diff --git a/gcc/ada/gcc-interface/utils.c b/gcc/ada/gcc-interface/utils.c
index eac87e0..dd06ca3 100644
--- a/gcc/ada/gcc-interface/utils.c
+++ b/gcc/ada/gcc-interface/utils.c
@@ -395,8 +395,8 @@ gnat_poplevel (void)
   else if (BLOCK_VARS (block) == NULL_TREE)
     {
       BLOCK_SUBBLOCKS (level->chain->block)
-	= chainon (BLOCK_SUBBLOCKS (block),
-		   BLOCK_SUBBLOCKS (level->chain->block));
+	= block_chainon (BLOCK_SUBBLOCKS (block),
+			 BLOCK_SUBBLOCKS (level->chain->block));
       BLOCK_CHAIN (block) = free_block_chain;
       free_block_chain = block;
     }
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index a0ef39f..de96ac2 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -794,7 +794,7 @@ poplevel (int keep, int reverse, int functionbody)
     }
   else if (block)
     current_binding_level->blocks
-      = chainon (current_binding_level->blocks, block);
+      = block_chainon (current_binding_level->blocks, block);
 
   /* If we did not make a block for the level just exited,
      any blocks made for inner levels
@@ -803,7 +803,7 @@ poplevel (int keep, int reverse, int functionbody)
      of something else.  */
   else if (subblocks)
     current_binding_level->blocks
-      = chainon (current_binding_level->blocks, subblocks);
+      = block_chainon (current_binding_level->blocks, subblocks);
 
   /* Each and every BLOCK node created here in `poplevel' is important
      (e.g. for proper debugging information) so if we created one
diff --git a/gcc/fortran/f95-lang.c b/gcc/fortran/f95-lang.c
index 687f60b..aebe397 100644
--- a/gcc/fortran/f95-lang.c
+++ b/gcc/fortran/f95-lang.c
@@ -444,7 +444,7 @@ poplevel (int keep, int reverse, int functionbody)
 
   /* Record the BLOCK node just built as the subblock its enclosing scope.  */
   for (subblock_node = subblock_chain; subblock_node;
-       subblock_node = TREE_CHAIN (subblock_node))
+       subblock_node = BLOCK_CHAIN (subblock_node))
     BLOCK_SUPERCONTEXT (subblock_node) = block_node;
 
   /* Clear out the meanings of the local variables of this level.  */
@@ -475,7 +475,7 @@ poplevel (int keep, int reverse, int functionbody)
   else if (block_node)
     {
       current_binding_level->blocks
-	= chainon (current_binding_level->blocks, block_node);
+	= block_chainon (current_binding_level->blocks, block_node);
     }
 
   /* If we did not make a block for the level just exited, any blocks made for
@@ -484,7 +484,7 @@ poplevel (int keep, int reverse, int functionbody)
      else.  */
   else if (subblock_chain)
     current_binding_level->blocks
-      = chainon (current_binding_level->blocks, subblock_chain);
+      = block_chainon (current_binding_level->blocks, subblock_chain);
   if (block_node)
     TREE_USED (block_node) = 1;
 
diff --git a/gcc/function.c b/gcc/function.c
index 3f721fb..094cf74 100644
--- a/gcc/function.c
+++ b/gcc/function.c
@@ -4167,6 +4167,34 @@ blocks_nreverse (tree t)
   return prev;
 }
 
+/* Concatenate two chains of blocks (chained through BLOCK_CHAIN)
+   by modifying the last node in chain 1 to point to chain 2.  */
+
+tree
+block_chainon (tree op1, tree op2)
+{
+  tree t1;
+
+  if (!op1)
+    return op2;
+  if (!op2)
+    return op1;
+
+  for (t1 = op1; BLOCK_CHAIN (t1); t1 = BLOCK_CHAIN (t1))
+    continue;
+  BLOCK_CHAIN (t1) = op2;
+
+#ifdef ENABLE_TREE_CHECKING
+  {
+    tree t2;
+    for (t2 = op2; t2; t2 = BLOCK_CHAIN (t2))
+      gcc_assert (t2 != t1);
+  }
+#endif
+
+  return op1;
+}
+
 /* Count the subblocks of the list starting with BLOCK.  If VECTOR is
    non-NULL, list them all into VECTOR, in a depth-first preorder
    traversal of the block tree.  Also clear TREE_ASM_WRITTEN in all
diff --git a/gcc/function.h b/gcc/function.h
index 6e7f539..73af294 100644
--- a/gcc/function.h
+++ b/gcc/function.h
@@ -713,6 +713,7 @@ extern void number_blocks (tree);
 
 extern void clear_block_marks (tree);
 extern tree blocks_nreverse (tree);
+extern tree block_chainon (tree, tree);
 
 /* Return size needed for stack frame based on slots so far allocated.
    This size counts from zero.  It is not rounded to STACK_BOUNDARY;
diff --git a/gcc/java/decl.c b/gcc/java/decl.c
index a17b826..6e94dff 100644
--- a/gcc/java/decl.c
+++ b/gcc/java/decl.c
@@ -1481,7 +1481,7 @@ poplevel (int keep, int reverse, int functionbody)
 
   /* In each subblock, record that this is its superior.  */
 
-  for (link = subblocks; link; link = TREE_CHAIN (link))
+  for (link = subblocks; link; link = BLOCK_CHAIN (link))
     BLOCK_SUPERCONTEXT (link) = block;
 
   /* Clear out the meanings of the local variables of this level.  */
@@ -1545,7 +1545,7 @@ poplevel (int keep, int reverse, int functionbody)
       if (block)
 	{
 	  current_binding_level->blocks
-	    = chainon (current_binding_level->blocks, block);
+	    = block_chainon (current_binding_level->blocks, block);
 	}
       /* If we did not make a block for the level just exited,
 	 any blocks made for inner levels
@@ -1554,7 +1554,7 @@ poplevel (int keep, int reverse, int functionbody)
 	 of something else.  */
       else if (subblocks)
 	current_binding_level->blocks
-	  = chainon (current_binding_level->blocks, subblocks);
+	  = block_chainon (current_binding_level->blocks, subblocks);
 
       if (bind)
 	java_add_stmt (bind);
