path: root/gcc/tree-ssa-threadbackward.c

/* SSA Jump Threading
   Copyright (C) 2005-2016 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 "params.h"
#include "tree-ssa-loop.h"
#include "cfganal.h"
#include "tree-pass.h"
#include "gimple-ssa.h"
#include "tree-phinodes.h"

static int max_threaded_paths;

/* 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;
}

/* Return true if the CFG contains at least one path from START_BB to END_BB.
   When a path is found, record in PATH the blocks from END_BB to START_BB.
   VISITED_BBS is used to make sure we don't fall into an infinite loop.  Bound
   the recursion to basic blocks belonging to LOOP.  */

static bool
fsm_find_thread_path (basic_block start_bb, basic_block end_bb,
		      vec<basic_block, va_gc> *&path,
		      hash_set<basic_block> *visited_bbs, loop_p loop)
{
  if (loop != start_bb->loop_father)
    return false;

  if (start_bb == end_bb)
    {
      vec_safe_push (path, start_bb);
      return true;
    }

  if (!visited_bbs->add (start_bb))
    {
      edge e;
      edge_iterator ei;
      FOR_EACH_EDGE (e, ei, start_bb->succs)
	if (fsm_find_thread_path (e->dest, end_bb, path, visited_bbs, loop))
	  {
	    vec_safe_push (path, start_bb);
	    return true;
	  }
    }

  return false;
}

/* Examine jump threading path PATH to which we want to add BBI.

   If the resulting path is profitable to thread, then return the
   final taken edge from the path, NULL otherwise.

   NAME is the SSA_NAME of the variable we found to have a constant
   value on PATH.  ARG is the value of that SSA_NAME.

   BBI will be appended to PATH when we have a profitable jump threading
   path.  Callers are responsible for removing BBI from PATH in that case. */

static edge
profitable_jump_thread_path (vec<basic_block, va_gc> *&path,
			     basic_block bbi, tree name, tree arg)
{
  /* Note BBI is not in the path yet, hence the +1 in the test below
     to make sure BBI is accounted for in the path length test.  */
  int path_length = path->length ();
  if (path_length + 1 > PARAM_VALUE (PARAM_MAX_FSM_THREAD_LENGTH))
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
	fprintf (dump_file, "FSM jump-thread path not considered: "
		 "the number of basic blocks on the path "
		 "exceeds PARAM_MAX_FSM_THREAD_LENGTH.\n");
      return NULL;
    }

  if (max_threaded_paths <= 0)
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
	fprintf (dump_file, "FSM jump-thread path not considered: "
		 "the number of previously recorded FSM paths to "
		 "thread exceeds PARAM_MAX_FSM_THREAD_PATHS.\n");
      return NULL;
    }

  /* Add BBI to the path.
     From this point onward, if we decide we the path is not profitable
     to thread, we must remove BBI from the path.  */
  vec_safe_push (path, bbi);
  ++path_length;

  int n_insns = 0;
  gimple_stmt_iterator gsi;
  int j;
  loop_p loop = (*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;

  /* 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 (j = 0; j < path_length; j++)
    {
      basic_block bb = (*path)[j];

      /* 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 < path_length - 1)
	{
	  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 ((SSA_NAME_VAR (dst) != SSA_NAME_VAR (name)
		       || !SSA_NAME_VAR (dst))
		      && !virtual_operand_p (dst))
		    ++n_insns;
		}
	    }

	  for (gsi = gsi_after_labels (bb);
	       !gsi_end_p (gsi);
	       gsi_next_nondebug (&gsi))
	    {
	      gimple *stmt = gsi_stmt (gsi);
	      /* 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;
	    }

	  /* 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;
    }

  /* 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.  */
  n_insns--;

  gimple *stmt = get_gimple_control_stmt ((*path)[0]);
  gcc_assert (stmt);
  /* We have found a constant value for ARG.  For GIMPLE_SWITCH
     and GIMPLE_GOTO, we use it as-is.  However, for a GIMPLE_COND
     we need to substitute, fold and simplify so we can determine
     the edge taken out of the last block.  */
  if (gimple_code (stmt) == GIMPLE_COND)
    {
      enum tree_code cond_code = gimple_cond_code (stmt);

      /* We know the underyling format of the condition.  */
      arg = fold_binary (cond_code, boolean_type_node,
			 arg, gimple_cond_rhs (stmt));
    }

  /* 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.

     We have to know the outgoing edge to figure this out.  */
  edge taken_edge = find_taken_edge ((*path)[0], arg);

  /* There are cases where we may not be able to extract the
     taken edge.  For example, a computed goto to an absolute
     address.  Handle those cases gracefully.  */
  if (taken_edge == NULL)
    {
      path->pop ();
      return NULL;
    }

  bool creates_irreducible_loop = false;
  if (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, "FSM jump-thread path not considered: "
		 "the path crosses loops.\n");
      path->pop ();
      return NULL;
    }

  if (n_insns >= PARAM_VALUE (PARAM_MAX_FSM_THREAD_PATH_INSNS))
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
	fprintf (dump_file, "FSM jump-thread path not considered: "
		 "the number of instructions on the path "
		 "exceeds PARAM_MAX_FSM_THREAD_PATH_INSNS.\n");
      path->pop ();
      return NULL;
    }

  /* 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
      && (n_insns * PARAM_VALUE (PARAM_FSM_SCALE_PATH_STMTS)
	  > path_length * PARAM_VALUE (PARAM_FSM_SCALE_PATH_BLOCKS)))

    {
      if (dump_file && (dump_flags & TDF_DETAILS))
	fprintf (dump_file,
		 "FSM would create irreducible loop without threading "
		 "multiway branch.\n");
      path->pop ();
      return NULL;
    }


  /* 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_VALUE (PARAM_FSM_SCALE_PATH_STMTS)
	  >= PARAM_VALUE (PARAM_MAX_JUMP_THREAD_DUPLICATION_STMTS)))
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
	fprintf (dump_file,
		 "FSM did not thread around loop and would copy too "
		 "many statements.\n");
      path->pop ();
      return NULL;
    }

  /* 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,
		 "FSM Thread through multiway branch without threading "
		 "a multiway branch.\n");
      path->pop ();
      return NULL;
    }
  return taken_edge;
}

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

   Convert that path into the form used by register_jump_thread and
   register the path.   */

static void
convert_and_register_jump_thread_path (vec<basic_block, va_gc> *&path,
				       edge taken_edge)
{
  vec<jump_thread_edge *> *jump_thread_path = new vec<jump_thread_edge *> ();

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

      /* This can happen when we have an SSA_NAME as a PHI argument and
	 its initialization block is the head of the PHI argument's
	 edge.  */
      if (bb1 == bb2)
	continue;

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

  /* As a consequence of the test for duplicate blocks in the path
     above, we can get a path with no blocks.  This happens if a
     conditional can be fully evaluated at compile time using just
     defining statements in the same block as the test.

     When we no longer push the block associated with a PHI argument
     onto the stack, then this as well as the test in the loop above
     can be removed.  */
  if (jump_thread_path->length () == 0)
    {
      jump_thread_path->release ();
      delete jump_thread_path;
      path->pop ();
      return;
    }

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

  register_jump_thread (jump_thread_path);
  --max_threaded_paths;

  /* Remove BBI from the path.  */
  path->pop ();
}

/* We trace the value of the SSA_NAME NAME back through any phi nodes looking
   for places where it gets a constant value and save the path.  Stop after
   having recorded MAX_PATHS jump threading paths.  */

static void
fsm_find_control_statement_thread_paths (tree name,
					 hash_set<basic_block> *visited_bbs,
					 vec<basic_block, va_gc> *&path,
					 bool seen_loop_phi)
{
  /* If NAME appears in an abnormal PHI, then don't try to trace its
     value back through PHI nodes.  */
  if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (name))
    return;

  gimple *def_stmt = SSA_NAME_DEF_STMT (name);
  basic_block var_bb = gimple_bb (def_stmt);

  if (var_bb == NULL)
    return;

  /* We allow the SSA chain to contains PHIs and simple copies and constant
     initializations.  */
  if (gimple_code (def_stmt) != GIMPLE_PHI
      && gimple_code (def_stmt) != GIMPLE_ASSIGN)
    return;

  if (gimple_code (def_stmt) == GIMPLE_PHI
      && (gimple_phi_num_args (def_stmt)
	  >= (unsigned) PARAM_VALUE (PARAM_FSM_MAXIMUM_PHI_ARGUMENTS)))
    return;

  if (gimple_code (def_stmt) == GIMPLE_ASSIGN
      && gimple_assign_rhs_code (def_stmt) != INTEGER_CST
      && gimple_assign_rhs_code (def_stmt) != SSA_NAME)
    return;

  /* Avoid infinite recursion.  */
  if (visited_bbs->add (var_bb))
    return;

  int next_path_length = 0;
  basic_block last_bb_in_path = path->last ();

  if (loop_containing_stmt (def_stmt)->header == gimple_bb (def_stmt))
    {
      /* Do not walk through more than one loop PHI node.  */
      if (seen_loop_phi)
	return;
      seen_loop_phi = true;
    }

  /* Following the chain of SSA_NAME definitions, we jumped from a definition in
     LAST_BB_IN_PATH to a definition in VAR_BB.  When these basic blocks are
     different, append to PATH the blocks from LAST_BB_IN_PATH to VAR_BB.  */
  if (var_bb != last_bb_in_path)
    {
      edge e;
      int e_count = 0;
      edge_iterator ei;
      vec<basic_block, va_gc> *next_path;
      vec_alloc (next_path, 10);

      /* When VAR_BB == LAST_BB_IN_PATH, then the first block in the path
	 will already be in VISITED_BBS.  When they are not equal, then we
	 must ensure that first block is accounted for to ensure we do not
	 create bogus jump threading paths.  */
      visited_bbs->add ((*path)[0]);
      FOR_EACH_EDGE (e, ei, last_bb_in_path->preds)
	{
	  hash_set<basic_block> *visited_bbs = new hash_set<basic_block>;

	  if (fsm_find_thread_path (var_bb, e->src, next_path, visited_bbs,
				    e->src->loop_father))
	    ++e_count;

	  delete visited_bbs;

	  /* If there is more than one path, stop.  */
	  if (e_count > 1)
	    {
	      vec_free (next_path);
	      return;
	    }
	}

      /* Stop if we have not found a path: this could occur when the recursion
	 is stopped by one of the bounds.  */
      if (e_count == 0)
	{
	  vec_free (next_path);
	  return;
	}

      /* Make sure we haven't already visited any of the nodes in
	 NEXT_PATH.  Don't add them here to avoid pollution.  */
      for (unsigned int i = 0; i < next_path->length () - 1; i++)
	{
	  if (visited_bbs->contains ((*next_path)[i]))
	    {
	      vec_free (next_path);
	      return;
	    }
	}

      /* Now add the nodes to VISISTED_BBS.  */
      for (unsigned int i = 0; i < next_path->length () - 1; i++)
	visited_bbs->add ((*next_path)[i]);

      /* Append all the nodes from NEXT_PATH to PATH.  */
      vec_safe_splice (path, next_path);
      next_path_length = next_path->length ();
      vec_free (next_path);
    }

  gcc_assert (path->last () == var_bb);

  /* Iterate over the arguments of PHI.  */
  unsigned int i;
  if (gimple_code (def_stmt) == GIMPLE_PHI)
    {
      gphi *phi = as_a <gphi *> (def_stmt);
      for (i = 0; i < gimple_phi_num_args (phi); i++)
	{
	  tree arg = gimple_phi_arg_def (phi, i);
	  basic_block bbi = gimple_phi_arg_edge (phi, i)->src;

	  /* Skip edges pointing outside the current loop.  */
	  if (!arg || var_bb->loop_father != bbi->loop_father)
	    continue;

	  if (TREE_CODE (arg) == SSA_NAME)
	    {
	      vec_safe_push (path, bbi);
	      /* Recursively follow SSA_NAMEs looking for a constant
		 definition.  */
	      fsm_find_control_statement_thread_paths (arg, visited_bbs, path,
						       seen_loop_phi);

	      path->pop ();
	      continue;
	    }

	  if (TREE_CODE (arg) != INTEGER_CST)
	    continue;

	  /* If this is a profitable jump thread path, then convert it
	     into the canonical form and register it.  */
	  edge taken_edge = profitable_jump_thread_path (path, bbi, name, arg);
	  if (taken_edge)
	    convert_and_register_jump_thread_path (path, taken_edge);
	}
    }
  else if (gimple_code (def_stmt) == GIMPLE_ASSIGN)
    {
      tree arg = gimple_assign_rhs1 (def_stmt);

      if (TREE_CODE (arg) == SSA_NAME)
	fsm_find_control_statement_thread_paths (arg, visited_bbs,
						 path, seen_loop_phi);

      else
	{
	  edge taken_edge = profitable_jump_thread_path (path, var_bb,
						     name, arg);
	  if (taken_edge)
	    convert_and_register_jump_thread_path (path, taken_edge);
	}
    }

  /* Remove all the nodes that we added from NEXT_PATH.  */
  if (next_path_length)
    vec_safe_truncate (path, (path->length () - next_path_length));
}

/* Search backwards from BB looking for paths where NAME (an SSA_NAME)
   is a constant.  Record such paths for jump threading.

   It is assumed that BB ends with a control statement and that by
   finding a path where NAME is a constant, we can thread the path.  */

void  
find_jump_threads_backwards (basic_block bb)
{     
  if (!flag_expensive_optimizations
      || optimize_function_for_size_p (cfun))
    return;

  gimple *stmt = get_gimple_control_stmt (bb);
  if (!stmt)
    return;

  enum gimple_code code = gimple_code (stmt);
  tree name = NULL;
  if (code == GIMPLE_SWITCH)
    name = gimple_switch_index (as_a <gswitch *> (stmt));
  else if (code == GIMPLE_GOTO)
    name = gimple_goto_dest (stmt);
  else if (code == GIMPLE_COND)
    {
      if (TREE_CODE (gimple_cond_lhs (stmt)) == SSA_NAME
	  && TREE_CODE (gimple_cond_rhs (stmt)) == INTEGER_CST
	  && (INTEGRAL_TYPE_P (TREE_TYPE (gimple_cond_lhs (stmt)))
	      || POINTER_TYPE_P (TREE_TYPE (gimple_cond_lhs (stmt)))))
	name = gimple_cond_lhs (stmt);
    }

  if (!name || TREE_CODE (name) != SSA_NAME)
    return;

  vec<basic_block, va_gc> *bb_path;
  vec_alloc (bb_path, 10);
  vec_safe_push (bb_path, bb);
  hash_set<basic_block> *visited_bbs = new hash_set<basic_block>;

  max_threaded_paths = PARAM_VALUE (PARAM_MAX_FSM_THREAD_PATHS);
  fsm_find_control_statement_thread_paths (name, visited_bbs, bb_path, false);

  delete visited_bbs;
  vec_free (bb_path);
}

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_cleanup_cfg | 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
	  && ! optimize_function_for_size_p (cfun));
}


unsigned int
pass_thread_jumps::execute (function *fun)
{
  /* Try to thread each block with more than one successor.  */
  basic_block bb;
  FOR_EACH_BB_FN (bb, fun)
    {
      if (EDGE_COUNT (bb->succs) > 1)
	find_jump_threads_backwards (bb);
    }
  thread_through_all_blocks (true);
  return 0;
}

}

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