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

[Doug Simon]

On Wed, 25 Nov 2020 01:48:17 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> 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?

The reason to go through Java JVMCI code is that JVMCI itself does not know which GCs are supported by a JVMCI compiler. For example, once Graal supports ZGC, the above `supported_gc` function will be incorrect.

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