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/* Thread edges through blocks and update the control flow and SSA graphs.

   Copyright (C) 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 "flags.h"

#include "rtl.h"

#include "tm_p.h"

#include "ggc.h"

#include "basic-block.h"

#include "output.h"

#include "expr.h"

#include "function.h"

#include "diagnostic.h"

#include "tree-flow.h"

#include "tree-dump.h"

#include "tree-pass.h"

#include "cfgloop.h"

/* Given a block B, update the CFG and SSA graph to reflect redirecting

   one or more in-edges to B to instead reach the destination of an

   out-edge from B while preserving any side effects in B.

   i.e., given A->B and B->C, change A->B to be A->C yet still preserve the

   side effects of executing B.

     1. Make a copy of B (including its outgoing edges and statements).  Call

        the copy B'.  Note B' has no incoming edges or PHIs at this time.

     2. Remove the control statement at the end of B' and all outgoing edges

        except B'->C.

     3. Add a new argument to each PHI in C with the same value as the existing

        argument associated with edge B->C.  Associate the new PHI arguments

        with the edge B'->C.

     4. For each PHI in B, find or create a PHI in B' with an identical

        PHI_RESULT.  Add an argument to the PHI in B' which has the same

        value as the PHI in B associated with the edge A->B.  Associate

        the new argument in the PHI in B' with the edge A->B.

     5. Change the edge A->B to A->B'.

        5a. This automatically deletes any PHI arguments associated with the

            edge A->B in B.

        5b. This automatically associates each new argument added in step 4

            with the edge A->B'.

     6. Repeat for other incoming edges into B.

     7. Put the duplicated resources in B and all the B' blocks into SSA form.

   Note that block duplication can be minimized by first collecting the

   set of unique destination blocks that the incoming edges should

   be threaded to.  Block duplication can be further minimized by using

   B instead of creating B' for one destination if all edges into B are

   going to be threaded to a successor of B.

   We further reduce the number of edges and statements we create by

   not copying all the outgoing edges and the control statement in

   step #1.  We instead create a template block without the outgoing

   edges and duplicate the template.  */

/* Steps #5 and #6 of the above algorithm are best implemented by walking

   all the incoming edges which thread to the same destination edge at

   the same time.  That avoids lots of table lookups to get information

   for the destination edge.

   To realize that implementation we create a list of incoming edges

   which thread to the same outgoing edge.  Thus to implement steps

   #5 and #6 we traverse our hash table of outgoing edge information.

   For each entry we walk the list of incoming edges which thread to

   the current outgoing edge.  */

struct el

{

  edge e;

  struct el *next;

};

/* Main data structure recording information regarding B's duplicate

   blocks.  */

/* We need to efficiently record the unique thread destinations of this

   block and specific information associated with those destinations.  We

   may have many incoming edges threaded to the same outgoing edge.  This

   can be naturally implemented with a hash table.  */

struct redirection_data

{

  /* A duplicate of B with the trailing control statement removed and which

     targets a single successor of B.  */

  basic_block dup_block;

  /* An outgoing edge from B.  DUP_BLOCK will have OUTGOING_EDGE->dest as

     its single successor.  */

  edge outgoing_edge;

  /* A list of incoming edges which we want to thread to

     OUTGOING_EDGE->dest.  */

  struct el *incoming_edges;

  /* Flag indicating whether or not we should create a duplicate block

     for this thread destination.  This is only true if we are threading

     all incoming edges and thus are using BB itself as a duplicate block.  */

  bool do_not_duplicate;

};

/* Main data structure to hold information for duplicates of BB.  */

static htab_t redirection_data;

/* Data structure of information to pass to hash table traversal routines.  */

struct local_info

{

  /* The current block we are working on.  */

  basic_block bb;

  /* A template copy of BB with no outgoing edges or control statement that

     we use for creating copies.  */

  basic_block template_block;

  /* TRUE if we thread one or more jumps, FALSE otherwise.  */

  bool jumps_threaded;

};

/* Passes which use the jump threading code register jump threading

   opportunities as they are discovered.  We keep the registered

   jump threading opportunities in this vector as edge pairs

   (original_edge, target_edge).  */

static VEC(edge,heap) *threaded_edges;

/* Jump threading statistics.  */

struct thread_stats_d

{

  unsigned long num_threaded_edges;

};

struct thread_stats_d thread_stats;

/* Remove the last statement in block BB if it is a control statement

   Also remove all outgoing edges except the edge which reaches DEST_BB.

   If DEST_BB is NULL, then remove all outgoing edges.  */

static void

remove_ctrl_stmt_and_useless_edges (basic_block bb, basic_block dest_bb)

{

  gimple_stmt_iterator gsi;

  edge e;

  edge_iterator ei;

  gsi = gsi_last_bb (bb);

  /* If the duplicate ends with a control statement, then remove it.

     Note that if we are duplicating the template block rather than the

     original basic block, then the duplicate might not have any real

     statements in it.  */

  if (!gsi_end_p (gsi)

      && gsi_stmt (gsi)

      && (gimple_code (gsi_stmt (gsi)) == GIMPLE_COND

          || gimple_code (gsi_stmt (gsi)) == GIMPLE_GOTO

          || gimple_code (gsi_stmt (gsi)) == GIMPLE_SWITCH))

    gsi_remove (&gsi, true);

  for (ei = ei_start (bb->succs); (e = ei_safe_edge (ei)); )

    {

      if (e->dest != dest_bb)

        remove_edge (e);

      else

        ei_next (&ei);

    }

}

/* Create a duplicate of BB which only reaches the destination of the edge

   stored in RD.  Record the duplicate block in RD.  */

static void

create_block_for_threading (basic_block bb, struct redirection_data *rd)

{

  /* We can use the generic block duplication code and simply remove

     the stuff we do not need.  */

  rd->dup_block = duplicate_block (bb, NULL, NULL);

  /* Zero out the profile, since the block is unreachable for now.  */

  rd->dup_block->frequency = 0;

  rd->dup_block->count = 0;

  /* The call to duplicate_block will copy everything, including the

     useless COND_EXPR or SWITCH_EXPR at the end of BB.  We just remove

     the useless COND_EXPR or SWITCH_EXPR here rather than having a

     specialized block copier.  We also remove all outgoing edges

     from the duplicate block.  The appropriate edge will be created

     later.  */

  remove_ctrl_stmt_and_useless_edges (rd->dup_block, NULL);

}

/* Hashing and equality routines for our hash table.  */

static hashval_t

redirection_data_hash (const void *p)

{

  edge e = ((const struct redirection_data *)p)->outgoing_edge;

  return e->dest->index;

}

static int

redirection_data_eq (const void *p1, const void *p2)

{

  edge e1 = ((const struct redirection_data *)p1)->outgoing_edge;

  edge e2 = ((const struct redirection_data *)p2)->outgoing_edge;

  return e1 == e2;

}

/* Given an outgoing edge E lookup and return its entry in our hash table.

   If INSERT is true, then we insert the entry into the hash table if

   it is not already present.  INCOMING_EDGE is added to the list of incoming

   edges associated with E in the hash table.  */

static struct redirection_data *

lookup_redirection_data (edge e, edge incoming_edge, enum insert_option insert)

{

  void **slot;

  struct redirection_data *elt;

 /* Build a hash table element so we can see if E is already

     in the table.  */

  elt = XNEW (struct redirection_data);

  elt->outgoing_edge = e;

  elt->dup_block = NULL;

  elt->do_not_duplicate = false;

  elt->incoming_edges = NULL;

  slot = htab_find_slot (redirection_data, elt, insert);

  /* This will only happen if INSERT is false and the entry is not

     in the hash table.  */

  if (slot == NULL)

    {

      free (elt);

      return NULL;

    }

  /* This will only happen if E was not in the hash table and

     INSERT is true.  */

  if (*slot == NULL)

    {

      *slot = (void *)elt;

      elt->incoming_edges = XNEW (struct el);

      elt->incoming_edges->e = incoming_edge;

      elt->incoming_edges->next = NULL;

      return elt;

    }

  /* E was in the hash table.  */

  else

    {

      /* Free ELT as we do not need it anymore, we will extract the

         relevant entry from the hash table itself.  */

      free (elt);

      /* Get the entry stored in the hash table.  */

      elt = (struct redirection_data *) *slot;

      /* If insertion was requested, then we need to add INCOMING_EDGE

         to the list of incoming edges associated with E.  */

      if (insert)

        {

          struct el *el = XNEW (struct el);

          el->next = elt->incoming_edges;

          el->e = incoming_edge;

          elt->incoming_edges = el;

        }

      return elt;

    }

}

/* Given a duplicate block and its single destination (both stored

   in RD).  Create an edge between the duplicate and its single

   destination.

   Add an additional argument to any PHI nodes at the single

   destination.  */

static void

create_edge_and_update_destination_phis (struct redirection_data *rd)

{

  edge e = make_edge (rd->dup_block, rd->outgoing_edge->dest, EDGE_FALLTHRU);

  gimple_stmt_iterator gsi;

  rescan_loop_exit (e, true, false);

  e->probability = REG_BR_PROB_BASE;

  e->count = rd->dup_block->count;

  e->aux = rd->outgoing_edge->aux;

  /* If there are any PHI nodes at the destination of the outgoing edge

     from the duplicate block, then we will need to add a new argument

     to them.  The argument should have the same value as the argument

     associated with the outgoing edge stored in RD.  */

  for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))

    {

      gimple phi = gsi_stmt (gsi);

      source_location locus;

      int indx = rd->outgoing_edge->dest_idx;

      locus = gimple_phi_arg_location (phi, indx);

      add_phi_arg (phi, gimple_phi_arg_def (phi, indx), e, locus);

    }

}

/* Hash table traversal callback routine to create duplicate blocks.  */

static int

create_duplicates (void **slot, void *data)

{

  struct redirection_data *rd = (struct redirection_data *) *slot;

  struct local_info *local_info = (struct local_info *)data;

  /* If this entry should not have a duplicate created, then there's

     nothing to do.  */

  if (rd->do_not_duplicate)

    return 1;

  /* Create a template block if we have not done so already.  Otherwise

     use the template to create a new block.  */

  if (local_info->template_block == NULL)

    {

      create_block_for_threading (local_info->bb, rd);

      local_info->template_block = rd->dup_block;

      /* We do not create any outgoing edges for the template.  We will

         take care of that in a later traversal.  That way we do not

         create edges that are going to just be deleted.  */

    }

  else

    {

      create_block_for_threading (local_info->template_block, rd);

      /* Go ahead and wire up outgoing edges and update PHIs for the duplicate

         block.  */

      create_edge_and_update_destination_phis (rd);

    }

  /* Keep walking the hash table.  */

  return 1;

}

/* We did not create any outgoing edges for the template block during

   block creation.  This hash table traversal callback creates the

   outgoing edge for the template block.  */

static int

fixup_template_block (void **slot, void *data)

{

  struct redirection_data *rd = (struct redirection_data *) *slot;

  struct local_info *local_info = (struct local_info *)data;

  /* If this is the template block, then create its outgoing edges

     and halt the hash table traversal.  */

  if (rd->dup_block && rd->dup_block == local_info->template_block)

    {

      create_edge_and_update_destination_phis (rd);

      return 0;

    }

  return 1;

}

/* Hash table traversal callback to redirect each incoming edge

   associated with this hash table element to its new destination.  */

static int

redirect_edges (void **slot, void *data)

{

  struct redirection_data *rd = (struct redirection_data *) *slot;

  struct local_info *local_info = (struct local_info *)data;

  struct el *next, *el;

  /* Walk over all the incoming edges associated associated with this

     hash table entry.  */

  for (el = rd->incoming_edges; el; el = next)

    {

      edge e = el->e;

      /* Go ahead and free this element from the list.  Doing this now

         avoids the need for another list walk when we destroy the hash

         table.  */

      next = el->next;

      free (el);

      /* Go ahead and clear E->aux.  It's not needed anymore and failure

         to clear it will cause all kinds of unpleasant problems later.  */

      e->aux = NULL;

      thread_stats.num_threaded_edges++;

      if (rd->dup_block)

        {

          edge e2;

          if (dump_file && (dump_flags & TDF_DETAILS))

            fprintf (dump_file, "  Threaded jump %d --> %d to %d\n",

                     e->src->index, e->dest->index, rd->dup_block->index);

          rd->dup_block->count += e->count;

          rd->dup_block->frequency += EDGE_FREQUENCY (e);

          EDGE_SUCC (rd->dup_block, 0)->count += e->count;

          /* Redirect the incoming edge to the appropriate duplicate

             block.  */

          e2 = redirect_edge_and_branch (e, rd->dup_block);

          gcc_assert (e == e2);

          flush_pending_stmts (e2);

        }

      else

        {

          if (dump_file && (dump_flags & TDF_DETAILS))

            fprintf (dump_file, "  Threaded jump %d --> %d to %d\n",

                     e->src->index, e->dest->index, local_info->bb->index);

          /* We are using BB as the duplicate.  Remove the unnecessary

             outgoing edges and statements from BB.  */

          remove_ctrl_stmt_and_useless_edges (local_info->bb,

                                              rd->outgoing_edge->dest);

          /* Fixup the flags on the single remaining edge.  */

          single_succ_edge (local_info->bb)->flags

            &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE | EDGE_ABNORMAL);

          single_succ_edge (local_info->bb)->flags |= EDGE_FALLTHRU;

          /* And adjust count and frequency on BB.  */

          local_info->bb->count = e->count;

          local_info->bb->frequency = EDGE_FREQUENCY (e);

        }

    }

  /* Indicate that we actually threaded one or more jumps.  */

  if (rd->incoming_edges)

    local_info->jumps_threaded = true;

  return 1;

}

/* Return true if this block has no executable statements other than

   a simple ctrl flow instruction.  When the number of outgoing edges

   is one, this is equivalent to a "forwarder" block.  */

static bool

redirection_block_p (basic_block bb)

{

  gimple_stmt_iterator gsi;

  /* Advance to the first executable statement.  */

  gsi = gsi_start_bb (bb);

  while (!gsi_end_p (gsi)

         && (gimple_code (gsi_stmt (gsi)) == GIMPLE_LABEL

             || is_gimple_debug (gsi_stmt (gsi))

             || gimple_nop_p (gsi_stmt (gsi))))

    gsi_next (&gsi);

  /* Check if this is an empty block.  */

  if (gsi_end_p (gsi))

    return true;

  /* Test that we've reached the terminating control statement.  */

  return gsi_stmt (gsi)

         && (gimple_code (gsi_stmt (gsi)) == GIMPLE_COND

             || gimple_code (gsi_stmt (gsi)) == GIMPLE_GOTO

             || gimple_code (gsi_stmt (gsi)) == GIMPLE_SWITCH);

}

/* BB is a block which ends with a COND_EXPR or SWITCH_EXPR and when BB

   is reached via one or more specific incoming edges, we know which

   outgoing edge from BB will be traversed.

   We want to redirect those incoming edges to the target of the

   appropriate outgoing edge.  Doing so avoids a conditional branch

   and may expose new optimization opportunities.  Note that we have

   to update dominator tree and SSA graph after such changes.

   The key to keeping the SSA graph update manageable is to duplicate

   the side effects occurring in BB so that those side effects still

   occur on the paths which bypass BB after redirecting edges.

   We accomplish this by creating duplicates of BB and arranging for

   the duplicates to unconditionally pass control to one specific

   successor of BB.  We then revector the incoming edges into BB to

   the appropriate duplicate of BB.

   If NOLOOP_ONLY is true, we only perform the threading as long as it

   does not affect the structure of the loops in a nontrivial way.  */

static bool

thread_block (basic_block bb, bool noloop_only)

{

  /* E is an incoming edge into BB that we may or may not want to

     redirect to a duplicate of BB.  */

  edge e, e2;

  edge_iterator ei;

  struct local_info local_info;

  struct loop *loop = bb->loop_father;

  /* ALL indicates whether or not all incoming edges into BB should

     be threaded to a duplicate of BB.  */

  bool all = true;

  /* To avoid scanning a linear array for the element we need we instead

     use a hash table.  For normal code there should be no noticeable

     difference.  However, if we have a block with a large number of

     incoming and outgoing edges such linear searches can get expensive.  */

  redirection_data = htab_create (EDGE_COUNT (bb->succs),

                                  redirection_data_hash,

                                  redirection_data_eq,

                                  free);

  /* If we thread the latch of the loop to its exit, the loop ceases to

     exist.  Make sure we do not restrict ourselves in order to preserve

     this loop.  */

  if (loop->header == bb)

    {

      e = loop_latch_edge (loop);

      e2 = (edge) e->aux;

      if (e2 && loop_exit_edge_p (loop, e2))

        {

          loop->header = NULL;

          loop->latch = NULL;

        }

    }

  /* Record each unique threaded destination into a hash table for

     efficient lookups.  */

  FOR_EACH_EDGE (e, ei, bb->preds)

    {

      e2 = (edge) e->aux;

      if (!e2

          /* If NOLOOP_ONLY is true, we only allow threading through the

             header of a loop to exit edges.  */

          || (noloop_only

              && bb == bb->loop_father->header

              && !loop_exit_edge_p (bb->loop_father, e2)))

        {

          all = false;

          continue;

        }

      update_bb_profile_for_threading (e->dest, EDGE_FREQUENCY (e),

                                       e->count, (edge) e->aux);

      /* Insert the outgoing edge into the hash table if it is not

         already in the hash table.  */

      lookup_redirection_data (e2, e, INSERT);

    }

  /* If we are going to thread all incoming edges to an outgoing edge, then

     BB will become unreachable.  Rather than just throwing it away, use

     it for one of the duplicates.  Mark the first incoming edge with the

     DO_NOT_DUPLICATE attribute.  */

  if (all)

    {

      edge e = (edge) EDGE_PRED (bb, 0)->aux;

      lookup_redirection_data (e, NULL, NO_INSERT)->do_not_duplicate = true;

    }

  /* We do not update dominance info.  */

  free_dominance_info (CDI_DOMINATORS);

  /* Now create duplicates of BB.

     Note that for a block with a high outgoing degree we can waste

     a lot of time and memory creating and destroying useless edges.

     So we first duplicate BB and remove the control structure at the

     tail of the duplicate as well as all outgoing edges from the

     duplicate.  We then use that duplicate block as a template for

     the rest of the duplicates.  */

  local_info.template_block = NULL;

  local_info.bb = bb;

  local_info.jumps_threaded = false;

  htab_traverse (redirection_data, create_duplicates, &local_info);

  /* The template does not have an outgoing edge.  Create that outgoing

     edge and update PHI nodes as the edge's target as necessary.

     We do this after creating all the duplicates to avoid creating

     unnecessary edges.  */

  htab_traverse (redirection_data, fixup_template_block, &local_info);

  /* The hash table traversals above created the duplicate blocks (and the

     statements within the duplicate blocks).  This loop creates PHI nodes for

     the duplicated blocks and redirects the incoming edges into BB to reach

     the duplicates of BB.  */

  htab_traverse (redirection_data, redirect_edges, &local_info);

  /* Done with this block.  Clear REDIRECTION_DATA.  */

  htab_delete (redirection_data);

  redirection_data = NULL;

  /* Indicate to our caller whether or not any jumps were threaded.  */

  return local_info.jumps_threaded;

}

/* Threads edge E through E->dest to the edge E->aux.  Returns the copy

   of E->dest created during threading, or E->dest if it was not necessary

   to copy it (E is its single predecessor).  */

static basic_block

thread_single_edge (edge e)

{

  basic_block bb = e->dest;

  edge eto = (edge) e->aux;

  struct redirection_data rd;

  e->aux = NULL;

  thread_stats.num_threaded_edges++;

  if (single_pred_p (bb))

    {

      /* If BB has just a single predecessor, we should only remove the

         control statements at its end, and successors except for ETO.  */

      remove_ctrl_stmt_and_useless_edges (bb, eto->dest);

      /* And fixup the flags on the single remaining edge.  */

      eto->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE | EDGE_ABNORMAL);

      eto->flags |= EDGE_FALLTHRU;

      return bb;

    }

  /* Otherwise, we need to create a copy.  */

  update_bb_profile_for_threading (bb, EDGE_FREQUENCY (e), e->count, eto);

  rd.outgoing_edge = eto;

  create_block_for_threading (bb, &rd);

  create_edge_and_update_destination_phis (&rd);

  if (dump_file && (dump_flags & TDF_DETAILS))

    fprintf (dump_file, "  Threaded jump %d --> %d to %d\n",

             e->src->index, e->dest->index, rd.dup_block->index);

  rd.dup_block->count = e->count;

  rd.dup_block->frequency = EDGE_FREQUENCY (e);

  single_succ_edge (rd.dup_block)->count = e->count;

  redirect_edge_and_branch (e, rd.dup_block);

  flush_pending_stmts (e);

  return rd.dup_block;

}

/* Callback for dfs_enumerate_from.  Returns true if BB is different

   from STOP and DBDS_CE_STOP.  */

static basic_block dbds_ce_stop;

static bool

dbds_continue_enumeration_p (const_basic_block bb, const void *stop)

{

  return (bb != (const_basic_block) stop

          && bb != dbds_ce_stop);

}

/* Evaluates the dominance relationship of latch of the LOOP and BB, and

   returns the state.  */

enum bb_dom_status

{

  /* BB does not dominate latch of the LOOP.  */

  DOMST_NONDOMINATING,

  /* The LOOP is broken (there is no path from the header to its latch.  */

  DOMST_LOOP_BROKEN,

  /* BB dominates the latch of the LOOP.  */

  DOMST_DOMINATING

};

static enum bb_dom_status

determine_bb_domination_status (struct loop *loop, basic_block bb)

{

  basic_block *bblocks;

  unsigned nblocks, i;

  bool bb_reachable = false;

  edge_iterator ei;

  edge e;

#ifdef ENABLE_CHECKING

  /* This function assumes BB is a successor of LOOP->header.  */

    {

      bool ok = false;

      FOR_EACH_EDGE (e, ei, bb->preds)

        {

          if (e->src == loop->header)

            {

              ok = true;

              break;

            }

        }

      gcc_assert (ok);

    }

#endif

  if (bb == loop->latch)

    return DOMST_DOMINATING;

  /* Check that BB dominates LOOP->latch, and that it is back-reachable

     from it.  */

  bblocks = XCNEWVEC (basic_block, loop->num_nodes);

  dbds_ce_stop = loop->header;

  nblocks = dfs_enumerate_from (loop->latch, 1, dbds_continue_enumeration_p,

                                bblocks, loop->num_nodes, bb);

  for (i = 0; i < nblocks; i++)

    FOR_EACH_EDGE (e, ei, bblocks[i]->preds)

      {

        if (e->src == loop->header)

          {

            free (bblocks);

            return DOMST_NONDOMINATING;

          }

        if (e->src == bb)

          bb_reachable = true;

      }

  free (bblocks);