[10?] RFR (XS): 8129440: G1 crash during concurrent root region scan

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that prevents C++ compilers
from generating code that reloads oop value from the heap instead of
using a local variable in concurrent marking closures. This can cause
issues if this reloading occurs after the originally loaded value has
been checked for NULL to prevent SIGSEGVs and the mutator changed the
value in the meantime.

E.g.

  oop o = load_from_heap(p);
  if (o == NULL) {
     return;
  }
  // do something with o; potential crash here when reloading from p
instead of actually reusing o

According to SAP particularly the IBM xlc compiler tends to to such
nasty things (which is completely okay from a language POV).

The fix is to make the load from the heap a volatile load using the new
AccessBarrier API in cases this can happen.

CR:
https://bugs.openjdk.java.net/browse/JDK-8129440
Webrev:
http://cr.openjdk.java.net/~tschatzl/8129440/webrev/
Testing:
hs tier1+2; note that this has only been reported for the IBM xlc
compiler for PPC which Oracle does not test for. I hope somebody at SAP
can confirm that it generates the correct code now.

Based on JDK-8193063 also out for review right now.

Thanks,
  Thomas