCs that don't have fully support for it.

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Marked as reviewed by stefank (Reviewer).

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