RFR (XS): JDK-8025728: Missing volatile specifier for field G1AllocRegion::_alloc_region

[Siebenborn, Axel]

Hi
We investigated some crashes on AIX using the G1 collector. We compiled the HotSpot with IBM XLC12. The reason for the crashes is, that XLC creates code which loads _alloc_region's value and compares it against NULL. Later on, it reloads _alloc_region from memory and uses that new value without another null-check. The problem is that meanwhile another thread has set the _alloc_region's value to NULL.
While one may argue that the code generated by XLC is not very elegant it is still correct from a C++ point of view. And we can't be sure that other compilers do not generate similar code.
Fortunately, this behavior can be easily prevented by declaring _alloc_region as volatile pointer.

Could you please review the following webrev:
http://www.sapjvm.com/as/webrevs/8025728/

Regards,
Axel
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