Make +UseZGC fail hard when it is unsupported

[Aleksey Shipilev]

On 06/14/2018 10:55 AM, Per Liden wrote:
> On 06/14/2018 10:45 AM, Aleksey Shipilev wrote:
>> I have build recent workspace, and forgot to enable zgc in configure. Yet, the runs proceeded as
>> usual, and G1 was automatically selected. Per, can you make this failure fatal?
> 
> This behavior is in line with how this mechanism works for all other GCs, so it's not ZGC specific.
> E.g. if you build a minimal VM you should see the same behavior if you try to enable, say G1.
> 
> However, it can certainly be debated whether this is a good behavior or not, and I tend to agree
> with you that it's not.

Sure. My point was that users would become exposed to this with optionally-compiled experimental GC
very often now. Hardly anyone uses minimal VM, but everyone would use server VM with optional GCs.
Would be sad to see reports against ZGC, Shenandoah, Epsilon that are not actually running those GCs.

Since you were redoing the gcConfig machinery, would you have cycles to make RFE that makes this
warning fatal?

-Aleksey