ImplicitNullChecks did not work on platforms where the zero
page is only write protected.
This change adds os property 'zero_page_read_protected' and extends
the ImplicitNullCheck optimization to consider only stores if
this property is not true. If a decoded compressed oop will access the
guard page before the heap, Loads work again.
This is needed on AIX, where the page at address '0' is only write-protected.
This is preparation for PPC64 integration: http://openjdk.java.net/jeps/175
This and following ppc64 changes will go into staging repository first and tested there: http://hg.openjdk.java.net/ppc-aix-port/stage/