RFR: 8205064: Fail immediately if an unsupported GC is selected

[Per Liden]

Updated webrev with the auto-select stuff removed.

http://cr.openjdk.java.net/~pliden/8205064/webrev.1

I need to check if this change requires a CSR, in which case this might 
or might not make it into 11, but I'll try.

/Per

On 06/15/2018 08:36 AM, Per Liden wrote:
> On 2018-06-15 07:51, Aleksey Shipilev wrote:
>> On 06/15/2018 12:15 AM, Roman Kennke wrote:
>>> Am 14.06.2018 um 22:34 schrieb Per Liden:
>>>> If an unsupported GC (i.e. a GC that is not built into the VM) is
>>>> selected by the user, the VM issues a warning and then continues and
>>>> (silently) selects a different GC. Aleksey brought this up on the ZGC
>>>> list [1]. I agree that this behavior seems dubious. With this patch we
>>>> instead fail immediately to avoid unnecessary confusion.
>>>>
>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8205064
>>>> Webrev: http://cr.openjdk.java.net/~pliden/8205064/webrev.0
>>
>> Thanks! Looks good, modulo the comment below:
>>
>>> I am not sure I'd ever set Epsilon as default, but it's the last in
>>> line, i.e. selected when built *only* with Epsilon, is that right?
>>> Why not include CMS in that list? If I build with CMS and Epsilon, I get
>>> Epsilon selected?
>>
>> Also, IIRC, if we do autoselect either ZGC or Epsilon, the argument 
>> checking would fail right away,
>> because we need to unlock them with UnlockExperimentalVMOptions first:
>>
>>    experimental(bool, UseEpsilonGC, 
>> false,                                   \
>>            "Use the Epsilon (no-op) garbage 
>> collector")                      \
>>                                                                              
>> \
>>    experimental(bool, UseZGC, 
>> false,                                         \
>>            "Use the Z garbage 
>> collector")                                    \
>>                                                                              
>> \
>>
>> I think we better avoid adding experimental GCs to auto-selection, and 
>> just leave
>> GCConfig::select_gc_ergonomically alone.
>>
> 
> Thanks for reviewing Aleksey. Dropping the auto-select part.
> 
> /Per
> 
>> -Aleksey
>>