[PATCH] Improve PR46590 compile-time, fix RPO VN regression

Richard Biener rguenther@suse.de
Mon Apr 1 08:54:00 GMT 2019


This fixes a quadraticness in region RPO VN which happens because
dom_walker computes whole-function RPO order in its constructor
even though in the end we do not use it.  That's an artifact of
C++ (we derive from dom_walker).

The fix is to lazily do costly initialization in the ::walk
method.  I'm not sure we can re-use previous edge flag state
for multiple ::walk invocations when the user said
REACHABLE_BLOCKS instead of REACHABLE_BLOCKS_PRESERVING_FLAGS
so I've gone for the safe side (but I'm quite sure we do not
have a consumer walking multiple times using the same class
instance anyways).

This cuts down GIMPLE copy-header time from 11% to 0% on the
second testcase from PR46590, leaving RTL invariant motion
as the worst offender at 50% of the compile-time (through it's use of DF).

Bootstrap / regtest running on x86_64-unknown-linux-gnu.

Richard.

2019-04-01  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/46590
	* domwalk.h (dom_walker::dom_walker): Consolidate constructors.
	(dom_walker::m_reachability): Add in place of...
	(dom_walker::m_skip_unreachable_blocks): ...this.
	* domwalk.c (dom_walker::dom_walker): Consoliate constructors.
	Move complex initialization ...
	(dom_walker::walk): Here.  Especially compute m_bb_to_rpo
	lazily and initialize edge flags on each invocation.
	(dom_walker::bb_reachable): Use m_reachability.

Index: gcc/domwalk.c
===================================================================
--- gcc/domwalk.c	(revision 270053)
+++ gcc/domwalk.c	(working copy)
@@ -190,69 +190,11 @@ dom_walker::dom_walker (cdi_direction di
 			enum reachability reachability,
 			int *bb_index_to_rpo)
   : m_dom_direction (direction),
-    m_skip_unreachable_blocks (reachability != ALL_BLOCKS),
-    m_user_bb_to_rpo (true),
+    m_reachability (reachability),
+    m_user_bb_to_rpo (bb_index_to_rpo != NULL),
     m_unreachable_dom (NULL),
     m_bb_to_rpo (bb_index_to_rpo)
 {
-  /* Set up edge flags if need be.  */
-  switch (reachability)
-    {
-    default:
-      gcc_unreachable ();
-    case ALL_BLOCKS:
-      /* No need to touch edge flags.  */
-      break;
-
-    case REACHABLE_BLOCKS:
-      set_all_edges_as_executable (cfun);
-      break;
-
-    case REACHABLE_BLOCKS_PRESERVING_FLAGS:
-      /* Preserve the edge flags.  */
-      break;
-    }
-}
-
-/* Constructor for a dom walker.  */
-
-dom_walker::dom_walker (cdi_direction direction,
-			enum reachability reachability)
-  : m_dom_direction (direction),
-    m_skip_unreachable_blocks (reachability != ALL_BLOCKS),
-    m_user_bb_to_rpo (false),
-    m_unreachable_dom (NULL),
-    m_bb_to_rpo (NULL)
-{
-  /* Compute the basic-block index to RPO mapping.  */
-  if (direction == CDI_DOMINATORS)
-    {
-      int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
-      int postorder_num = pre_and_rev_post_order_compute (NULL, postorder,
-							  true);
-      m_bb_to_rpo = XNEWVEC (int, last_basic_block_for_fn (cfun));
-      for (int i = 0; i < postorder_num; ++i)
-	m_bb_to_rpo[postorder[i]] = i;
-      free (postorder);
-    }
-
-  /* Set up edge flags if need be.  */
-  switch (reachability)
-    {
-    default:
-      gcc_unreachable ();
-    case ALL_BLOCKS:
-      /* No need to touch edge flags.  */
-      break;
-
-    case REACHABLE_BLOCKS:
-      set_all_edges_as_executable (cfun);
-      break;
-
-    case REACHABLE_BLOCKS_PRESERVING_FLAGS:
-      /* Preserve the edge flags.  */
-      break;
-    }
 }
 
 /* Destructor.  */
@@ -270,7 +212,7 @@ dom_walker::bb_reachable (struct functio
 {
   /* If we're not skipping unreachable blocks, then assume everything
      is reachable.  */
-  if (!m_skip_unreachable_blocks)
+  if (m_reachability == ALL_BLOCKS)
     return true;
 
   /* If any of the predecessor edges that do not come from blocks dominated
@@ -331,6 +273,23 @@ const edge dom_walker::STOP = (edge)-1;
 void
 dom_walker::walk (basic_block bb)
 {
+  /* Compute the basic-block index to RPO mapping lazily.  */
+  if (!m_bb_to_rpo
+      && m_dom_direction == CDI_DOMINATORS)
+    {
+      int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
+      int postorder_num = pre_and_rev_post_order_compute (NULL, postorder,
+							  true);
+      m_bb_to_rpo = XNEWVEC (int, last_basic_block_for_fn (cfun));
+      for (int i = 0; i < postorder_num; ++i)
+	m_bb_to_rpo[postorder[i]] = i;
+      free (postorder);
+    }
+
+  /* Set up edge flags if need be.  */
+  if (m_reachability == REACHABLE_BLOCKS)
+    set_all_edges_as_executable (cfun);
+
   basic_block dest;
   basic_block *worklist = XNEWVEC (basic_block,
 				   n_basic_blocks_for_fn (cfun) * 2);
Index: gcc/domwalk.h
===================================================================
--- gcc/domwalk.h	(revision 270053)
+++ gcc/domwalk.h	(working copy)
@@ -60,13 +60,12 @@ public:
     REACHABLE_BLOCKS_PRESERVING_FLAGS
   };
 
-  dom_walker (cdi_direction direction, enum reachability = ALL_BLOCKS);
-
   /* You can provide a mapping of basic-block index to RPO if you
      have that readily available or you do multiple walks.  If you
      specify NULL as BB_INDEX_TO_RPO dominator children will not be
      walked in RPO order.  */
-  dom_walker (cdi_direction direction, enum reachability, int *bb_index_to_rpo);
+  dom_walker (cdi_direction direction, enum reachability = ALL_BLOCKS,
+	      int *bb_index_to_rpo = NULL);
 
   ~dom_walker ();
 
@@ -94,7 +93,7 @@ private:
      if it is set to CDI_POST_DOMINATORS, then we walk the post
      dominator tree.  */
   const ENUM_BITFIELD (cdi_direction) m_dom_direction : 2;
-  bool m_skip_unreachable_blocks;
+  const ENUM_BITFIELD (reachability) m_reachability : 2;
   bool m_user_bb_to_rpo;
   basic_block m_unreachable_dom;
   int *m_bb_to_rpo;



More information about the Gcc-patches mailing list