Stack allocate DFS::scc_stack and DFS::worklist

[Jan Hubicka]

Hi,
another common (ab)use of malloc/free is the DEF worklist and stack
used in streaming out.  Since most of SCCs are small, we could easily
use stack for them in majority of time.

Bootstrapped/regtested x86_64-linux, OK?

        * lto-streamer-out.c (DFS::sccstack): Turn into auto-vec;
        preallocate for 32 entries.
        (DFS::worklist_vec): Likewise.
        (DFS::DFS): Do not initialize sccstack and worklist.
        (DFS::~DFS): Do not release sccstack.

Index: lto-streamer-out.c
===================================================================
--- lto-streamer-out.c  (revision 278464)
+++ lto-streamer-out.c  (working copy)
@@ -514,7 +514,7 @@ public:
     tree t;
     hashval_t hash;
   };
-  vec<scc_entry> sccstack;
+  auto_vec<scc_entry, 32> sccstack;
 
 private:
   struct sccs
@@ -544,7 +544,7 @@ private:
            bool ref_p, bool this_ref_p);
 
   hash_map<tree, sccs *> sccstate;
-  vec<worklist> worklist_vec;
+  auto_vec<worklist, 32> worklist_vec;
   struct obstack sccstate_obstack;
 };
 
@@ -558,9 +558,7 @@ DFS::DFS (struct output_block *ob, tree
          bool single_p)
 {
   unsigned int next_dfs_num = 1;
-  sccstack.create (0);
   gcc_obstack_init (&sccstate_obstack);
-  worklist_vec = vNULL;
   DFS_write_tree (ob, NULL, expr, ref_p, this_ref_p);
   while (!worklist_vec.is_empty ())
     {
@@ -735,12 +733,10 @@ DFS::DFS (struct output_block *ob, tree
        from_state->low = MIN (cstate->dfsnum, from_state->low);
       worklist_vec.pop ();
     }
-  worklist_vec.release ();
 }
 
 DFS::~DFS ()
 {
-  sccstack.release ();
   obstack_free (&sccstate_obstack, NULL);
 }