RFR 8201785: Make ModRefBarrierSetAssembler abstract on all platforms

[Per Liden]

Looks good to me!

/Per

On 04/18/2018 01:57 PM, Aleksey Shipilev wrote:
> RFE:
>    https://bugs.openjdk.java.net/browse/JDK-8201785
> 
> Webrev:
>    http://cr.openjdk.java.net/~shade/8201785/webrev.02/
> 
> Found this when while rebasing Epsilon to the recent jdk/jdk. ModRefBSAssembler is abstract on some
> platforms (x86, sparc), but not on the others (aarch64, ppc). I think this allows missing the
> required overrides of oop_store_at in ModRefBSAssembler subclasses, which should be caught
> mechanically. (Epsilon used to defer to ModRefBSAssembler, and it was successfully instantiated!)
> 
> PPC folks, would you like to give it a spin on your platforms?
> 
> Testing: x86_64 and aarch64 non-PCH builds, Epsilon tests
> 
> Thanks,
> -Aleksey
>