RFR: Fix dangling write-barrier in acmp

[Aleksey Shipilev]

On 12/12/18 1:00 PM, Roman Kennke wrote:
> This is needed to fix the recent failure that we have observed with
> new-shaped acmp code in jdk8u:
> 
> http://cr.openjdk.java.net/~rkennke/fixacmp-jdk11/webrev.00/

This is shared code path, needs to be protected with INCLUDE_SHENANDOAHGC and UseShenandoahGC?

For example:

#ifdef INCLUDE_SHENANDOAHGC
      if (UseShenandoahGC && igvn != NULL) {
        set_req_X(1, lhs, igvn);
        set_req_X(2, rhs, igvn);
      } else
#endif
      {
        set_req(1, lhs);
        set_req(2, rhs);
      }


> The webrev is against sh/jdk11, where the code is also missing (but bug
> hasn't showed up in testing). I intend to push the equivalent patch to
> sh/jdk8u at the same time.

Remember, there is no INCLUDE_SHENANDOAHGC macro in 8u.

-Aleksey