6716785: implicit null checks not triggering with CompressedOops

Allocate alignment-sized page(s) below java heap so that memory accesses at heap_base+1page give signal and cause an implicit null check

Reviewed-by: kvn, jmasa, phh, jcoomes

hotspot/src/share/vm/asm/assembler.cpp

@@ -246,6 +246,24 @@ void AbstractAssembler::block_comment(const char* comment) {
   }
 }
 
+bool MacroAssembler::needs_explicit_null_check(intptr_t offset) {
+  // Exception handler checks the nmethod's implicit null checks table
+  // only when this method returns false.
+#ifndef SPARC
+  // Sparc does not have based addressing
+  if (UseCompressedOops) {
+    // The first page after heap_base is unmapped and
+    // the 'offset' is equal to [heap_base + offset] for
+    // narrow oop implicit null checks.
+    uintptr_t heap_base = (uintptr_t)Universe::heap_base();
+    if ((uintptr_t)offset >= heap_base) {
+      // Normalize offset for the next check.
+      offset = (intptr_t)(pointer_delta((void*)offset, (void*)heap_base, 1));
+    }
+  }
+#endif // SPARC
+  return offset < 0 || os::vm_page_size() <= offset;
+}
 
 #ifndef PRODUCT
 void Label::print_instructions(MacroAssembler* masm) const {