src/share/vm/opto/matcher.hpp

rev 7386 : 8064457: Introduce compressed oops mode disjoint base and improve compressed heap handling.

@@ -431,10 +431,17 @@
   // decode narrow_oop_reg, oop_reg // only 'shift'
   // [oop_reg + offset]
   // NullCheck oop_reg
   //
   inline static bool gen_narrow_oop_implicit_null_checks() {
+    // Advice matcher to perform null checks on the narrow oop side.
+    // Implicit checks are not possible on the uncompressed oop side anyway
+    // (at least not for read accesses).
+    // Performs significantly better (especially on Power 6).
+    if (!os::zero_page_read_protected()) {
+      return true;
+    }
     return Universe::narrow_oop_use_implicit_null_checks() &&
            (narrow_oop_use_complex_address() ||
             Universe::narrow_oop_base() != NULL);
   }