--- a/hotspot/src/cpu/sparc/vm/assembler_sparc.cpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/cpu/sparc/vm/assembler_sparc.cpp	Wed May 28 21:06:24 2008 -0700
@@ -3421,13 +3421,15 @@
   set((intptr_t)markOopDesc::prototype()->copy_set_hash(0x2), t2);
   st_ptr(t2, top, oopDesc::mark_offset_in_bytes()); // set up the mark word
   // set klass to intArrayKlass
-  set((intptr_t)Universe::intArrayKlassObj_addr(), t2);
-  ld_ptr(t2, 0, t2);
-  store_klass(t2, top);
   sub(t1, typeArrayOopDesc::header_size(T_INT), t1);
   add(t1, ThreadLocalAllocBuffer::alignment_reserve(), t1);
   sll_ptr(t1, log2_intptr(HeapWordSize/sizeof(jint)), t1);
   st(t1, top, arrayOopDesc::length_offset_in_bytes());
+  set((intptr_t)Universe::intArrayKlassObj_addr(), t2);
+  ld_ptr(t2, 0, t2);
+  // store klass last.  concurrent gcs assumes klass length is valid if
+  // klass field is not null.
+  store_klass(t2, top);
   verify_oop(top);
 
   // refill the tlab with an eden allocation
@@ -3568,13 +3570,19 @@
   if (UseCompressedOops) {
     assert(dst_oop != klass, "not enough registers");
     encode_heap_oop_not_null(klass);
-    sllx(klass, BitsPerInt, klass);
-    stx(klass, dst_oop, oopDesc::klass_offset_in_bytes());
+    st(klass, dst_oop, oopDesc::klass_offset_in_bytes());
   } else {
     st_ptr(klass, dst_oop, oopDesc::klass_offset_in_bytes());
   }
 }
 
+void MacroAssembler::store_klass_gap(Register s, Register d) {
+  if (UseCompressedOops) {
+    assert(s != d, "not enough registers");
+    st(s, d, oopDesc::klass_gap_offset_in_bytes());
+  }
+}
+
 void MacroAssembler::load_heap_oop(const Address& s, Register d, int offset) {
   if (UseCompressedOops) {
     lduw(s, d, offset);

--- a/hotspot/src/cpu/sparc/vm/assembler_sparc.hpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/cpu/sparc/vm/assembler_sparc.hpp	Wed May 28 21:06:24 2008 -0700
@@ -1979,6 +1979,7 @@
   // klass oop manipulations if compressed
   void load_klass(Register src_oop, Register klass);
   void store_klass(Register klass, Register dst_oop);
+  void store_klass_gap(Register s, Register dst_oop);
 
    // oop manipulations
   void load_heap_oop(const Address& s, Register d, int offset = 0);

--- a/hotspot/src/cpu/sparc/vm/templateTable_sparc.cpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/cpu/sparc/vm/templateTable_sparc.cpp	Wed May 28 21:06:24 2008 -0700
@@ -3222,7 +3222,8 @@
     __ set((intptr_t)markOopDesc::prototype(), G4_scratch);
   }
   __ st_ptr(G4_scratch, RallocatedObject, oopDesc::mark_offset_in_bytes());       // mark
-  __ store_klass(RinstanceKlass, RallocatedObject); // klass
+  __ store_klass_gap(G0, RallocatedObject);         // klass gap if compressed
+  __ store_klass(RinstanceKlass, RallocatedObject); // klass (last for cms)
 
   {
     SkipIfEqual skip_if(

--- a/hotspot/src/cpu/x86/vm/assembler_x86_64.cpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/cpu/x86/vm/assembler_x86_64.cpp	Wed May 28 21:06:24 2008 -0700
@@ -4935,6 +4935,8 @@
   movq(Address(top, arrayOopDesc::length_offset_in_bytes()), t1);
   // set klass to intArrayKlass
   movptr(t1, ExternalAddress((address) Universe::intArrayKlassObj_addr()));
+  // store klass last.  concurrent gcs assumes klass length is valid if
+  // klass field is not null.
   store_klass(top, t1);
 
   // refill the tlab with an eden allocation
@@ -5159,9 +5161,17 @@
 void MacroAssembler::store_klass(Register dst, Register src) {
   if (UseCompressedOops) {
     encode_heap_oop_not_null(src);
-    // Store to the wide klass field to zero the gap.
+    movl(Address(dst, oopDesc::klass_offset_in_bytes()), src);
+  } else {
+    movq(Address(dst, oopDesc::klass_offset_in_bytes()), src);
   }
-  movq(Address(dst, oopDesc::klass_offset_in_bytes()), src);
+}
+
+void MacroAssembler::store_klass_gap(Register dst, Register src) {
+  if (UseCompressedOops) {
+    // Store to klass gap in destination
+    movl(Address(dst, oopDesc::klass_gap_offset_in_bytes()), src);
+  }
 }
 
 void MacroAssembler::load_heap_oop(Register dst, Address src) {

--- a/hotspot/src/cpu/x86/vm/assembler_x86_64.hpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/cpu/x86/vm/assembler_x86_64.hpp	Wed May 28 21:06:24 2008 -0700
@@ -1109,6 +1109,7 @@
   // oop manipulations
   void load_klass(Register dst, Register src);
   void store_klass(Register dst, Register src);
+  void store_klass_gap(Register dst, Register src);
 
   void load_heap_oop(Register dst, Address src);
   void store_heap_oop(Address dst, Register src);

--- a/hotspot/src/cpu/x86/vm/templateTable_x86_64.cpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/cpu/x86/vm/templateTable_x86_64.cpp	Wed May 28 21:06:24 2008 -0700
@@ -3163,7 +3163,9 @@
       __ movptr(Address(rax, oopDesc::mark_offset_in_bytes()),
                (intptr_t) markOopDesc::prototype()); // header (address 0x1)
     }
-    __ store_klass(rax, rsi);  // klass
+    __ xorl(rcx, rcx); // use zero reg to clear memory (shorter code)
+    __ store_klass_gap(rax, rcx);  // zero klass gap for compressed oops
+    __ store_klass(rax, rsi);      // store klass last
     __ jmp(done);
   }
 

--- a/hotspot/src/share/vm/gc_implementation/parNew/parNewGeneration.cpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/share/vm/gc_implementation/parNew/parNewGeneration.cpp	Wed May 28 21:06:24 2008 -0700
@@ -1169,18 +1169,18 @@
   // Trim off a prefix of at most objsFromOverflow items
   int i = 1;
   oop cur = prefix;
-  while (i < objsFromOverflow && cur->klass() != NULL) {
+  while (i < objsFromOverflow && cur->klass_or_null() != NULL) {
     i++; cur = oop(cur->klass());
   }
 
   // Reattach remaining (suffix) to overflow list
-  if (cur->klass() != NULL) {
+  if (cur->klass_or_null() != NULL) {
     oop suffix = oop(cur->klass());
     cur->set_klass_to_list_ptr(NULL);
 
     // Find last item of suffix list
     oop last = suffix;
-    while (last->klass() != NULL) {
+    while (last->klass_or_null() != NULL) {
       last = oop(last->klass());
     }
     // Atomically prepend suffix to current overflow list

--- a/hotspot/src/share/vm/gc_interface/collectedHeap.inline.hpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/share/vm/gc_interface/collectedHeap.inline.hpp	Wed May 28 21:06:24 2008 -0700
@@ -90,11 +90,12 @@
                                                 HeapWord* obj,
                                                 size_t size,
                                                 int length) {
+  // Set array length before setting the _klass field
+  // in post_allocation_setup_common() because the klass field
+  // indicates that the object is parsable by concurrent GC.
   assert(length >= 0, "length should be non-negative");
+  ((arrayOop)obj)->set_length(length);
   post_allocation_setup_common(klass, obj, size);
-  // Must set length after installing klass as set_klass zeros the length
-  // field in UseCompressedOops
-  ((arrayOop)obj)->set_length(length);
   assert(((oop)obj)->blueprint()->oop_is_array(), "must be an array");
   // notify jvmti and dtrace (must be after length is set for dtrace)
   post_allocation_notify(klass, (oop)obj);
@@ -224,6 +225,7 @@
   assert(obj != NULL, "cannot initialize NULL object");
   const size_t hs = oopDesc::header_size();
   assert(size >= hs, "unexpected object size");
+  ((oop)obj)->set_klass_gap(0);
   Copy::fill_to_aligned_words(obj + hs, size - hs);
 }
 

--- a/hotspot/src/share/vm/interpreter/bytecodeInterpreter.cpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/share/vm/interpreter/bytecodeInterpreter.cpp	Wed May 28 21:06:24 2008 -0700
@@ -1931,6 +1931,7 @@
               } else {
                 result->set_mark(markOopDesc::prototype());
               }
+              result->set_klass_gap(0);
               result->set_klass(k_entry);
               SET_STACK_OBJECT(result, 0);
               UPDATE_PC_AND_TOS_AND_CONTINUE(3, 1);

--- a/hotspot/src/share/vm/memory/space.cpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/share/vm/memory/space.cpp	Wed May 28 21:06:24 2008 -0700
@@ -815,6 +815,7 @@
            "size for smallest fake object doesn't match");
     instanceOop obj = (instanceOop) allocate(size);
     obj->set_mark(markOopDesc::prototype());
+    obj->set_klass_gap(0);
     obj->set_klass(SystemDictionary::object_klass());
   }
 }

--- a/hotspot/src/share/vm/oops/oop.hpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/share/vm/oops/oop.hpp	Wed May 28 21:06:24 2008 -0700
@@ -77,10 +77,15 @@
   void init_mark();
 
   klassOop klass() const;
+  klassOop klass_or_null() const volatile;
   oop* klass_addr();
   narrowOop* compressed_klass_addr();
 
   void set_klass(klassOop k);
+
+  // For klass field compression
+  int klass_gap() const;
+  void set_klass_gap(int z);
   // For when the klass pointer is being used as a linked list "next" field.
   void set_klass_to_list_ptr(oop k);
 

--- a/hotspot/src/share/vm/oops/oop.inline.hpp	Wed May 28 12:42:34 2008 -0700
+++ b/hotspot/src/share/vm/oops/oop.inline.hpp	Wed May 28 21:06:24 2008 -0700
@@ -36,7 +36,15 @@
 inline klassOop oopDesc::klass() const {
   if (UseCompressedOops) {
     return (klassOop)decode_heap_oop_not_null(_metadata._compressed_klass);
-      // can be NULL in CMS, but isn't supported on CMS yet.
+  } else {
+    return _metadata._klass;
+  }
+}
+
+inline klassOop oopDesc::klass_or_null() const volatile {
+  // can be NULL in CMS
+  if (UseCompressedOops) {
+    return (klassOop)decode_heap_oop(_metadata._compressed_klass);
   } else {
     return _metadata._klass;
   }
@@ -64,15 +72,22 @@
   assert(Universe::is_bootstrapping() || k != NULL, "must be a real klassOop");
   assert(Universe::is_bootstrapping() || k->is_klass(), "not a klassOop");
   if (UseCompressedOops) {
-    // zero the gap when the klass is set, by zeroing the pointer sized
-    // part of the union.
-    _metadata._klass = NULL;
     oop_store_without_check(compressed_klass_addr(), (oop)k);
   } else {
     oop_store_without_check(klass_addr(), (oop) k);
   }
 }
 
+inline int oopDesc::klass_gap() const {
+  return *(int*)(((intptr_t)this) + klass_gap_offset_in_bytes());
+}
+
+inline void oopDesc::set_klass_gap(int v) {
+  if (UseCompressedOops) {
+    *(int*)(((intptr_t)this) + klass_gap_offset_in_bytes()) = v;
+  }
+}
+
 inline void oopDesc::set_klass_to_list_ptr(oop k) {
   // This is only to be used during GC, for from-space objects, so no
   // barrier is needed.
@@ -505,7 +520,7 @@
   // try to find metaclass cycle safely without seg faulting on bad input
   // we should reach klassKlassObj by following klass link at most 3 times
   for (int i = 0; i < 3; i++) {
-    obj = obj->klass();
+    obj = obj->klass_or_null();
     // klass should be aligned and in permspace
     if (!check_obj_alignment(obj)) return false;
     if (!Universe::heap()->is_in_permanent(obj)) return false;