loop splitting

(approved at https://gcc.gnu.org/ml/gcc-patches/2015-12/msg00648.html )

	* common.opt (-fsplit-loops): New flag.
	* passes.def (pass_loop_split): Add.
	* opts.c (default_options_table): Add OPT_fsplit_loops entry at -O3.
	(enable_fdo_optimizations): Add loop splitting.
	* timevar.def (TV_LOOP_SPLIT): Add.
	* tree-pass.h (make_pass_loop_split): Declare.
	* tree-ssa-loop-manip.h (rewrite_into_loop_closed_ssa_1): Declare.
	* tree-ssa-loop-unswitch.c: Include tree-ssa-loop-manip.h,
	* tree-ssa-loop-split.c: New file.
	* Makefile.in (OBJS): Add tree-ssa-loop-split.o.
	* doc/invoke.texi (fsplit-loops): Document.
	* doc/passes.texi (Loop optimization): Add paragraph about loop
	splitting.

testsuite/
	* gcc.dg/loop-split.c: New test.
	* gcc.dg/tree-ssa/ifc-9.c: Update.
	* gcc.dg/tree-ssa/ifc-10.c: Update.

From-SVN: r241374

1 files changed, 687 insertions, 0 deletions

diff --git a/gcc/tree-ssa-loop-split.c b/gcc/tree-ssa-loop-split.c
new file mode 100644
index 0000000..e2bfd78
--- /dev/null
+++ b/gcc/tree-ssa-loop-split.c
@@ -0,0 +1,687 @@
+/* Loop splitting.
+   Copyright (C) 2015 Free Software Foundation, Inc.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it
+under the terms of the GNU General Public License as published by the
+Free Software Foundation; either version 3, or (at your option) any
+later version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT
+ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "backend.h"
+#include "tree.h"
+#include "gimple.h"
+#include "tree-pass.h"
+#include "ssa.h"
+#include "fold-const.h"
+#include "tree-cfg.h"
+#include "tree-ssa.h"
+#include "tree-ssa-loop-niter.h"
+#include "tree-ssa-loop.h"
+#include "tree-ssa-loop-manip.h"
+#include "tree-into-ssa.h"
+#include "cfgloop.h"
+#include "tree-scalar-evolution.h"
+#include "gimple-iterator.h"
+#include "gimple-pretty-print.h"
+#include "cfghooks.h"
+#include "gimple-fold.h"
+#include "gimplify-me.h"
+
+/* This file implements loop splitting, i.e. transformation of loops like
+
+   for (i = 0; i < 100; i++)
+     {
+       if (i < 50)
+         A;
+       else
+         B;
+     }
+
+   into:
+
+   for (i = 0; i < 50; i++)
+     {
+       A;
+     }
+   for (; i < 100; i++)
+     {
+       B;
+     }
+
+   */
+
+/* Return true when BB inside LOOP is a potential iteration space
+   split point, i.e. ends with a condition like "IV < comp", which
+   is true on one side of the iteration space and false on the other,
+   and the split point can be computed.  If so, also return the border
+   point in *BORDER and the comparison induction variable in IV.  */
+
+static tree
+split_at_bb_p (struct loop *loop, basic_block bb, tree *border, affine_iv *iv)
+{
+  gimple *last;
+  gcond *stmt;
+  affine_iv iv2;
+
+  /* BB must end in a simple conditional jump.  */
+  last = last_stmt (bb);
+  if (!last || gimple_code (last) != GIMPLE_COND)
+    return NULL_TREE;
+  stmt = as_a <gcond *> (last);
+
+  enum tree_code code = gimple_cond_code (stmt);
+
+  /* Only handle relational comparisons, for equality and non-equality
+     we'd have to split the loop into two loops and a middle statement.  */
+  switch (code)
+    {
+      case LT_EXPR:
+      case LE_EXPR:
+      case GT_EXPR:
+      case GE_EXPR:
+	break;
+      default:
+	return NULL_TREE;
+    }
+
+  if (loop_exits_from_bb_p (loop, bb))
+    return NULL_TREE;
+
+  tree op0 = gimple_cond_lhs (stmt);
+  tree op1 = gimple_cond_rhs (stmt);
+
+  if (!simple_iv (loop, loop, op0, iv, false))
+    return NULL_TREE;
+  if (!simple_iv (loop, loop, op1, &iv2, false))
+    return NULL_TREE;
+
+  /* Make it so that the first argument of the condition is
+     the looping one.  */
+  if (!integer_zerop (iv2.step))
+    {
+      std::swap (op0, op1);
+      std::swap (*iv, iv2);
+      code = swap_tree_comparison (code);
+      gimple_cond_set_condition (stmt, code, op0, op1);
+      update_stmt (stmt);
+    }
+  else if (integer_zerop (iv->step))
+    return NULL_TREE;
+  if (!integer_zerop (iv2.step))
+    return NULL_TREE;
+
+  if (dump_file && (dump_flags & TDF_DETAILS))
+    {
+      fprintf (dump_file, "Found potential split point: ");
+      print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
+      fprintf (dump_file, " { ");
+      print_generic_expr (dump_file, iv->base, TDF_SLIM);
+      fprintf (dump_file, " + I*");
+      print_generic_expr (dump_file, iv->step, TDF_SLIM);
+      fprintf (dump_file, " } %s ", get_tree_code_name (code));
+      print_generic_expr (dump_file, iv2.base, TDF_SLIM);
+      fprintf (dump_file, "\n");
+    }
+
+  *border = iv2.base;
+  return op0;
+}
+
+/* Given a GUARD conditional stmt inside LOOP, which we want to make always
+   true or false depending on INITIAL_TRUE, and adjusted values NEXTVAL
+   (a post-increment IV) and NEWBOUND (the comparator) adjust the loop
+   exit test statement to loop back only if the GUARD statement will
+   also be true/false in the next iteration.  */
+
+static void
+patch_loop_exit (struct loop *loop, gcond *guard, tree nextval, tree newbound,
+		 bool initial_true)
+{
+  edge exit = single_exit (loop);
+  gcond *stmt = as_a <gcond *> (last_stmt (exit->src));
+  gimple_cond_set_condition (stmt, gimple_cond_code (guard),
+			     nextval, newbound);
+  update_stmt (stmt);
+
+  edge stay = single_pred_edge (loop->latch);
+
+  exit->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
+  stay->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
+
+  if (initial_true)
+    {
+      exit->flags |= EDGE_FALSE_VALUE;
+      stay->flags |= EDGE_TRUE_VALUE;
+    }
+  else
+    {
+      exit->flags |= EDGE_TRUE_VALUE;
+      stay->flags |= EDGE_FALSE_VALUE;
+    }
+}
+
+/* Give an induction variable GUARD_IV, and its affine descriptor IV,
+   find the loop phi node in LOOP defining it directly, or create
+   such phi node.  Return that phi node.  */
+
+static gphi *
+find_or_create_guard_phi (struct loop *loop, tree guard_iv, affine_iv * /*iv*/)
+{
+  gimple *def = SSA_NAME_DEF_STMT (guard_iv);
+  gphi *phi;
+  if ((phi = dyn_cast <gphi *> (def))
+      && gimple_bb (phi) == loop->header)
+    return phi;
+
+  /* XXX Create the PHI instead.  */
+  return NULL;
+}
+
+/* This function updates the SSA form after connect_loops made a new
+   edge NEW_E leading from LOOP1 exit to LOOP2 (via in intermediate
+   conditional).  I.e. the second loop can now be entered either
+   via the original entry or via NEW_E, so the entry values of LOOP2
+   phi nodes are either the original ones or those at the exit
+   of LOOP1.  Insert new phi nodes in LOOP2 pre-header reflecting
+   this.  */
+
+static void
+connect_loop_phis (struct loop *loop1, struct loop *loop2, edge new_e)
+{
+  basic_block rest = loop_preheader_edge (loop2)->src;
+  gcc_assert (new_e->dest == rest);
+  edge skip_first = EDGE_PRED (rest, EDGE_PRED (rest, 0) == new_e);
+
+  edge firste = loop_preheader_edge (loop1);
+  edge seconde = loop_preheader_edge (loop2);
+  edge firstn = loop_latch_edge (loop1);
+  gphi_iterator psi_first, psi_second;
+  for (psi_first = gsi_start_phis (loop1->header),
+       psi_second = gsi_start_phis (loop2->header);
+       !gsi_end_p (psi_first);
+       gsi_next (&psi_first), gsi_next (&psi_second))
+    {
+      tree init, next, new_init;
+      use_operand_p op;
+      gphi *phi_first = psi_first.phi ();
+      gphi *phi_second = psi_second.phi ();
+
+      init = PHI_ARG_DEF_FROM_EDGE (phi_first, firste);
+      next = PHI_ARG_DEF_FROM_EDGE (phi_first, firstn);
+      op = PHI_ARG_DEF_PTR_FROM_EDGE (phi_second, seconde);
+      gcc_assert (operand_equal_for_phi_arg_p (init, USE_FROM_PTR (op)));
+
+      /* Prefer using original variable as a base for the new ssa name.
+	 This is necessary for virtual ops, and useful in order to avoid
+	 losing debug info for real ops.  */
+      if (TREE_CODE (next) == SSA_NAME
+	  && useless_type_conversion_p (TREE_TYPE (next),
+					TREE_TYPE (init)))
+	new_init = copy_ssa_name (next);
+      else if (TREE_CODE (init) == SSA_NAME
+	       && useless_type_conversion_p (TREE_TYPE (init),
+					     TREE_TYPE (next)))
+	new_init = copy_ssa_name (init);
+      else if (useless_type_conversion_p (TREE_TYPE (next),
+					  TREE_TYPE (init)))
+	new_init = make_temp_ssa_name (TREE_TYPE (next), NULL,
+				       "unrinittmp");
+      else
+	new_init = make_temp_ssa_name (TREE_TYPE (init), NULL,
+				       "unrinittmp");
+
+      gphi * newphi = create_phi_node (new_init, rest);
+      add_phi_arg (newphi, init, skip_first, UNKNOWN_LOCATION);
+      add_phi_arg (newphi, next, new_e, UNKNOWN_LOCATION);
+      SET_USE (op, new_init);
+    }
+}
+
+/* The two loops LOOP1 and LOOP2 were just created by loop versioning,
+   they are still equivalent and placed in two arms of a diamond, like so:
+
+               .------if (cond)------.
+               v                     v
+             pre1                   pre2
+              |                      |
+        .--->h1                     h2<----.
+        |     |                      |     |
+        |    ex1---.            .---ex2    |
+        |    /     |            |     \    |
+        '---l1     X            |     l2---'
+                   |            |
+                   |            |
+                   '--->join<---'
+
+   This function transforms the program such that LOOP1 is conditionally
+   falling through to LOOP2, or skipping it.  This is done by splitting
+   the ex1->join edge at X in the diagram above, and inserting a condition
+   whose one arm goes to pre2, resulting in this situation:
+   
+               .------if (cond)------.
+               v                     v
+             pre1       .---------->pre2
+              |         |            |
+        .--->h1         |           h2<----.
+        |     |         |            |     |
+        |    ex1---.    |       .---ex2    |
+        |    /     v    |       |     \    |
+        '---l1   skip---'       |     l2---'
+                   |            |
+                   |            |
+                   '--->join<---'
+
+   
+   The condition used is the exit condition of LOOP1, which effectively means
+   that when the first loop exits (for whatever reason) but the real original
+   exit expression is still false the second loop will be entered.
+   The function returns the new edge cond->pre2.
+   
+   This doesn't update the SSA form, see connect_loop_phis for that.  */
+
+static edge
+connect_loops (struct loop *loop1, struct loop *loop2)
+{
+  edge exit = single_exit (loop1);
+  basic_block skip_bb = split_edge (exit);
+  gcond *skip_stmt;
+  gimple_stmt_iterator gsi;
+  edge new_e, skip_e;
+
+  gimple *stmt = last_stmt (exit->src);
+  skip_stmt = gimple_build_cond (gimple_cond_code (stmt),
+				 gimple_cond_lhs (stmt),
+				 gimple_cond_rhs (stmt),
+				 NULL_TREE, NULL_TREE);
+  gsi = gsi_last_bb (skip_bb);
+  gsi_insert_after (&gsi, skip_stmt, GSI_NEW_STMT);
+
+  skip_e = EDGE_SUCC (skip_bb, 0);
+  skip_e->flags &= ~EDGE_FALLTHRU;
+  new_e = make_edge (skip_bb, loop_preheader_edge (loop2)->src, 0);
+  if (exit->flags & EDGE_TRUE_VALUE)
+    {
+      skip_e->flags |= EDGE_TRUE_VALUE;
+      new_e->flags |= EDGE_FALSE_VALUE;
+    }
+  else
+    {
+      skip_e->flags |= EDGE_FALSE_VALUE;
+      new_e->flags |= EDGE_TRUE_VALUE;
+    }
+
+  new_e->count = skip_bb->count;
+  new_e->probability = PROB_LIKELY;
+  new_e->count = apply_probability (skip_e->count, PROB_LIKELY);
+  skip_e->count -= new_e->count;
+  skip_e->probability = inverse_probability (PROB_LIKELY);
+
+  return new_e;
+}
+
+/* This returns the new bound for iterations given the original iteration
+   space in NITER, an arbitrary new bound BORDER, assumed to be some
+   comparison value with a different IV, the initial value GUARD_INIT of
+   that other IV, and the comparison code GUARD_CODE that compares
+   that other IV with BORDER.  We return an SSA name, and place any
+   necessary statements for that computation into *STMTS.
+
+   For example for such a loop:
+
+     for (i = beg, j = guard_init; i < end; i++, j++)
+       if (j < border)  // this is supposed to be true/false
+         ...
+
+   we want to return a new bound (on j) that makes the loop iterate
+   as long as the condition j < border stays true.  We also don't want
+   to iterate more often than the original loop, so we have to introduce
+   some cut-off as well (via min/max), effectively resulting in:
+
+     newend = min (end+guard_init-beg, border)
+     for (i = beg; j = guard_init; j < newend; i++, j++)
+       if (j < c)
+         ...
+
+   Depending on the direction of the IVs and if the exit tests
+   are strict or non-strict we need to use MIN or MAX,
+   and add or subtract 1.  This routine computes newend above.  */
+
+static tree
+compute_new_first_bound (gimple_seq *stmts, struct tree_niter_desc *niter,
+			 tree border,
+			 enum tree_code guard_code, tree guard_init)
+{
+  /* The niter structure contains the after-increment IV, we need
+     the loop-enter base, so subtract STEP once.  */
+  tree controlbase = force_gimple_operand (niter->control.base,
+					   stmts, true, NULL_TREE);
+  tree controlstep = niter->control.step;
+  tree enddiff;
+  if (POINTER_TYPE_P (TREE_TYPE (controlbase)))
+    {
+      controlstep = gimple_build (stmts, NEGATE_EXPR,
+				  TREE_TYPE (controlstep), controlstep);
+      enddiff = gimple_build (stmts, POINTER_PLUS_EXPR,
+			      TREE_TYPE (controlbase),
+			      controlbase, controlstep);
+    }
+  else
+    enddiff = gimple_build (stmts, MINUS_EXPR,
+			    TREE_TYPE (controlbase),
+			    controlbase, controlstep);
+
+  /* Compute beg-guard_init.  */
+  if (POINTER_TYPE_P (TREE_TYPE (enddiff)))
+    {
+      tree tem = gimple_convert (stmts, sizetype, guard_init);
+      tem = gimple_build (stmts, NEGATE_EXPR, sizetype, tem);
+      enddiff = gimple_build (stmts, POINTER_PLUS_EXPR,
+			      TREE_TYPE (enddiff),
+			      enddiff, tem);
+    }
+  else
+    enddiff = gimple_build (stmts, MINUS_EXPR, TREE_TYPE (enddiff),
+			    enddiff, guard_init);
+
+  /* Compute end-(beg-guard_init).  */
+  gimple_seq stmts2;
+  tree newbound = force_gimple_operand (niter->bound, &stmts2,
+					true, NULL_TREE);
+  gimple_seq_add_seq_without_update (stmts, stmts2);
+
+  if (POINTER_TYPE_P (TREE_TYPE (enddiff))
+      || POINTER_TYPE_P (TREE_TYPE (newbound)))
+    {
+      enddiff = gimple_convert (stmts, sizetype, enddiff);
+      enddiff = gimple_build (stmts, NEGATE_EXPR, sizetype, enddiff);
+      newbound = gimple_build (stmts, POINTER_PLUS_EXPR,
+			       TREE_TYPE (newbound),
+			       newbound, enddiff);
+    }
+  else
+    newbound = gimple_build (stmts, MINUS_EXPR, TREE_TYPE (enddiff),
+			     newbound, enddiff);
+
+  /* Depending on the direction of the IVs the new bound for the first
+     loop is the minimum or maximum of old bound and border.
+     Also, if the guard condition isn't strictly less or greater,
+     we need to adjust the bound.  */ 
+  int addbound = 0;
+  enum tree_code minmax;
+  if (niter->cmp == LT_EXPR)
+    {
+      /* GT and LE are the same, inverted.  */
+      if (guard_code == GT_EXPR || guard_code == LE_EXPR)
+	addbound = -1;
+      minmax = MIN_EXPR;
+    }
+  else
+    {
+      gcc_assert (niter->cmp == GT_EXPR);
+      if (guard_code == GE_EXPR || guard_code == LT_EXPR)
+	addbound = 1;
+      minmax = MAX_EXPR;
+    }
+
+  if (addbound)
+    {
+      tree type2 = TREE_TYPE (newbound);
+      if (POINTER_TYPE_P (type2))
+	type2 = sizetype;
+      newbound = gimple_build (stmts,
+			       POINTER_TYPE_P (TREE_TYPE (newbound))
+			       ? POINTER_PLUS_EXPR : PLUS_EXPR,
+			       TREE_TYPE (newbound),
+			       newbound,
+			       build_int_cst (type2, addbound));
+    }
+
+  newbound = gimple_convert (stmts, TREE_TYPE (border), newbound);
+  tree newend = gimple_build (stmts, minmax, TREE_TYPE (border),
+			      border, newbound);
+  return newend;
+}
+
+/* Checks if LOOP contains an conditional block whose condition
+   depends on which side in the iteration space it is, and if so
+   splits the iteration space into two loops.  Returns true if the
+   loop was split.  NITER must contain the iteration descriptor for the
+   single exit of LOOP.  */
+
+static bool
+split_loop (struct loop *loop1, struct tree_niter_desc *niter)
+{
+  basic_block *bbs;
+  unsigned i;
+  bool changed = false;
+  tree guard_iv;
+  tree border;
+  affine_iv iv;
+
+  bbs = get_loop_body (loop1);
+
+  /* Find a splitting opportunity.  */
+  for (i = 0; i < loop1->num_nodes; i++)
+    if ((guard_iv = split_at_bb_p (loop1, bbs[i], &border, &iv)))
+      {
+	/* Handling opposite steps is not implemented yet.  Neither
+	   is handling different step sizes.  */
+	if ((tree_int_cst_sign_bit (iv.step)
+	     != tree_int_cst_sign_bit (niter->control.step))
+	    || !tree_int_cst_equal (iv.step, niter->control.step))
+	  continue;
+
+	/* Find a loop PHI node that defines guard_iv directly,
+	   or create one doing that.  */
+	gphi *phi = find_or_create_guard_phi (loop1, guard_iv, &iv);
+	if (!phi)
+	  continue;
+	gcond *guard_stmt = as_a<gcond *> (last_stmt (bbs[i]));
+	tree guard_init = PHI_ARG_DEF_FROM_EDGE (phi,
+						 loop_preheader_edge (loop1));
+	enum tree_code guard_code = gimple_cond_code (guard_stmt);
+
+	/* Loop splitting is implemented by versioning the loop, placing
+	   the new loop after the old loop, make the first loop iterate
+	   as long as the conditional stays true (or false) and let the
+	   second (new) loop handle the rest of the iterations.
+
+	   First we need to determine if the condition will start being true
+	   or false in the first loop.  */
+	bool initial_true;
+	switch (guard_code)
+	  {
+	    case LT_EXPR:
+	    case LE_EXPR:
+	      initial_true = !tree_int_cst_sign_bit (iv.step);
+	      break;
+	    case GT_EXPR:
+	    case GE_EXPR:
+	      initial_true = tree_int_cst_sign_bit (iv.step);
+	      break;
+	    default:
+	      gcc_unreachable ();
+	  }
+
+	/* Build a condition that will skip the first loop when the
+	   guard condition won't ever be true (or false).  */
+	gimple_seq stmts2;
+	border = force_gimple_operand (border, &stmts2, true, NULL_TREE);
+	if (stmts2)
+	  gsi_insert_seq_on_edge_immediate (loop_preheader_edge (loop1),
+					    stmts2);
+	tree cond = build2 (guard_code, boolean_type_node, guard_init, border);
+	if (!initial_true)
+	  cond = fold_build1 (TRUTH_NOT_EXPR, boolean_type_node, cond); 
+
+	/* Now version the loop, placing loop2 after loop1 connecting
+	   them, and fix up SSA form for that.  */
+	initialize_original_copy_tables ();
+	basic_block cond_bb;
+	struct loop *loop2 = loop_version (loop1, cond, &cond_bb,
+					   REG_BR_PROB_BASE, REG_BR_PROB_BASE,
+					   REG_BR_PROB_BASE, true);
+	gcc_assert (loop2);
+	update_ssa (TODO_update_ssa);
+
+	edge new_e = connect_loops (loop1, loop2);
+	connect_loop_phis (loop1, loop2, new_e);
+
+	/* The iterations of the second loop is now already
+	   exactly those that the first loop didn't do, but the
+	   iteration space of the first loop is still the original one.
+	   Compute the new bound for the guarding IV and patch the
+	   loop exit to use it instead of original IV and bound.  */
+	gimple_seq stmts = NULL;
+	tree newend = compute_new_first_bound (&stmts, niter, border,
+					       guard_code, guard_init);
+	if (stmts)
+	  gsi_insert_seq_on_edge_immediate (loop_preheader_edge (loop1),
+					    stmts);
+	tree guard_next = PHI_ARG_DEF_FROM_EDGE (phi, loop_latch_edge (loop1));
+	patch_loop_exit (loop1, guard_stmt, guard_next, newend, initial_true);
+
+	/* Finally patch out the two copies of the condition to be always
+	   true/false (or opposite).  */
+	gcond *force_true = as_a<gcond *> (last_stmt (bbs[i]));
+	gcond *force_false = as_a<gcond *> (last_stmt (get_bb_copy (bbs[i])));
+	if (!initial_true)
+	  std::swap (force_true, force_false);
+	gimple_cond_make_true (force_true);
+	gimple_cond_make_false (force_false);
+	update_stmt (force_true);
+	update_stmt (force_false);
+
+	free_original_copy_tables ();
+
+	/* We destroyed LCSSA form above.  Eventually we might be able
+	   to fix it on the fly, for now simply punt and use the helper.  */
+	rewrite_into_loop_closed_ssa_1 (NULL, 0, SSA_OP_USE, loop1);
+
+	changed = true;
+	if (dump_file && (dump_flags & TDF_DETAILS))
+	  fprintf (dump_file, ";; Loop split.\n");
+
+	/* Only deal with the first opportunity.  */
+	break;
+      }
+
+  free (bbs);
+  return changed;
+}
+
+/* Main entry point.  Perform loop splitting on all suitable loops.  */
+
+static unsigned int
+tree_ssa_split_loops (void)
+{
+  struct loop *loop;
+  bool changed = false;
+
+  gcc_assert (scev_initialized_p ());
+  FOR_EACH_LOOP (loop, 0)
+    loop->aux = NULL;
+
+  /* Go through all loops starting from innermost.  */
+  FOR_EACH_LOOP (loop, LI_FROM_INNERMOST)
+    {
+      struct tree_niter_desc niter;
+      if (loop->aux)
+	{
+	  /* If any of our inner loops was split, don't split us,
+	     and mark our containing loop as having had splits as well.  */
+	  loop_outer (loop)->aux = loop;
+	  continue;
+	}
+
+      if (single_exit (loop)
+	  /* ??? We could handle non-empty latches when we split
+	     the latch edge (not the exit edge), and put the new
+	     exit condition in the new block.  OTOH this executes some
+	     code unconditionally that might have been skipped by the
+	     original exit before.  */
+	  && empty_block_p (loop->latch)
+	  && !optimize_loop_for_size_p (loop)
+	  && number_of_iterations_exit (loop, single_exit (loop), &niter,
+					false, true)
+	  && niter.cmp != ERROR_MARK
+	  /* We can't yet handle loops controlled by a != predicate.  */
+	  && niter.cmp != NE_EXPR)
+	{
+	  if (split_loop (loop, &niter))
+	    {
+	      /* Mark our containing loop as having had some split inner
+	         loops.  */
+	      loop_outer (loop)->aux = loop;
+	      changed = true;
+	    }
+	}
+    }
+
+  FOR_EACH_LOOP (loop, 0)
+    loop->aux = NULL;
+
+  if (changed)
+    return TODO_cleanup_cfg;
+  return 0;
+}
+
+/* Loop splitting pass.  */
+
+namespace {
+
+const pass_data pass_data_loop_split =
+{
+  GIMPLE_PASS, /* type */
+  "lsplit", /* name */
+  OPTGROUP_LOOP, /* optinfo_flags */
+  TV_LOOP_SPLIT, /* tv_id */
+  PROP_cfg, /* properties_required */
+  0, /* properties_provided */
+  0, /* properties_destroyed */
+  0, /* todo_flags_start */
+  0, /* todo_flags_finish */
+};
+
+class pass_loop_split : public gimple_opt_pass
+{
+public:
+  pass_loop_split (gcc::context *ctxt)
+    : gimple_opt_pass (pass_data_loop_split, ctxt)
+  {}
+
+  /* opt_pass methods: */
+  virtual bool gate (function *) { return flag_split_loops != 0; }
+  virtual unsigned int execute (function *);
+
+}; // class pass_loop_split
+
+unsigned int
+pass_loop_split::execute (function *fun)
+{
+  if (number_of_loops (fun) <= 1)
+    return 0;
+
+  return tree_ssa_split_loops ();
+}
+
+} // anon namespace
+
+gimple_opt_pass *
+make_pass_loop_split (gcc::context *ctxt)
+{
+  return new pass_loop_split (ctxt);
+}