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

[Doug Simon]

On Wed, 25 Nov 2020 08:29:01 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> Could this be done without adding JVMCI code to GCConfig::is_gc_supported?

This has to go through the WhiteBox API as JVMCI is not accessible to normal Java code (such as VMProps).
We could restrict the JVMCI interposition to WhiteBox. This is what I did [initially](https://github.com/openjdk/jdk/pull/1423/commits/191d61d996dd08f43c2c946f02b0cce0d96f2ef4). However, I then saw that `GCConfig::is_gc_supported` is only called from whitebox.cpp so [moved](https://github.com/openjdk/jdk/pull/1423/commits/54b7ba8463dfee9a679abb09a5cc898ba8550d85) the logic directly into `GCConfig`. I think this is better because as soon as there's another caller of `GCConfig::is_gc_supported`, they should get an accurate answer and not have to worry about implications of `EnableJVMCI` separately.

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