[Bug tree-optimization/44563] GCC uses a lot of RAM when compiling a large numbers of functions

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44563

--- Comment #29 from Richard Biener <rguenth at gcc dot gnu.org> ---
Sth like

@@ -672,8 +650,18 @@ cleanup_tree_cfg_bb (basic_block bb)
   if (single_succ_p (bb)
       && can_merge_blocks_p (bb, single_succ (bb)))
     {
-      merge_blocks (bb, single_succ (bb));
-      return true;
+      /* If there is a merge opportunity with the predecessor
+         do nothing now but wait until we process the predecessor.
+        This happens when we visit BBs in a non-optimal order and
+        avoids quadratic behavior with adjusting stmts BB pointer.  */
+      if (single_pred_p (bb)
+         && can_merge_blocks_p (single_pred (bb), bb))
+       ;
+      else
+       {
+         merge_blocks (bb, single_succ (bb));
+         return true;
+       }
     }

   return retval;

in addition should do the job.  Iteration on the predecessor should
cover both merges (so we don't actually need to revisit this block itself).