Avoid full DOM walk in LIM fill_always_executed_in

This avoids a full DOM walk via get_loop_body_in_dom_order in the
loop body walk of fill_always_executed_in which is often terminating
the walk of a loop body early by integrating the DOM walk of
get_loop_body_in_dom_order with the actual processing done by
fill_always_executed_in.  This trades the fully populated loop
body array with a worklist allocation of the same size and thus
should be a strict improvement over the recursive approach of
get_loop_body_in_dom_order.

2021-09-09  Richard Biener  <rguenther@suse.de>

	* tree-ssa-loop-im.c (fill_always_executed_in_1): Integrate
	DOM walk from get_loop_body_in_dom_order using a worklist
	approach.

diff --git a/gcc/tree-ssa-loop-im.c b/gcc/tree-ssa-loop-im.c
index 01f3fc2f7f03127aebfeec1cb3bc299090781c77..5d6845478e76c70cc3f2baec06d59a37b53902c2 100644 (file)
--- a/gcc/tree-ssa-loop-im.c
+++ b/gcc/tree-ssa-loop-im.c
@@ -3030,19 +3030,19 @@ do_store_motion (void)
 static void
 fill_always_executed_in_1 (class loop *loop, sbitmap contains_call)
 {
-  basic_block bb = NULL, *bbs, last = NULL;
-  unsigned i;
+  basic_block bb = NULL, last = NULL;
   edge e;
   class loop *inn_loop = loop;
 
   if (ALWAYS_EXECUTED_IN (loop->header) == NULL)
     {
-      bbs = get_loop_body_in_dom_order (loop);
-
-      for (i = 0; i < loop->num_nodes; i++)
+      auto_vec<basic_block, 64> worklist;
+      worklist.reserve_exact (loop->num_nodes);
+      worklist.quick_push (loop->header);
+      do
        {
          edge_iterator ei;
-         bb = bbs[i];
+         bb = worklist.pop ();
 
          if (!flow_bb_inside_loop_p (inn_loop, bb))
            {
@@ -3083,7 +3083,32 @@ fill_always_executed_in_1 (class loop *loop, sbitmap contains_call)
                 since it might not be finite.  */
              inn_loop = bb->loop_father;
            }
+
+         /* Walk the body of LOOP sorted by dominance relation.  Additionally,
+            if a basic block S dominates the latch, then only blocks dominated
+            by S are after it.
+            This is get_loop_body_in_dom_order using a worklist algorithm and
+            stopping once we are no longer interested in visiting further
+            blocks.  */
+         unsigned old_len = worklist.length ();
+         unsigned postpone = 0;
+         for (basic_block son = first_dom_son (CDI_DOMINATORS, bb);
+              son;
+              son = next_dom_son (CDI_DOMINATORS, son))
+           {
+             if (!flow_bb_inside_loop_p (loop, son))
+               continue;
+             if (dominated_by_p (CDI_DOMINATORS, loop->latch, son))
+               postpone = worklist.length ();
+             worklist.quick_push (son);
+           }
+         if (postpone)
+           /* Postponing the block that dominates the latch means
+              processing it last and thus putting it earliest in the
+              worklist.  */
+           std::swap (worklist[old_len], worklist[postpone]);
        }
+      while (!worklist.is_empty ());
 
       while (1)
        {
@@ -3095,8 +3120,6 @@ fill_always_executed_in_1 (class loop *loop, sbitmap contains_call)
            break;
          last = get_immediate_dominator (CDI_DOMINATORS, last);
        }
-
-      free (bbs);
     }
 
   for (loop = loop->inner; loop; loop = loop->next)