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

[Vladimir Kozlov]

On Wed, 25 Nov 2020 01:39:11 GMT, Vladimir Kozlov <kvn 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
>
> For metropolis I have next code in jvmci_globals.cpp which switch off JVMCI if test uses unsupported GC:
> 
> static bool supported_gc() {
>   return (UseSerialGC || UseParallelGC || UseG1GC);
> }
> 
> void JVMCIGlobals::check_jvmci_supported_gc() {
>   if (EnableJVMCI) {
>     // Check if selected GC is supported by JVMCI and Java compiler
>     if (!supported_gc()) {
>       if (FLAG_IS_DEFAULT(UseJVMCINativeLibrary) && !UseJVMCINativeLibrary) { // Just disable Graal when libgraal is present
>         vm_exit_during_initialization("JVMCI Compiler does not support selected GC", GCConfig::hs_err_name());
>       }
>       FLAG_SET_DEFAULT(EnableJVMCI, false);
>       FLAG_SET_DEFAULT(UseJVMCICompiler, false);
>       FLAG_SET_DEFAULT(UseJVMCINativeLibrary, false);
>     }
>   }
> }

Why not call such `JVMCIGlobals::supported_gc()` from `GCConfig::is_gc_supported`?
I don't understand why you need to go through Java JVMCI code for that?

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