Index: basic-block.h
===================================================================
--- basic-block.h	(revision 194362)
+++ basic-block.h	(working copy)
@@ -35,7 +35,9 @@ typedef unsigned HOST_WIDEST_INT gcov_type_unsigne
 
 /* Control flow edge information.  */
 struct GTY((user)) edge_def {
-  /* The two blocks at the ends of the edge.  */
+  /* The two blocks at the ends of the edge.  The 'user' marker function
+     should ignore them for GC, because basic_blocks get visited via the
+     basic_block_info array in struct control_flow_graph.  */
   basic_block src;
   basic_block dest;
 
@@ -177,23 +179,25 @@ struct GTY(()) gimple_bb_info {
    basic blocks.  */
 
 /* Basic block information indexed by block number.  */
-struct GTY((chain_next ("%h.next_bb"), chain_prev ("%h.prev_bb"))) basic_block_def {
+struct GTY (()) basic_block_def {
   /* The edges into and out of the block.  */
   vec<edge, va_gc> *preds;
   vec<edge, va_gc> *succs;
 
   /* Auxiliary info specific to a pass.  */
-  PTR GTY ((skip (""))) aux;
+  PTR GTY ((skip)) aux;
 
   /* Innermost loop containing the block.  */
   struct loop *loop_father;
 
   /* The dominance and postdominance information node.  */
-  struct et_node * GTY ((skip (""))) dom[2];
+  struct et_node * GTY ((skip)) dom[2];
 
-  /* Previous and next blocks in the chain.  */
-  basic_block prev_bb;
-  basic_block next_bb;
+  /* Previous and next blocks in the chain.  Ignore them for GC,
+     they get visited via the basic_block_info array in struct
+     control_flow_graph.  */
+  basic_block GTY ((skip)) prev_bb;
+  basic_block GTY ((skip)) next_bb;
 
   union basic_block_il_dependent {
       struct gimple_bb_info GTY ((tag ("0"))) gimple;
@@ -289,8 +293,8 @@ enum profile_status_d
 struct GTY(()) control_flow_graph {
   /* Block pointers for the exit and entry of a function.
      These are always the head and tail of the basic block list.  */
-  basic_block x_entry_block_ptr;
-  basic_block x_exit_block_ptr;
+  basic_block GTY ((skip)) x_entry_block_ptr;
+  basic_block GTY ((skip)) x_exit_block_ptr;
 
   /* Index by basic block number, get basic block struct info.  */
   vec<basic_block, va_gc> *x_basic_block_info;
Index: gimple.h
===================================================================
--- gimple.h	(revision 194362)
+++ gimple.h	(working copy)
@@ -195,8 +195,10 @@ struct GTY((chain_next ("%h.next"))) gimple_statem
   unsigned num_ops;
 
   /* [ WORD 3 ]
-     Basic block holding this statement.  */
-  basic_block bb;
+     Basic block holding this statement.
+     Marking for GC happens via the basic_block_info array in
+     struct control_flow_graph.  */
+  basic_block GTY((skip)) bb;
 
   /* [ WORD 4-5 ]
      Linked lists of gimple statements.  The next pointers form
Index: gengtype.c
===================================================================
--- gengtype.c	(revision 194362)
+++ gengtype.c	(working copy)
@@ -1289,6 +1289,10 @@ adjust_field_rtx_def (type_p t, options_p ARG_UNUS
 	    subfields->opt = 
 	      create_string_option (subfields->opt, "desc",
 				    "CONSTANT_POOL_ADDRESS_P (&%0)");
+	  if (t == basic_block_tp)
+	    subfields->opt =
+	      create_string_option (subfields->opt, "skip",
+				    "");
 	}
 
       if (i == SYMBOL_REF)
Index: tree-cfg.c
===================================================================
--- tree-cfg.c	(revision 194362)
+++ tree-cfg.c	(working copy)
@@ -8023,45 +8023,33 @@ extern void gt_ggc_mx (basic_block&);
 void
 gt_ggc_mx (edge_def *e)
 {
-  tree block = LOCATION_BLOCK (e->goto_locus);
-  gt_ggc_mx (e->src);
-  gt_ggc_mx (e->dest);
+  /* e->src and e->dest are not marked here.  All basic blocks
+     are marked via the basic_block_info array in the CFG data
+     structure hanging from each struct function.  */
+
+  /* So, only mark the other fields.  */
   if (current_ir_type () == IR_GIMPLE)
     gt_ggc_mx (e->insns.g);
   else
     gt_ggc_mx (e->insns.r);
+
+  tree block = LOCATION_BLOCK (e->goto_locus);
   gt_ggc_mx (block);
 }
 
-/* PCH support for edge_def.  */
+/* PCH support, or rather lack thereof, for edge_def.
+   CFG data structures should never end up in a PCH.  */
 
-extern void gt_pch_nx (tree&);
-extern void gt_pch_nx (gimple&);
-extern void gt_pch_nx (rtx&);
-extern void gt_pch_nx (basic_block&);
-
 void
-gt_pch_nx (edge_def *e)
+gt_pch_nx (edge_def *e ATTRIBUTE_UNUSED)
 {
-  tree block = LOCATION_BLOCK (e->goto_locus);
-  gt_pch_nx (e->src);
-  gt_pch_nx (e->dest);
-  if (current_ir_type () == IR_GIMPLE)
-    gt_pch_nx (e->insns.g);
-  else
-    gt_pch_nx (e->insns.r);
-  gt_pch_nx (block);
+  gcc_unreachable ();
 }
 
 void
-gt_pch_nx (edge_def *e, gt_pointer_operator op, void *cookie)
+gt_pch_nx (edge_def *e ATTRIBUTE_UNUSED,
+	   gt_pointer_operator op ATTRIBUTE_UNUSED,
+	   void *cookie ATTRIBUTE_UNUSED)
 {
-  tree block = LOCATION_BLOCK (e->goto_locus);
-  op (&(e->src), cookie);
-  op (&(e->dest), cookie);
-  if (current_ir_type () == IR_GIMPLE)
-    op (&(e->insns.g), cookie);
-  else
-    op (&(e->insns.r), cookie);
-  op (&(block), cookie);
+  gcc_unreachable ();
 }