We have observed that C2 generates implicit null checks for write accesses to the heap base even though it was not protected (narrow_oop_use_implicit_null_checks = false).
The problem showed up on AIX with -XX:HeapBaseMinAddress=32g. ReservedHeapSpace::establish_noaccess_prefix protects the base area on almost all platforms, but not on AIX (in some case).
PhaseCFG::implicit_null_check needs to skip the transformation in this case.