68 

    69 bool MacroAssembler::needs_explicit_null_check(intptr_t offset) {

    70   // Exception handler checks the nmethod's implicit null checks table

    71   // only when this method returns false.

    72   if (UseCompressedOops) {

    73     // The first page after heap_base is unmapped and

    74     // the 'offset' is equal to [heap_base + offset] for

    75     // narrow oop implicit null checks.

    76     uintptr_t heap_base = (uintptr_t)Universe::heap_base();

    77     if ((uintptr_t)offset >= heap_base) {

    78       // Normalize offset for the next check.

    79       offset = (intptr_t)(pointer_delta((void*)offset, (void*)heap_base, 1));

    80     }

    81   }

    82   // Linux kernel guarantees that the first page is always unmapped. Don't

    83   // assume anything more than that.

    84   bool offset_in_first_page =   0 <= offset  &&  offset < os::vm_page_size();

    85   return !offset_in_first_page;

    86 }