1 files changed, 63 insertions, 26 deletions

diff --git a/src/coreclr/gc/gc.cpp b/src/coreclr/gc/gc.cpp
index c7f2b177f52..7db0faacc26 100644
--- a/src/coreclr/gc/gc.cpp
+++ b/src/coreclr/gc/gc.cpp
@@ -4165,7 +4165,16 @@ size_t gcard_of ( uint8_t*);
 // We only do this when we decide to compact.
 #define BGC_MARKED_BY_FGC (size_t)0x2
 #define MAKE_FREE_OBJ_IN_COMPACT (size_t)0x4
-#endif //DOUBLY_LINKED_FL
+#define ALLOWED_SPECIAL_HEADER_BITS (GC_MARKED|BGC_MARKED_BY_FGC|MAKE_FREE_OBJ_IN_COMPACT)
+#else //DOUBLY_LINKED_FL
+#define ALLOWED_SPECIAL_HEADER_BITS (GC_MARKED)
+#endif //!DOUBLY_LINKED_FL
+
+#ifdef HOST_64BIT
+#define SPECIAL_HEADER_BITS (0x7)
+#else
+#define SPECIAL_HEADER_BITS (0x3)
+#endif
 
 #define slot(i, j) ((uint8_t**)(i))[(j)+1]
 
@@ -4267,11 +4276,7 @@ public:
 
     MethodTable    *GetMethodTable() const
     {
-        return( (MethodTable *) (((size_t) RawGetMethodTable()) & (~(GC_MARKED
-#ifdef DOUBLY_LINKED_FL
-            | BGC_MARKED_BY_FGC | MAKE_FREE_OBJ_IN_COMPACT
-#endif //DOUBLY_LINKED_FL
-            ))));
+        return( (MethodTable *) (((size_t) RawGetMethodTable()) & (~SPECIAL_HEADER_BITS)));
     }
 
     void SetMarked()
@@ -4339,6 +4344,26 @@ public:
     }
 #endif //DOUBLY_LINKED_FL
 
+    size_t ClearSpecialBits()
+    {
+        size_t special_bits = ((size_t)RawGetMethodTable()) & SPECIAL_HEADER_BITS;
+        if (special_bits != 0)
+        {
+            assert ((special_bits & (~ALLOWED_SPECIAL_HEADER_BITS)) == 0);
+            RawSetMethodTable ((MethodTable*)(((size_t)RawGetMethodTable()) & ~(SPECIAL_HEADER_BITS)));
+        }
+        return special_bits;
+    }
+
+    void SetSpecialBits (size_t special_bits)
+    {
+        assert ((special_bits & (~ALLOWED_SPECIAL_HEADER_BITS)) == 0);
+        if (special_bits != 0)
+        {
+            RawSetMethodTable ((MethodTable*)(((size_t)RawGetMethodTable()) | special_bits));
+        }
+    }
+
     CGCDesc *GetSlotMap ()
     {
         assert (GetMethodTable()->ContainsPointers());
@@ -4506,6 +4531,17 @@ inline
 BOOL is_plug_padded (uint8_t* node){return FALSE;}
 #endif //SHORT_PLUGS
 
+inline
+size_t clear_special_bits (uint8_t* node)
+{
+    return header(node)->ClearSpecialBits();
+}
+
+inline
+void set_special_bits (uint8_t* node, size_t special_bits)
+{
+    header(node)->SetSpecialBits (special_bits);
+}
 
 inline size_t unused_array_size(uint8_t * p)
 {
@@ -20978,16 +21014,16 @@ void gc_heap::enque_pinned_plug (uint8_t* plug,
 
     if (save_pre_plug_info_p)
     {
-#ifdef SHORT_PLUGS
-        BOOL is_padded = is_plug_padded (last_object_in_last_plug);
-        if (is_padded)
-            clear_plug_padded (last_object_in_last_plug);
-#endif //SHORT_PLUGS
+        // In the case of short plugs or doubly linked free lists, there may be extra bits
+        // set in the method table pointer.
+        // Clear these bits for the copy saved in saved_pre_plug, but not for the copy
+        // saved in saved_pre_plug_reloc.
+        // This is because we need these bits for compaction, but not for mark & sweep.
+        size_t special_bits = clear_special_bits (last_object_in_last_plug);
+        // now copy the bits over
         memcpy (&(m.saved_pre_plug), &(((plug_and_gap*)plug)[-1]), sizeof (gap_reloc_pair));
-#ifdef SHORT_PLUGS
-        if (is_padded)
-            set_plug_padded (last_object_in_last_plug);
-#endif //SHORT_PLUGS
+        // restore the bits in the original
+        set_special_bits (last_object_in_last_plug, special_bits);
 
         memcpy (&(m.saved_pre_plug_reloc), &(((plug_and_gap*)plug)[-1]), sizeof (gap_reloc_pair));
 
@@ -20997,7 +21033,7 @@ void gc_heap::enque_pinned_plug (uint8_t* plug,
         {
             record_interesting_data_point (idp_pre_short);
 #ifdef SHORT_PLUGS
-            if (is_padded)
+            if (is_plug_padded (last_object_in_last_plug))
                 record_interesting_data_point (idp_pre_short_padded);
 #endif //SHORT_PLUGS
             dprintf (3, ("encountered a short object %Ix right before pinned plug %Ix!",
@@ -21041,16 +21077,17 @@ void gc_heap::save_post_plug_info (uint8_t* last_pinned_plug, uint8_t* last_obje
     assert (last_pinned_plug == m.first);
     m.saved_post_plug_info_start = (uint8_t*)&(((plug_and_gap*)post_plug)[-1]);
 
-#ifdef SHORT_PLUGS
-    BOOL is_padded = is_plug_padded (last_object_in_last_plug);
-    if (is_padded)
-        clear_plug_padded (last_object_in_last_plug);
-#endif //SHORT_PLUGS
+    // In the case of short plugs or doubly linked free lists, there may be extra bits
+    // set in the method table pointer.
+    // Clear these bits for the copy saved in saved_post_plug, but not for the copy
+    // saved in saved_post_plug_reloc.
+    // This is because we need these bits for compaction, but not for mark & sweep.
+    // Note that currently none of these bits will ever be set in the object saved *after*
+    // a pinned plug - this object is currently pinned along with the pinned object before it
+    size_t special_bits = clear_special_bits (last_object_in_last_plug);
     memcpy (&(m.saved_post_plug), m.saved_post_plug_info_start, sizeof (gap_reloc_pair));
-#ifdef SHORT_PLUGS
-    if (is_padded)
-        set_plug_padded (last_object_in_last_plug);
-#endif //SHORT_PLUGS
+    // restore the bits in the original
+    set_special_bits (last_object_in_last_plug, special_bits);
 
     memcpy (&(m.saved_post_plug_reloc), m.saved_post_plug_info_start, sizeof (gap_reloc_pair));
 
@@ -21069,7 +21106,7 @@ void gc_heap::save_post_plug_info (uint8_t* last_pinned_plug, uint8_t* last_obje
         dprintf (3, ("PP %Ix last obj %Ix is too short", last_pinned_plug, last_object_in_last_plug));
         record_interesting_data_point (idp_post_short);
 #ifdef SHORT_PLUGS
-        if (is_padded)
+        if (is_plug_padded (last_object_in_last_plug))
             record_interesting_data_point (idp_post_short_padded);
 #endif //SHORT_PLUGS
         m.set_post_short();