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[official-gcc.git] / gcc / tree-ssa-threadbackward.c

/* SSA Jump Threading
   Copyright (C) 2005-2021 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 "predict.h"
#include "tree.h"
#include "gimple.h"
#include "fold-const.h"
#include "cfgloop.h"
#include "gimple-iterator.h"
#include "tree-cfg.h"
#include "tree-ssa-threadupdate.h"
#include "tree-ssa-loop.h"
#include "cfganal.h"
#include "tree-pass.h"
#include "gimple-ssa.h"
#include "tree-phinodes.h"
#include "tree-inline.h"
#include "tree-vectorizer.h"
#include "value-range.h"
#include "gimple-range.h"
#include "tree-ssa-threadedge.h"
#include "gimple-range-path.h"
#include "ssa.h"
#include "tree-cfgcleanup.h"

// Path registry for the backwards threader.  After all paths have been
// registered with register_path(), thread_through_all_blocks() is called
// to modify the CFG.

class back_threader_registry
{
public:
  back_threader_registry (int max_allowable_paths);
  bool register_path (const vec<basic_block> &, edge taken);
  bool thread_through_all_blocks ();
private:
  jump_thread_path_registry m_lowlevel_registry;
  const int m_max_allowable_paths;
  int m_threaded_paths;
};

// Class to abstract the profitability code for the backwards threader.

class back_threader_profitability
{
public:
  back_threader_profitability (bool speed_p)
    : m_speed_p (speed_p)
  { }
  bool profitable_path_p (const vec<basic_block> &, tree name, edge taken,
                          bool *irreducible_loop = NULL);
private:
  const bool m_speed_p;
};

class back_threader
{
public:
  back_threader (bool speed_p);
  ~back_threader ();
  void maybe_thread_block (basic_block bb);
  bool thread_through_all_blocks ();
private:
  void find_paths (basic_block bb, tree name);
  void maybe_register_path (edge taken_edge);
  bool find_paths_to_names (basic_block bb, bitmap imports);
  bool resolve_def (tree name, bitmap interesting, vec<tree> &worklist);
  bool resolve_phi (gphi *phi, bitmap imports);
  edge find_taken_edge (const vec<basic_block> &path);
  edge find_taken_edge_cond (const vec<basic_block> &path, gcond *);
  edge find_taken_edge_switch (const vec<basic_block> &path, gswitch *);

  back_threader_registry m_registry;
  back_threader_profitability m_profit;
  gimple_ranger m_ranger;
  path_range_query m_solver;

  // Current path being analyzed.
  auto_vec<basic_block> m_path;
  // Hash to mark visited BBs while analyzing a path.
  hash_set<basic_block> m_visited_bbs;
  // The set of SSA names, any of which could potentially change the
  // value of the final conditional in a path.
  bitmap m_imports;
  // The last statement in the path.
  gimple *m_last_stmt;
  // This is a bit of a wart.  It's used to pass the LHS SSA name to
  // the profitability engine.
  tree m_name;
  // Marker to differentiate unreachable edges.
  static const edge UNREACHABLE_EDGE;
};

// Used to differentiate unreachable edges, so we may stop the search
// in a the given direction.
const edge back_threader::UNREACHABLE_EDGE = (edge) -1;

back_threader::back_threader (bool speed_p)
  : m_registry (param_max_fsm_thread_paths),
    m_profit (speed_p),
    m_solver (m_ranger)
{
  m_last_stmt = NULL;
  m_imports = BITMAP_ALLOC (NULL);
}

back_threader::~back_threader ()
{
  m_path.release ();
  BITMAP_FREE (m_imports);
}

// Register the current path for jump threading if it's profitable to
// do so.  TAKEN_EDGE is the known edge out of the path.

void
back_threader::maybe_register_path (edge taken_edge)
{
  bool irreducible = false;
  bool profitable
    = m_profit.profitable_path_p (m_path, m_name, taken_edge, &irreducible);

  if (profitable)
    {
      m_registry.register_path (m_path, taken_edge);

      if (irreducible)
        vect_free_loop_info_assumptions (m_path[0]->loop_father);
    }
}

// Return the known taken edge out of a path.  If the path can be
// determined to be unreachable, return UNREACHABLE_EDGE.  If no
// outgoing edge can be calculated, return NULL.

edge
back_threader::find_taken_edge (const vec<basic_block> &path)
{
  gcc_checking_assert (path.length () > 1);
  switch (gimple_code (m_last_stmt))
    {
    case GIMPLE_COND:
      return find_taken_edge_cond (path, as_a<gcond *> (m_last_stmt));

    case GIMPLE_SWITCH:
      return find_taken_edge_switch (path, as_a<gswitch *> (m_last_stmt));

    default:
      return NULL;
    }
}

// Same as find_taken_edge, but for paths ending in a switch.

edge
back_threader::find_taken_edge_switch (const vec<basic_block> &path,
                                       gswitch *sw)
{
  tree name = gimple_switch_index (sw);
  int_range_max r;

  m_solver.precompute_ranges (path, m_imports);
  m_solver.range_of_expr (r, name, sw);

  if (r.undefined_p ())
    return UNREACHABLE_EDGE;

  if (r.varying_p ())
    return NULL;

  tree val;
  if (r.singleton_p (&val))
    return ::find_taken_edge (gimple_bb (sw), val);

  return NULL;
}

// Same as find_taken_edge, but for paths ending in a GIMPLE_COND.

edge
back_threader::find_taken_edge_cond (const vec<basic_block> &path,
                                     gcond *cond)
{
  m_solver.precompute_ranges (path, m_imports);

  // Check if either operand is unreachable since this knowledge could
  // help the caller cut down the search space.
  int_range_max r;
  m_solver.range_of_expr (r, gimple_cond_lhs (cond));
  if (r.undefined_p ())
    return UNREACHABLE_EDGE;
  m_solver.range_of_expr (r, gimple_cond_rhs (cond));
  if (r.undefined_p ())
    return UNREACHABLE_EDGE;

  m_solver.range_of_stmt (r, cond);

  int_range<2> true_range (boolean_true_node, boolean_true_node);
  int_range<2> false_range (boolean_false_node, boolean_false_node);

  if (r == true_range || r == false_range)
    {
      edge e_true, e_false;
      basic_block bb = gimple_bb (cond);
      extract_true_false_edges_from_block (bb, &e_true, &e_false);
      return r == true_range ? e_true : e_false;
    }
  return NULL;
}

// Populate a vector of trees from a bitmap.

static inline void
populate_worklist (vec<tree> &worklist, bitmap bits)
{
  bitmap_iterator bi;
  unsigned i;

  EXECUTE_IF_SET_IN_BITMAP (bits, 0, i, bi)
    {
      tree name = ssa_name (i);
      worklist.quick_push (name);
    }
}

// If taking any of the incoming edges to a PHI causes the final
// conditional of the current path to be constant, register the
// path(s), and return TRUE.

bool
back_threader::resolve_phi (gphi *phi, bitmap interesting)
{
  if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (gimple_phi_result (phi)))
    return true;

  bool done = false;
  for (size_t i = 0; i < gimple_phi_num_args (phi); ++i)
    {
      edge e = gimple_phi_arg_edge (phi, i);

      // This is like path_crosses_loops in profitable_path_p but more
      // restrictive, since profitable_path_p allows threading the
      // first block because it would be redirected anyhow.
      //
      // If we loosened the restriction and used profitable_path_p()
      // here instead, we would peel off the first iterations of loops
      // in places like tree-ssa/pr14341.c.
      bool profitable_p = m_path[0]->loop_father == e->src->loop_father;
      if (!profitable_p)
        {
          if (dump_file && (dump_flags & TDF_DETAILS))
            fprintf (dump_file,
                     "  FAIL: path through PHI in bb%d (incoming bb:%d) crosses loop\n",
                     e->dest->index, e->src->index);
          continue;
        }

      tree arg = gimple_phi_arg_def (phi, i);
      if (TREE_CODE (arg) == SSA_NAME)
        {
          unsigned v = SSA_NAME_VERSION (arg);

          // Avoid loops as in: x_5 = PHI <x_5(2), ...>.
          if (bitmap_bit_p (interesting, v))
            continue;

          bitmap_set_bit (interesting, v);
          bitmap_set_bit (m_imports, v);
          done |= find_paths_to_names (e->src, interesting);
          bitmap_clear_bit (interesting, v);
        }
      else if (TREE_CODE (arg) == INTEGER_CST)
        {
          m_path.safe_push (e->src);
          edge taken_edge = find_taken_edge (m_path);
          if (taken_edge && taken_edge != UNREACHABLE_EDGE)
            {
              maybe_register_path (taken_edge);
              done = true;
            }
          m_path.pop ();
        }
    }
  return done;
}

// If the definition of NAME causes the final conditional of the
// current path to be constant, register the path, and return TRUE.

bool
back_threader::resolve_def (tree name, bitmap interesting, vec<tree> &worklist)
{
  gimple *def_stmt = SSA_NAME_DEF_STMT (name);

  // Handle PHIs.
  if (is_a<gphi *> (def_stmt)
      && resolve_phi (as_a<gphi *> (def_stmt), interesting))
    return true;

  // Defer copies of SSAs by adding the source to the worklist.
  if (gimple_assign_single_p (def_stmt)
      && TREE_CODE (gimple_assign_rhs1 (def_stmt)) == SSA_NAME)
    {
      tree rhs = gimple_assign_rhs1 (def_stmt);
      bitmap_set_bit (m_imports, SSA_NAME_VERSION (rhs));
      bitmap_set_bit (interesting, SSA_NAME_VERSION (rhs));
      worklist.safe_push (rhs);
    }
  return false;
}

// Find jump threading paths to any of the SSA names in the
// INTERESTING bitmap, and register any such paths.
//
// Return TRUE if no further processing past this block is necessary.
// This is because we've either registered a path, or because there is
// nothing of interesting beyond this block.
//
// BB is the current path being processed.

bool
back_threader::find_paths_to_names (basic_block bb, bitmap interesting)
{
  if (m_visited_bbs.add (bb))
    return true;

  m_path.safe_push (bb);

  if (m_path.length () > 1
      && !m_profit.profitable_path_p (m_path, m_name, NULL))
    {
      m_path.pop ();
      m_visited_bbs.remove (bb);
      return false;
    }

  auto_bitmap processed;
  unsigned i;
  bool done = false;

  // We use a worklist instead of iterating through the bitmap,
  // because we may add new items in-flight.
  auto_vec<tree> worklist (bitmap_count_bits (interesting));
  populate_worklist (worklist, interesting);
  while (!worklist.is_empty ())
    {
      tree name = worklist.pop ();
      unsigned i = SSA_NAME_VERSION (name);
      basic_block def_bb = gimple_bb (SSA_NAME_DEF_STMT (name));

      // Process any names defined in this block.
      if (def_bb == bb)
        {
          bitmap_set_bit (processed, i);

          if (resolve_def (name, interesting, worklist))
            {
              done = true;
              goto leave_bb;
            }
        }
      // Examine blocks that define or export an interesting SSA,
      // since they may compute a range which resolve this path.
      if ((def_bb == bb
           || bitmap_bit_p (m_ranger.gori ().exports (bb), i))
          && m_path.length () > 1)
        {
          edge taken_edge = find_taken_edge (m_path);
          if (taken_edge)
            {
              if (taken_edge != UNREACHABLE_EDGE)
                maybe_register_path (taken_edge);

              done = true;
              goto leave_bb;
            }
        }
    }

  // If there are interesting names not yet processed, keep looking.
  bitmap_and_compl_into (interesting, processed);
  if (!bitmap_empty_p (interesting))
    {
      edge_iterator iter;
      edge e;
      FOR_EACH_EDGE (e, iter, bb->preds)
        if ((e->flags & EDGE_ABNORMAL) == 0)
          done |= find_paths_to_names (e->src, interesting);
    }

 leave_bb:
  bitmap_iterator bi;
  EXECUTE_IF_SET_IN_BITMAP (processed, 0, i, bi)
    bitmap_set_bit (interesting, i);

  m_path.pop ();
  m_visited_bbs.remove (bb);
  return done;
}

// Search backwards from BB looking for paths where the final
// conditional out of BB can be determined.  NAME is the LHS of the
// final conditional.  Register such paths for jump threading.

void
back_threader::find_paths (basic_block bb, tree name)
{
  gimple *stmt = last_stmt (bb);
  if (!stmt
      || (gimple_code (stmt) != GIMPLE_COND
          && gimple_code (stmt) != GIMPLE_SWITCH))
    return;

  if (EDGE_COUNT (bb->succs) > 1
      || single_succ_to_potentially_threadable_block (bb))
    {
      m_last_stmt = stmt;
      m_visited_bbs.empty ();
      m_path.truncate (0);
      m_name = name;
      bitmap_clear (m_imports);

      auto_bitmap interesting;
      bitmap_copy (m_imports, m_ranger.gori ().imports (bb));
      bitmap_copy (interesting, m_imports);
      find_paths_to_names (bb, interesting);
    }
}

// Simple helper to get the last statement from BB, which is assumed
// to be a control statement.  Return NULL if the last statement is
// not a control statement.

static gimple *
get_gimple_control_stmt (basic_block bb)
{
  gimple_stmt_iterator gsi = gsi_last_nondebug_bb (bb);

  if (gsi_end_p (gsi))
    return NULL;

  gimple *stmt = gsi_stmt (gsi);
  enum gimple_code code = gimple_code (stmt);
  if (code == GIMPLE_COND || code == GIMPLE_SWITCH || code == GIMPLE_GOTO)
    return stmt;
  return NULL;
}

// Search backwards from BB looking for paths where the final
// conditional maybe threaded to a successor block.  Record such paths
// for jump threading.

void
back_threader::maybe_thread_block (basic_block bb)
{
  gimple *stmt = get_gimple_control_stmt (bb);
  if (!stmt)
    return;

  enum gimple_code code = gimple_code (stmt);
  tree name;
  if (code == GIMPLE_SWITCH)
    name = gimple_switch_index (as_a <gswitch *> (stmt));
  else if (code == GIMPLE_COND)
    name = gimple_cond_lhs (stmt);
  else if (code == GIMPLE_GOTO)
    name = gimple_goto_dest (stmt);
  else
    name = NULL;

  find_paths (bb, name);
}

// Perform the actual jump threading for the all queued paths.

bool
back_threader::thread_through_all_blocks ()
{
  return m_registry.thread_through_all_blocks ();
}

// Dump a sequence of BBs through the CFG.

DEBUG_FUNCTION void
dump_path (FILE *dump_file, const vec<basic_block> &path)
{
  for (size_t i = 0; i < path.length (); ++i)
    {
      fprintf (dump_file, "BB%d", path[i]->index);
      if (i + 1 < path.length ())
        fprintf (dump_file, " <- ");
    }
  fprintf (dump_file, "\n");
}

DEBUG_FUNCTION void
debug (const vec <basic_block> &path)
{
  dump_path (stderr, path);
}

back_threader_registry::back_threader_registry (int max_allowable_paths)
  : m_max_allowable_paths (max_allowable_paths)
{
  m_threaded_paths = 0;
}

bool
back_threader_registry::thread_through_all_blocks ()
{
  return m_lowlevel_registry.thread_through_all_blocks (true);
}

/* Examine jump threading path PATH and return TRUE if it is profitable to
   thread it, otherwise return FALSE.

   NAME is the SSA_NAME of the variable we found to have a constant
   value on PATH.  If unknown, SSA_NAME is NULL.

   If the taken edge out of the path is known ahead of time it is passed in
   TAKEN_EDGE, otherwise it is NULL.

   CREATES_IRREDUCIBLE_LOOP, if non-null is set to TRUE if threading this path
   would create an irreducible loop.  */

bool
back_threader_profitability::profitable_path_p (const vec<basic_block> &m_path,
                                                tree name,
                                                edge taken_edge,
                                                bool *creates_irreducible_loop)
{
  gcc_checking_assert (!m_path.is_empty ());

  /* We can an empty path here (excluding the DEF block) when the
     statement that makes a conditional generate a compile-time
     constant result is in the same block as the conditional.

     That's not really a jump threading opportunity, but instead is
     simple cprop & simplification.  We could handle it here if we
     wanted by wiring up all the incoming edges.  If we run this
     early in IPA, that might be worth doing.   For now we just
     reject that case.  */
  if (m_path.length () <= 1)
      return false;

  if (m_path.length () > (unsigned) param_max_fsm_thread_length)
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file, "  FAIL: FSM jump-thread path not considered: "
                 "the number of basic blocks on the path "
                 "exceeds PARAM_MAX_FSM_THREAD_LENGTH.\n");
      return false;
    }

  int n_insns = 0;
  gimple_stmt_iterator gsi;
  loop_p loop = m_path[0]->loop_father;
  bool path_crosses_loops = false;
  bool threaded_through_latch = false;
  bool multiway_branch_in_path = false;
  bool threaded_multiway_branch = false;
  bool contains_hot_bb = false;

  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "Checking profitability of path (backwards): ");

  /* Count the number of instructions on the path: as these instructions
     will have to be duplicated, we will not record the path if there
     are too many instructions on the path.  Also check that all the
     blocks in the path belong to a single loop.  */
  for (unsigned j = 0; j < m_path.length (); j++)
    {
      basic_block bb = m_path[j];

      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file, " bb:%i", bb->index);
      /* Remember, blocks in the path are stored in opposite order
         in the PATH array.  The last entry in the array represents
         the block with an outgoing edge that we will redirect to the
         jump threading path.  Thus we don't care about that block's
         loop father, nor how many statements are in that block because
         it will not be copied or whether or not it ends in a multiway
         branch.  */
      if (j < m_path.length () - 1)
        {
          int orig_n_insns = n_insns;
          if (bb->loop_father != loop)
            {
              path_crosses_loops = true;
              break;
            }

          /* PHIs in the path will create degenerate PHIS in the
             copied path which will then get propagated away, so
             looking at just the duplicate path the PHIs would
             seem unimportant.

             But those PHIs, because they're assignments to objects
             typically with lives that exist outside the thread path,
             will tend to generate PHIs (or at least new PHI arguments)
             at points where we leave the thread path and rejoin
             the original blocks.  So we do want to account for them.

             We ignore virtual PHIs.  We also ignore cases where BB
             has a single incoming edge.  That's the most common
             degenerate PHI we'll see here.  Finally we ignore PHIs
             that are associated with the value we're tracking as
             that object likely dies.  */
          if (EDGE_COUNT (bb->succs) > 1 && EDGE_COUNT (bb->preds) > 1)
            {
              for (gphi_iterator gsip = gsi_start_phis (bb);
                   !gsi_end_p (gsip);
                   gsi_next (&gsip))
                {
                  gphi *phi = gsip.phi ();
                  tree dst = gimple_phi_result (phi);

                  /* Note that if both NAME and DST are anonymous
                     SSA_NAMEs, then we do not have enough information
                     to consider them associated.  */
                  if (dst != name
                      && name
                      && TREE_CODE (name) == SSA_NAME
                      && (SSA_NAME_VAR (dst) != SSA_NAME_VAR (name)
                          || !SSA_NAME_VAR (dst))
                      && !virtual_operand_p (dst))
                    ++n_insns;
                }
            }

          if (!contains_hot_bb && m_speed_p)
            contains_hot_bb |= optimize_bb_for_speed_p (bb);
          for (gsi = gsi_after_labels (bb);
               !gsi_end_p (gsi);
               gsi_next_nondebug (&gsi))
            {
              /* Do not allow OpenACC loop markers and __builtin_constant_p on
                 threading paths.  The latter is disallowed, because an
                 expression might be constant on two threading paths, and
                 become non-constant (i.e.: phi) when they merge.  */
              gimple *stmt = gsi_stmt (gsi);
              if (gimple_call_internal_p (stmt, IFN_UNIQUE)
                  || gimple_call_builtin_p (stmt, BUILT_IN_CONSTANT_P))
                return false;
              /* Do not count empty statements and labels.  */
              if (gimple_code (stmt) != GIMPLE_NOP
                  && !(gimple_code (stmt) == GIMPLE_ASSIGN
                       && gimple_assign_rhs_code (stmt) == ASSERT_EXPR)
                  && !is_gimple_debug (stmt))
                n_insns += estimate_num_insns (stmt, &eni_size_weights);
            }
          if (dump_file && (dump_flags & TDF_DETAILS))
            fprintf (dump_file, " (%i insns)", n_insns-orig_n_insns);

          /* We do not look at the block with the threaded branch
             in this loop.  So if any block with a last statement that
             is a GIMPLE_SWITCH or GIMPLE_GOTO is seen, then we have a
             multiway branch on our path.

             The block in PATH[0] is special, it's the block were we're
             going to be able to eliminate its branch.  */
          gimple *last = last_stmt (bb);
          if (last && (gimple_code (last) == GIMPLE_SWITCH
                       || gimple_code (last) == GIMPLE_GOTO))
            {
              if (j == 0)
                threaded_multiway_branch = true;
              else
                multiway_branch_in_path = true;
            }
        }

      /* Note if we thread through the latch, we will want to include
         the last entry in the array when determining if we thread
         through the loop latch.  */
      if (loop->latch == bb)
        threaded_through_latch = true;
    }

  gimple *stmt = get_gimple_control_stmt (m_path[0]);
  gcc_assert (stmt);

  /* We are going to remove the control statement at the end of the
     last block in the threading path.  So don't count it against our
     statement count.  */

  int stmt_insns = estimate_num_insns (stmt, &eni_size_weights);
  n_insns-= stmt_insns;

  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "\n  Control statement insns: %i\n"
             "  Overall: %i insns\n",
             stmt_insns, n_insns);

  if (creates_irreducible_loop)
    {
      /* If this path threaded through the loop latch back into the
         same loop and the destination does not dominate the loop
         latch, then this thread would create an irreducible loop.  */
      *creates_irreducible_loop = false;
      if (taken_edge
          && threaded_through_latch
          && loop == taken_edge->dest->loop_father
          && (determine_bb_domination_status (loop, taken_edge->dest)
              == DOMST_NONDOMINATING))
        *creates_irreducible_loop = true;
    }

  if (path_crosses_loops)
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file, "  FAIL: FSM jump-thread path not considered: "
                 "the path crosses loops.\n");
      return false;
    }

  /* Threading is profitable if the path duplicated is hot but also
     in a case we separate cold path from hot path and permit optimization
     of the hot path later.  Be on the agressive side here. In some testcases,
     as in PR 78407 this leads to noticeable improvements.  */
  if (m_speed_p
      && ((taken_edge && optimize_edge_for_speed_p (taken_edge))
          || contains_hot_bb))
    {
      if (n_insns >= param_max_fsm_thread_path_insns)
        {
          if (dump_file && (dump_flags & TDF_DETAILS))
            fprintf (dump_file, "  FAIL: FSM jump-thread path not considered: "
                     "the number of instructions on the path "
                     "exceeds PARAM_MAX_FSM_THREAD_PATH_INSNS.\n");
          return false;
        }
    }
  else if (!m_speed_p && n_insns > 1)
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file, "  FAIL: FSM jump-thread path not considered: "
                 "duplication of %i insns is needed and optimizing for size.\n",
                 n_insns);
      return false;
    }

  /* We avoid creating irreducible inner loops unless we thread through
     a multiway branch, in which case we have deemed it worth losing
     other loop optimizations later.

     We also consider it worth creating an irreducible inner loop if
     the number of copied statement is low relative to the length of
     the path -- in that case there's little the traditional loop
     optimizer would have done anyway, so an irreducible loop is not
     so bad.  */
  if (!threaded_multiway_branch
      && creates_irreducible_loop
      && *creates_irreducible_loop
      && (n_insns * (unsigned) param_fsm_scale_path_stmts
          > (m_path.length () *
             (unsigned) param_fsm_scale_path_blocks)))

    {
      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file,
                 "  FAIL: FSM would create irreducible loop without threading "
                 "multiway branch.\n");
      return false;
    }

  /* If this path does not thread through the loop latch, then we are
     using the FSM threader to find old style jump threads.  This
     is good, except the FSM threader does not re-use an existing
     threading path to reduce code duplication.

     So for that case, drastically reduce the number of statements
     we are allowed to copy.  */
  if (!(threaded_through_latch && threaded_multiway_branch)
      && (n_insns * param_fsm_scale_path_stmts
          >= param_max_jump_thread_duplication_stmts))
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file,
                 "  FAIL: FSM did not thread around loop and would copy too "
                 "many statements.\n");
      return false;
    }

  /* When there is a multi-way branch on the path, then threading can
     explode the CFG due to duplicating the edges for that multi-way
     branch.  So like above, only allow a multi-way branch on the path
     if we actually thread a multi-way branch.  */
  if (!threaded_multiway_branch && multiway_branch_in_path)
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file,
                 "  FAIL: FSM Thread through multiway branch without threading "
                 "a multiway branch.\n");
      return false;
    }
  return true;
}

/* The current path PATH is a vector of blocks forming a jump threading
   path in reverse order.  TAKEN_EDGE is the edge taken from path[0].

   Convert the current path into the form used by register_jump_thread and
   register it.

   Return TRUE if successful or FALSE otherwise.  */

bool
back_threader_registry::register_path (const vec<basic_block> &m_path,
                                       edge taken_edge)
{
  if (m_threaded_paths > m_max_allowable_paths)
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file, "  FAIL: FSM jump-thread path not considered: "
                 "the number of previously recorded FSM paths to "
                 "thread exceeds PARAM_MAX_FSM_THREAD_PATHS.\n");
      return false;
    }

  vec<jump_thread_edge *> *jump_thread_path
    = m_lowlevel_registry.allocate_thread_path ();

  /* Record the edges between the blocks in PATH.  */
  for (unsigned int j = 0; j + 1 < m_path.length (); j++)
    {
      basic_block bb1 = m_path[m_path.length () - j - 1];
      basic_block bb2 = m_path[m_path.length () - j - 2];

      edge e = find_edge (bb1, bb2);
      gcc_assert (e);
      jump_thread_edge *x
        = m_lowlevel_registry.allocate_thread_edge (e, EDGE_FSM_THREAD);
      jump_thread_path->safe_push (x);
    }

  /* Add the edge taken when the control variable has value ARG.  */
  jump_thread_edge *x
    = m_lowlevel_registry.allocate_thread_edge (taken_edge,
                                                EDGE_NO_COPY_SRC_BLOCK);
  jump_thread_path->safe_push (x);

  if (m_lowlevel_registry.register_jump_thread (jump_thread_path))
    ++m_threaded_paths;
  return true;
}

namespace {

const pass_data pass_data_thread_jumps =
{
  GIMPLE_PASS,
  "thread",
  OPTGROUP_NONE,
  TV_TREE_SSA_THREAD_JUMPS,
  ( PROP_cfg | PROP_ssa ),
  0,
  0,
  0,
  TODO_update_ssa,
};

class pass_thread_jumps : public gimple_opt_pass
{
public:
  pass_thread_jumps (gcc::context *ctxt)
    : gimple_opt_pass (pass_data_thread_jumps, ctxt)
  {}

  opt_pass * clone (void) { return new pass_thread_jumps (m_ctxt); }
  virtual bool gate (function *);
  virtual unsigned int execute (function *);
};

bool
pass_thread_jumps::gate (function *fun ATTRIBUTE_UNUSED)
{
  return flag_expensive_optimizations;
}

// Try to thread blocks in FUN.  Return TRUE if any jump thread paths were
// registered.

static bool
try_thread_blocks (function *fun)
{
  /* Try to thread each block with more than one successor.  */
  back_threader threader (/*speed_p=*/true);
  basic_block bb;
  FOR_EACH_BB_FN (bb, fun)
    {
      if (EDGE_COUNT (bb->succs) > 1)
        threader.maybe_thread_block (bb);
    }
  return threader.thread_through_all_blocks ();
}

unsigned int
pass_thread_jumps::execute (function *fun)
{
  loop_optimizer_init (LOOPS_HAVE_PREHEADERS | LOOPS_HAVE_SIMPLE_LATCHES);

  bool changed = try_thread_blocks (fun);

  loop_optimizer_finalize ();

  return changed ? TODO_cleanup_cfg : 0;
}

}

gimple_opt_pass *
make_pass_thread_jumps (gcc::context *ctxt)
{
  return new pass_thread_jumps (ctxt);
}

namespace {

const pass_data pass_data_early_thread_jumps =
{
  GIMPLE_PASS,
  "ethread",
  OPTGROUP_NONE,
  TV_TREE_SSA_THREAD_JUMPS,
  ( PROP_cfg | PROP_ssa ),
  0,
  0,
  0,
  ( TODO_cleanup_cfg | TODO_update_ssa ),
};

class pass_early_thread_jumps : public gimple_opt_pass
{
public:
  pass_early_thread_jumps (gcc::context *ctxt)
    : gimple_opt_pass (pass_data_early_thread_jumps, ctxt)
  {}

  opt_pass * clone (void) { return new pass_early_thread_jumps (m_ctxt); }
  virtual bool gate (function *);
  virtual unsigned int execute (function *);
};

bool
pass_early_thread_jumps::gate (function *fun ATTRIBUTE_UNUSED)
{
  return true;
}

unsigned int
pass_early_thread_jumps::execute (function *fun)
{
  loop_optimizer_init (AVOID_CFG_MODIFICATIONS);

  /* Try to thread each block with more than one successor.  */
  back_threader threader (/*speed_p=*/false);
  basic_block bb;
  FOR_EACH_BB_FN (bb, fun)
    {
      if (EDGE_COUNT (bb->succs) > 1)
        threader.maybe_thread_block (bb);
    }
  threader.thread_through_all_blocks ();

  loop_optimizer_finalize ();
  return 0;
}

}

gimple_opt_pass *
make_pass_early_thread_jumps (gcc::context *ctxt)
{
  return new pass_early_thread_jumps (ctxt);
}