diff --git a/src/hotspot/share/gc/parallel/psParallelCompact.cpp b/src/hotspot/share/gc/parallel/psParallelCompact.cpp
index d321c4746f410..0c3ea4b4ba3e4 100644
--- a/src/hotspot/share/gc/parallel/psParallelCompact.cpp
+++ b/src/hotspot/share/gc/parallel/psParallelCompact.cpp
@@ -773,6 +773,8 @@ void PSParallelCompact::fill_dense_prefix_end(SpaceId id) {
   //
   // The size of the filler (min-obj-size) is 2 heap words with the default
   // MinObjAlignment, since both markword and klass take 1 heap word.
+  // With +UseCompactObjectHeaders, the minimum filler size is only one word,
+  // because both Klass* gets encoded in the mark-word.
   //
   // The size of the gap (if any) right before dense-prefix-end is
   // MinObjAlignment.
@@ -781,17 +783,13 @@ void PSParallelCompact::fill_dense_prefix_end(SpaceId id) {
   // filler obj will extend to next region.
 
   // Note: If min-fill-size decreases to 1, this whole method becomes redundant.
-  if (UseCompactObjectHeaders) {
-    // The gap is always equal to min-fill-size, so nothing to do.
-    return;
-  }
   assert(CollectedHeap::min_fill_size() >= 2, "inv");
 #ifndef _LP64
   // In 32-bit system, each heap word is 4 bytes, so MinObjAlignment == 2.
   // The gap is always equal to min-fill-size, so nothing to do.
   return;
 #endif
-  if (MinObjAlignment > 1) {
+  if (MinObjAlignment >= checked_cast<int>(CollectedHeap::min_fill_size())) {
     return;
   }
   assert(CollectedHeap::min_fill_size() == 2, "inv");
diff --git a/src/hotspot/share/gc/shared/collectedHeap.cpp b/src/hotspot/share/gc/shared/collectedHeap.cpp
index 7c5ba66f28178..9e08449659be1 100644
--- a/src/hotspot/share/gc/shared/collectedHeap.cpp
+++ b/src/hotspot/share/gc/shared/collectedHeap.cpp
@@ -220,7 +220,7 @@ bool CollectedHeap::supports_concurrent_gc_breakpoints() const {
   return false;
 }
 
-bool klass_is_sane(oop object) {
+static bool klass_is_sane(oop object) {
   Klass* klass;
   if (UseCompactObjectHeaders) {
     // With compact headers, we can't safely access the Klass* when
@@ -229,17 +229,16 @@ bool klass_is_sane(oop object) {
     // the forwarding pointer, and here we have no way to make a
     // distinction between Full-GC and regular GC forwarding.
     markWord mark = object->mark();
-    if (!mark.is_forwarded()) {
-      klass = mark.klass_without_asserts();
-    } else {
+    if (mark.is_forwarded()) {
       // We can't access the Klass*. We optimistically assume that
       // it is ok. This happens very rarely.
-      klass = nullptr;
+      return true;
     }
-  } else {
-    klass = object->klass_without_asserts();
+
+    return Metaspace::contains(mark.klass_without_asserts());
   }
-  return klass == nullptr || Metaspace::contains(klass);
+
+  return Metaspace::contains(object->klass_without_asserts());
 }
 
 bool CollectedHeap::is_oop(oop object) const {
diff --git a/src/hotspot/share/jvmci/vmStructs_jvmci.cpp b/src/hotspot/share/jvmci/vmStructs_jvmci.cpp
index 8f94b73e92f0a..5781b1660363f 100644
--- a/src/hotspot/share/jvmci/vmStructs_jvmci.cpp
+++ b/src/hotspot/share/jvmci/vmStructs_jvmci.cpp
@@ -279,6 +279,7 @@
   nonstatic_field(Klass,                       _bitmap,                                       uintx)                                 \
   nonstatic_field(Klass,                       _hash_slot,                                    uint8_t)                               \
   nonstatic_field(Klass,                       _misc_flags._flags,                            u1)                                    \
+  nonstatic_field(Klass,                       _prototype_header,                             markWord)                              \
                                                                                                                                      \
   nonstatic_field(LocalVariableTableElement,   start_bci,                                     u2)                                    \
   nonstatic_field(LocalVariableTableElement,   length,                                        u2)                                    \
diff --git a/src/hotspot/share/oops/compressedKlass.cpp b/src/hotspot/share/oops/compressedKlass.cpp
index f8b08bed2f398..fd25bf3421cef 100644
--- a/src/hotspot/share/oops/compressedKlass.cpp
+++ b/src/hotspot/share/oops/compressedKlass.cpp
@@ -25,7 +25,6 @@
 #include "precompiled.hpp"
 #include "logging/log.hpp"
 #include "memory/metaspace.hpp"
-#include "oops/klass.hpp"
 #include "oops/compressedKlass.inline.hpp"
 #include "runtime/globals.hpp"
 #include "runtime/java.hpp"
diff --git a/src/hotspot/share/oops/markWord.cpp b/src/hotspot/share/oops/markWord.cpp
index a44f0185f6c59..a9b1a7b026ac6 100644
--- a/src/hotspot/share/oops/markWord.cpp
+++ b/src/hotspot/share/oops/markWord.cpp
@@ -31,7 +31,7 @@
 
 #ifdef _LP64
 STATIC_ASSERT(markWord::klass_shift + markWord::klass_bits == 64);
-// The hash (preceding nKlass) shall be a direct neighbor but not interleave
+// The hash (preceding klass bits) shall be a direct neighbor but not interleave
 STATIC_ASSERT(markWord::klass_shift == markWord::hash_bits + markWord::hash_shift);
 #endif
 
diff --git a/src/hotspot/share/oops/markWord.inline.hpp b/src/hotspot/share/oops/markWord.inline.hpp
index ded1e10a57c77..ba394ab4b6e7e 100644
--- a/src/hotspot/share/oops/markWord.inline.hpp
+++ b/src/hotspot/share/oops/markWord.inline.hpp
@@ -31,7 +31,6 @@
 narrowKlass markWord::narrow_klass() const {
 #ifdef _LP64
   assert(UseCompactObjectHeaders, "only used with compact object headers");
-  const narrowKlass nk = value() >> klass_shift;
   return narrowKlass(value() >> klass_shift);
 #else
   ShouldNotReachHere();