path: root/gcc/tree-cfgcleanup.c

/* CFG cleanup for trees.
   Copyright (C) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008
   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 "tm.h"
#include "tree.h"
#include "rtl.h"
#include "tm_p.h"
#include "hard-reg-set.h"
#include "basic-block.h"
#include "output.h"
#include "toplev.h"
#include "flags.h"
#include "function.h"
#include "expr.h"
#include "ggc.h"
#include "langhooks.h"
#include "diagnostic.h"
#include "tree-flow.h"
#include "timevar.h"
#include "tree-dump.h"
#include "tree-pass.h"
#include "toplev.h"
#include "except.h"
#include "cfgloop.h"
#include "cfglayout.h"
#include "hashtab.h"
#include "tree-ssa-propagate.h"
#include "tree-scalar-evolution.h"

/* The set of blocks in that at least one of the following changes happened:
   -- the statement at the end of the block was changed
   -- the block was newly created
   -- the set of the predecessors of the block changed
   -- the set of the successors of the block changed
   ??? Maybe we could track these changes separately, since they determine
       what cleanups it makes sense to try on the block.  */
bitmap cfgcleanup_altered_bbs;

/* Remove any fallthru edge from EV.  Return true if an edge was removed.  */

static bool
remove_fallthru_edge (VEC(edge,gc) *ev)
{
  edge_iterator ei;
  edge e;

  FOR_EACH_EDGE (e, ei, ev)
    if ((e->flags & EDGE_FALLTHRU) != 0)
      {
	remove_edge_and_dominated_blocks (e);
	return true;
      }
  return false;
}


/* Disconnect an unreachable block in the control expression starting
   at block BB.  */

static bool
cleanup_control_expr_graph (basic_block bb, gimple_stmt_iterator gsi)
{
  edge taken_edge;
  bool retval = false;
  gimple stmt = gsi_stmt (gsi);
  tree val;

  if (!single_succ_p (bb))
    {
      edge e;
      edge_iterator ei;
      bool warned;

      fold_defer_overflow_warnings ();
      val = gimple_fold (stmt);
      taken_edge = find_taken_edge (bb, val);
      if (!taken_edge)
	{
	  fold_undefer_and_ignore_overflow_warnings ();
	  return false;
	}

      /* Remove all the edges except the one that is always executed.  */
      warned = false;
      for (ei = ei_start (bb->succs); (e = ei_safe_edge (ei)); )
	{
	  if (e != taken_edge)
	    {
	      if (!warned)
		{
		  fold_undefer_overflow_warnings
		    (true, stmt, WARN_STRICT_OVERFLOW_CONDITIONAL);
		  warned = true;
		}

	      taken_edge->probability += e->probability;
	      taken_edge->count += e->count;
	      remove_edge_and_dominated_blocks (e);
	      retval = true;
	    }
	  else
	    ei_next (&ei);
	}
      if (!warned)
	fold_undefer_and_ignore_overflow_warnings ();
      if (taken_edge->probability > REG_BR_PROB_BASE)
	taken_edge->probability = REG_BR_PROB_BASE;
    }
  else
    taken_edge = single_succ_edge (bb);

  bitmap_set_bit (cfgcleanup_altered_bbs, bb->index);
  gsi_remove (&gsi, true);
  taken_edge->flags = EDGE_FALLTHRU;

  return retval;
}

/* Try to remove superfluous control structures in basic block BB.  Returns
   true if anything changes.  */

static bool
cleanup_control_flow_bb (basic_block bb)
{
  gimple_stmt_iterator gsi;
  bool retval = false;
  gimple stmt;

  /* If the last statement of the block could throw and now cannot,
     we need to prune cfg.  */
  retval |= gimple_purge_dead_eh_edges (bb);

  gsi = gsi_last_bb (bb);
  if (gsi_end_p (gsi))
    return retval;

  stmt = gsi_stmt (gsi);

  if (gimple_code (stmt) == GIMPLE_COND
      || gimple_code (stmt) == GIMPLE_SWITCH)
    retval |= cleanup_control_expr_graph (bb, gsi);
  else if (gimple_code (stmt) == GIMPLE_GOTO
	   && TREE_CODE (gimple_goto_dest (stmt)) == ADDR_EXPR
	   && (TREE_CODE (TREE_OPERAND (gimple_goto_dest (stmt), 0))
	       == LABEL_DECL))
    {
      /* If we had a computed goto which has a compile-time determinable
	 destination, then we can eliminate the goto.  */
      edge e;
      tree label;
      edge_iterator ei;
      basic_block target_block;

      /* First look at all the outgoing edges.  Delete any outgoing
	 edges which do not go to the right block.  For the one
	 edge which goes to the right block, fix up its flags.  */
      label = TREE_OPERAND (gimple_goto_dest (stmt), 0);
      target_block = label_to_block (label);
      for (ei = ei_start (bb->succs); (e = ei_safe_edge (ei)); )
	{
	  if (e->dest != target_block)
	    remove_edge_and_dominated_blocks (e);
	  else
	    {
	      /* Turn off the EDGE_ABNORMAL flag.  */
	      e->flags &= ~EDGE_ABNORMAL;

	      /* And set EDGE_FALLTHRU.  */
	      e->flags |= EDGE_FALLTHRU;
	      ei_next (&ei);
	    }
	}

      bitmap_set_bit (cfgcleanup_altered_bbs, bb->index);
      bitmap_set_bit (cfgcleanup_altered_bbs, target_block->index);

      /* Remove the GOTO_EXPR as it is not needed.  The CFG has all the
	 relevant information we need.  */
      gsi_remove (&gsi, true);
      retval = true;
    }

  /* Check for indirect calls that have been turned into
     noreturn calls.  */
  else if (is_gimple_call (stmt)
           && gimple_call_noreturn_p (stmt)
           && remove_fallthru_edge (bb->succs))
    retval = true;

  return retval;
}

/* Return true if basic block BB does nothing except pass control
   flow to another block and that we can safely insert a label at
   the start of the successor block.

   As a precondition, we require that BB be not equal to
   ENTRY_BLOCK_PTR.  */

static bool
tree_forwarder_block_p (basic_block bb, bool phi_wanted)
{
  gimple_stmt_iterator gsi;

  /* BB must have a single outgoing edge.  */
  if (single_succ_p (bb) != 1
      /* If PHI_WANTED is false, BB must not have any PHI nodes.
	 Otherwise, BB must have PHI nodes.  */
      || gimple_seq_empty_p (phi_nodes (bb)) == phi_wanted
      /* BB may not be a predecessor of EXIT_BLOCK_PTR.  */
      || single_succ (bb) == EXIT_BLOCK_PTR
      /* Nor should this be an infinite loop.  */
      || single_succ (bb) == bb
      /* BB may not have an abnormal outgoing edge.  */
      || (single_succ_edge (bb)->flags & EDGE_ABNORMAL))
    return false;

#if ENABLE_CHECKING
  gcc_assert (bb != ENTRY_BLOCK_PTR);
#endif

  /* There should not be an edge coming from entry, or an EH edge.  */
  {
    edge_iterator ei;
    edge e;

    FOR_EACH_EDGE (e, ei, bb->preds)
      if (e->src == ENTRY_BLOCK_PTR || (e->flags & EDGE_EH))
	return false;
  }

  /* Now walk through the statements backward.  We can ignore labels,
     anything else means this is not a forwarder block.  */
  for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi_prev (&gsi))
    {
      gimple stmt = gsi_stmt (gsi);

      switch (gimple_code (stmt))
	{
	case GIMPLE_LABEL:
	  if (DECL_NONLOCAL (gimple_label_label (stmt)))
	    return false;
	  break;

	  /* ??? For now, hope there's a corresponding debug
	     assignment at the destination.  */
	case GIMPLE_DEBUG:
	  break;

	default:
	  return false;
	}
    }

  if (current_loops)
    {
      basic_block dest;
      /* Protect loop latches, headers and preheaders.  */
      if (bb->loop_father->header == bb)
	return false;
      dest = EDGE_SUCC (bb, 0)->dest;

      if (dest->loop_father->header == dest)
	return false;
    }
  return true;
}

/* Return true if BB has at least one abnormal incoming edge.  */

static inline bool
has_abnormal_incoming_edge_p (basic_block bb)
{
  edge e;
  edge_iterator ei;

  FOR_EACH_EDGE (e, ei, bb->preds)
    if (e->flags & EDGE_ABNORMAL)
      return true;

  return false;
}

/* If all the PHI nodes in DEST have alternatives for E1 and E2 and
   those alternatives are equal in each of the PHI nodes, then return
   true, else return false.  */

static bool
phi_alternatives_equal (basic_block dest, edge e1, edge e2)
{
  int n1 = e1->dest_idx;
  int n2 = e2->dest_idx;
  gimple_stmt_iterator gsi;

  for (gsi = gsi_start_phis (dest); !gsi_end_p (gsi); gsi_next (&gsi))
    {
      gimple phi = gsi_stmt (gsi);
      tree val1 = gimple_phi_arg_def (phi, n1);
      tree val2 = gimple_phi_arg_def (phi, n2);

      gcc_assert (val1 != NULL_TREE);
      gcc_assert (val2 != NULL_TREE);

      if (!operand_equal_for_phi_arg_p (val1, val2))
	return false;
    }

  return true;
}

/* Removes forwarder block BB.  Returns false if this failed.  */

static bool
remove_forwarder_block (basic_block bb)
{
  edge succ = single_succ_edge (bb), e, s;
  basic_block dest = succ->dest;
  gimple label;
  edge_iterator ei;
  gimple_stmt_iterator gsi, gsi_to;
  bool seen_abnormal_edge = false;

  /* We check for infinite loops already in tree_forwarder_block_p.
     However it may happen that the infinite loop is created
     afterwards due to removal of forwarders.  */
  if (dest == bb)
    return false;

  /* If the destination block consists of a nonlocal label, do not merge
     it.  */
  label = first_stmt (dest);
  if (label
      && gimple_code (label) == GIMPLE_LABEL
      && DECL_NONLOCAL (gimple_label_label (label)))
    return false;

  /* If there is an abnormal edge to basic block BB, but not into
     dest, problems might occur during removal of the phi node at out
     of ssa due to overlapping live ranges of registers.

     If there is an abnormal edge in DEST, the problems would occur
     anyway since cleanup_dead_labels would then merge the labels for
     two different eh regions, and rest of exception handling code
     does not like it.

     So if there is an abnormal edge to BB, proceed only if there is
     no abnormal edge to DEST and there are no phi nodes in DEST.  */
  if (has_abnormal_incoming_edge_p (bb))
    {
      seen_abnormal_edge = true;

      if (has_abnormal_incoming_edge_p (dest)
	  || !gimple_seq_empty_p (phi_nodes (dest)))
	return false;
    }

  /* If there are phi nodes in DEST, and some of the blocks that are
     predecessors of BB are also predecessors of DEST, check that the
     phi node arguments match.  */
  if (!gimple_seq_empty_p (phi_nodes (dest)))
    {
      FOR_EACH_EDGE (e, ei, bb->preds)
	{
	  s = find_edge (e->src, dest);
	  if (!s)
	    continue;

	  if (!phi_alternatives_equal (dest, succ, s))
	    return false;
	}
    }

  /* Redirect the edges.  */
  for (ei = ei_start (bb->preds); (e = ei_safe_edge (ei)); )
    {
      bitmap_set_bit (cfgcleanup_altered_bbs, e->src->index);

      if (e->flags & EDGE_ABNORMAL)
	{
	  /* If there is an abnormal edge, redirect it anyway, and
	     move the labels to the new block to make it legal.  */
	  s = redirect_edge_succ_nodup (e, dest);
	}
      else
	s = redirect_edge_and_branch (e, dest);

      if (s == e)
	{
	  /* Create arguments for the phi nodes, since the edge was not
	     here before.  */
	  for (gsi = gsi_start_phis (dest);
	       !gsi_end_p (gsi);
	       gsi_next (&gsi))
	    {
	      gimple phi = gsi_stmt (gsi);
	      source_location l = gimple_phi_arg_location_from_edge (phi, succ);
	      add_phi_arg (phi, gimple_phi_arg_def (phi, succ->dest_idx), s, l);
	    }
	}
    }

  if (seen_abnormal_edge)
    {
      /* Move the labels to the new block, so that the redirection of
	 the abnormal edges works.  */
      gsi_to = gsi_start_bb (dest);
      for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); )
	{
	  label = gsi_stmt (gsi);
	  gcc_assert (gimple_code (label) == GIMPLE_LABEL
		      || is_gimple_debug (label));
	  gsi_remove (&gsi, false);
	  gsi_insert_before (&gsi_to, label, GSI_SAME_STMT);
	}
    }

  bitmap_set_bit (cfgcleanup_altered_bbs, dest->index);

  /* Update the dominators.  */
  if (dom_info_available_p (CDI_DOMINATORS))
    {
      basic_block dom, dombb, domdest;

      dombb = get_immediate_dominator (CDI_DOMINATORS, bb);
      domdest = get_immediate_dominator (CDI_DOMINATORS, dest);
      if (domdest == bb)
	{
	  /* Shortcut to avoid calling (relatively expensive)
	     nearest_common_dominator unless necessary.  */
	  dom = dombb;
	}
      else
	dom = nearest_common_dominator (CDI_DOMINATORS, domdest, dombb);

      set_immediate_dominator (CDI_DOMINATORS, dest, dom);
    }

  /* And kill the forwarder block.  */
  delete_basic_block (bb);

  return true;
}

/* Split basic blocks on calls in the middle of a basic block that are now
   known not to return, and remove the unreachable code.  */

static bool
split_bbs_on_noreturn_calls (void)
{
  bool changed = false;
  gimple stmt;
  basic_block bb;

  /* Detect cases where a mid-block call is now known not to return.  */
  if (cfun->gimple_df)
    while (VEC_length (gimple, MODIFIED_NORETURN_CALLS (cfun)))
      {
	stmt = VEC_pop (gimple, MODIFIED_NORETURN_CALLS (cfun));
	bb = gimple_bb (stmt);
	/* BB might be deleted at this point, so verify first
	   BB is present in the cfg.  */
	if (bb == NULL
	    || bb->index < NUM_FIXED_BLOCKS
	    || bb->index >= n_basic_blocks
	    || BASIC_BLOCK (bb->index) != bb
	    || last_stmt (bb) == stmt
	    || !gimple_call_noreturn_p (stmt))
	  continue;

	changed = true;
	split_block (bb, stmt);
	remove_fallthru_edge (bb->succs);
      }

  return changed;
}

/* If GIMPLE_OMP_RETURN in basic block BB is unreachable, remove it.  */

static bool
cleanup_omp_return (basic_block bb)
{
  gimple stmt = last_stmt (bb);
  basic_block control_bb;

  if (stmt == NULL
      || gimple_code (stmt) != GIMPLE_OMP_RETURN
      || !single_pred_p (bb))
    return false;

  control_bb = single_pred (bb);
  stmt = last_stmt (control_bb);

  if (gimple_code (stmt) != GIMPLE_OMP_SECTIONS_SWITCH)
    return false;

  /* The block with the control statement normally has two entry edges -- one
     from entry, one from continue.  If continue is removed, return is
     unreachable, so we remove it here as well.  */
  if (EDGE_COUNT (control_bb->preds) == 2)
    return false;

  gcc_assert (EDGE_COUNT (control_bb->preds) == 1);
  remove_edge_and_dominated_blocks (single_pred_edge (bb));
  return true;
}

/* Tries to cleanup cfg in basic block BB.  Returns true if anything
   changes.  */

static bool
cleanup_tree_cfg_bb (basic_block bb)
{
  bool retval = false;

  if (cleanup_omp_return (bb))
    return true;

  retval = cleanup_control_flow_bb (bb);
  
  /* Forwarder blocks can carry line number information which is
     useful when debugging, so we only clean them up when
     optimizing.  */
  if (optimize > 0
      && tree_forwarder_block_p (bb, false)
      && remove_forwarder_block (bb))
    return true;

  /* Merging the blocks may create new opportunities for folding
     conditional branches (due to the elimination of single-valued PHI
     nodes).  */
  if (single_succ_p (bb)
      && can_merge_blocks_p (bb, single_succ (bb)))
    {
      merge_blocks (bb, single_succ (bb));
      return true;
    }

  return retval;
}

/* Iterate the cfg cleanups, while anything changes.  */

static bool
cleanup_tree_cfg_1 (void)
{
  bool retval = false;
  basic_block bb;
  unsigned i, n;

  retval |= split_bbs_on_noreturn_calls ();

  /* Prepare the worklists of altered blocks.  */
  cfgcleanup_altered_bbs = BITMAP_ALLOC (NULL);

  /* During forwarder block cleanup, we may redirect edges out of
     SWITCH_EXPRs, which can get expensive.  So we want to enable
     recording of edge to CASE_LABEL_EXPR.  */
  start_recording_case_labels ();

  /* Start by iterating over all basic blocks.  We cannot use FOR_EACH_BB,
     since the basic blocks may get removed.  */
  n = last_basic_block;
  for (i = NUM_FIXED_BLOCKS; i < n; i++)
    {
      bb = BASIC_BLOCK (i);
      if (bb)
	retval |= cleanup_tree_cfg_bb (bb);
    }

  /* Now process the altered blocks, as long as any are available.  */
  while (!bitmap_empty_p (cfgcleanup_altered_bbs))
    {
      i = bitmap_first_set_bit (cfgcleanup_altered_bbs);
      bitmap_clear_bit (cfgcleanup_altered_bbs, i);
      if (i < NUM_FIXED_BLOCKS)
	continue;

      bb = BASIC_BLOCK (i);
      if (!bb)
	continue;

      retval |= cleanup_tree_cfg_bb (bb);

      /* Rerun split_bbs_on_noreturn_calls, in case we have altered any noreturn
	 calls.  */
      retval |= split_bbs_on_noreturn_calls ();
    }
  
  end_recording_case_labels ();
  BITMAP_FREE (cfgcleanup_altered_bbs);
  return retval;
}


/* Remove unreachable blocks and other miscellaneous clean up work.
   Return true if the flowgraph was modified, false otherwise.  */

static bool
cleanup_tree_cfg_noloop (void)
{
  bool changed;

  timevar_push (TV_TREE_CLEANUP_CFG);

  /* Iterate until there are no more cleanups left to do.  If any
     iteration changed the flowgraph, set CHANGED to true.

     If dominance information is available, there cannot be any unreachable
     blocks.  */
  if (!dom_info_available_p (CDI_DOMINATORS))
    {
      changed = delete_unreachable_blocks ();
      calculate_dominance_info (CDI_DOMINATORS);
    }
  else
    {
#ifdef ENABLE_CHECKING
      verify_dominators (CDI_DOMINATORS);
#endif
      changed = false;
    }

  changed |= cleanup_tree_cfg_1 ();

  gcc_assert (dom_info_available_p (CDI_DOMINATORS));
  compact_blocks ();

#ifdef ENABLE_CHECKING
  verify_flow_info ();
#endif

  timevar_pop (TV_TREE_CLEANUP_CFG);

  if (changed && current_loops)
    loops_state_set (LOOPS_NEED_FIXUP);

  return changed;
}

/* Repairs loop structures.  */

static void
repair_loop_structures (void)
{
  bitmap changed_bbs = BITMAP_ALLOC (NULL);
  fix_loop_structure (changed_bbs);

  /* This usually does nothing.  But sometimes parts of cfg that originally
     were inside a loop get out of it due to edge removal (since they
     become unreachable by back edges from latch).  */
  if (loops_state_satisfies_p (LOOP_CLOSED_SSA))
    rewrite_into_loop_closed_ssa (changed_bbs, TODO_update_ssa);

  BITMAP_FREE (changed_bbs);

#ifdef ENABLE_CHECKING
  verify_loop_structure ();
#endif
  scev_reset ();

  loops_state_clear (LOOPS_NEED_FIXUP);
}

/* Cleanup cfg and repair loop structures.  */

bool
cleanup_tree_cfg (void)
{
  bool changed = cleanup_tree_cfg_noloop ();

  if (current_loops != NULL
      && loops_state_satisfies_p (LOOPS_NEED_FIXUP))
    repair_loop_structures ();

  return changed;
}

/* Merge the PHI nodes at BB into those at BB's sole successor.  */

static void
remove_forwarder_block_with_phi (basic_block bb)
{
  edge succ = single_succ_edge (bb);
  basic_block dest = succ->dest;
  gimple label;
  basic_block dombb, domdest, dom;

  /* We check for infinite loops already in tree_forwarder_block_p.
     However it may happen that the infinite loop is created
     afterwards due to removal of forwarders.  */
  if (dest == bb)
    return;

  /* If the destination block consists of a nonlocal label, do not
     merge it.  */
  label = first_stmt (dest);
  if (label
      && gimple_code (label) == GIMPLE_LABEL
      && DECL_NONLOCAL (gimple_label_label (label)))
    return;

  /* Redirect each incoming edge to BB to DEST.  */
  while (EDGE_COUNT (bb->preds) > 0)
    {
      edge e = EDGE_PRED (bb, 0), s;
      gimple_stmt_iterator gsi;

      s = find_edge (e->src, dest);
      if (s)
	{
	  /* We already have an edge S from E->src to DEST.  If S and
	     E->dest's sole successor edge have the same PHI arguments
	     at DEST, redirect S to DEST.  */
	  if (phi_alternatives_equal (dest, s, succ))
	    {
	      e = redirect_edge_and_branch (e, dest);
	      redirect_edge_var_map_clear (e);
	      continue;
	    }

	  /* PHI arguments are different.  Create a forwarder block by
	     splitting E so that we can merge PHI arguments on E to
	     DEST.  */
	  e = single_succ_edge (split_edge (e));
	}

      s = redirect_edge_and_branch (e, dest);

      /* redirect_edge_and_branch must not create a new edge.  */
      gcc_assert (s == e);

      /* Add to the PHI nodes at DEST each PHI argument removed at the
	 destination of E.  */
      for (gsi = gsi_start_phis (dest);
	   !gsi_end_p (gsi);
	   gsi_next (&gsi))
	{
	  gimple phi = gsi_stmt (gsi);
	  tree def = gimple_phi_arg_def (phi, succ->dest_idx);
	  source_location locus = gimple_phi_arg_location_from_edge (phi, succ);

	  if (TREE_CODE (def) == SSA_NAME)
	    {
	      edge_var_map_vector head;
	      edge_var_map *vm;
	      size_t i;

	      /* If DEF is one of the results of PHI nodes removed during
		 redirection, replace it with the PHI argument that used
		 to be on E.  */
	      head = redirect_edge_var_map_vector (e);
	      for (i = 0; VEC_iterate (edge_var_map, head, i, vm); ++i)
		{
		  tree old_arg = redirect_edge_var_map_result (vm);
		  tree new_arg = redirect_edge_var_map_def (vm);

		  if (def == old_arg)
		    {
		      def = new_arg;
		      locus = redirect_edge_var_map_location (vm);
		      break;
		    }
		}
	    }

	  add_phi_arg (phi, def, s, locus);
	}

      redirect_edge_var_map_clear (e);
    }

  /* Update the dominators.  */
  dombb = get_immediate_dominator (CDI_DOMINATORS, bb);
  domdest = get_immediate_dominator (CDI_DOMINATORS, dest);
  if (domdest == bb)
    {
      /* Shortcut to avoid calling (relatively expensive)
	 nearest_common_dominator unless necessary.  */
      dom = dombb;
    }
  else
    dom = nearest_common_dominator (CDI_DOMINATORS, domdest, dombb);

  set_immediate_dominator (CDI_DOMINATORS, dest, dom);

  /* Remove BB since all of BB's incoming edges have been redirected
     to DEST.  */
  delete_basic_block (bb);
}

/* This pass merges PHI nodes if one feeds into another.  For example,
   suppose we have the following:

  goto <bb 9> (<L9>);

<L8>:;
  tem_17 = foo ();

  # tem_6 = PHI <tem_17(8), tem_23(7)>;
<L9>:;

  # tem_3 = PHI <tem_6(9), tem_2(5)>;
<L10>:;

  Then we merge the first PHI node into the second one like so:

  goto <bb 9> (<L10>);

<L8>:;
  tem_17 = foo ();

  # tem_3 = PHI <tem_23(7), tem_2(5), tem_17(8)>;
<L10>:;
*/

static unsigned int
merge_phi_nodes (void)
{
  basic_block *worklist = XNEWVEC (basic_block, n_basic_blocks);
  basic_block *current = worklist;
  basic_block bb;

  calculate_dominance_info (CDI_DOMINATORS);

  /* Find all PHI nodes that we may be able to merge.  */
  FOR_EACH_BB (bb)
    {
      basic_block dest;

      /* Look for a forwarder block with PHI nodes.  */
      if (!tree_forwarder_block_p (bb, true))
	continue;

      dest = single_succ (bb);

      /* We have to feed into another basic block with PHI
	 nodes.  */
      if (!phi_nodes (dest)
	  /* We don't want to deal with a basic block with
	     abnormal edges.  */
	  || has_abnormal_incoming_edge_p (bb))
	continue;

      if (!dominated_by_p (CDI_DOMINATORS, dest, bb))
	{
	  /* If BB does not dominate DEST, then the PHI nodes at
	     DEST must be the only users of the results of the PHI
	     nodes at BB.  */
	  *current++ = bb;
	}
      else
	{
	  gimple_stmt_iterator gsi;
	  unsigned int dest_idx = single_succ_edge (bb)->dest_idx;

	  /* BB dominates DEST.  There may be many users of the PHI
	     nodes in BB.  However, there is still a trivial case we
	     can handle.  If the result of every PHI in BB is used
	     only by a PHI in DEST, then we can trivially merge the
	     PHI nodes from BB into DEST.  */
	  for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi);
	       gsi_next (&gsi))
	    {
	      gimple phi = gsi_stmt (gsi);
	      tree result = gimple_phi_result (phi);
	      use_operand_p imm_use;
	      gimple use_stmt;

	      /* If the PHI's result is never used, then we can just
		 ignore it.  */
	      if (has_zero_uses (result))
		continue;

	      /* Get the single use of the result of this PHI node.  */
  	      if (!single_imm_use (result, &imm_use, &use_stmt)
		  || gimple_code (use_stmt) != GIMPLE_PHI
		  || gimple_bb (use_stmt) != dest
		  || gimple_phi_arg_def (use_stmt, dest_idx) != result)
		break;
	    }

	  /* If the loop above iterated through all the PHI nodes
	     in BB, then we can merge the PHIs from BB into DEST.  */
	  if (gsi_end_p (gsi))
	    *current++ = bb;
	}
    }

  /* Now let's drain WORKLIST.  */
  while (current != worklist)
    {
      bb = *--current;
      remove_forwarder_block_with_phi (bb);
    }

  free (worklist);
  return 0;
}

static bool
gate_merge_phi (void)
{
  return 1;
}

struct gimple_opt_pass pass_merge_phi = 
{
 {
  GIMPLE_PASS,
  "mergephi",			/* name */
  gate_merge_phi,		/* gate */
  merge_phi_nodes,		/* execute */
  NULL,				/* sub */
  NULL,				/* next */
  0,				/* static_pass_number */
  TV_TREE_MERGE_PHI,		/* tv_id */
  PROP_cfg | PROP_ssa,		/* properties_required */
  0,				/* properties_provided */
  0,				/* properties_destroyed */
  0,				/* todo_flags_start */
  TODO_dump_func | TODO_ggc_collect	/* todo_flags_finish */
  | TODO_verify_ssa
 }
};