tree-ssa-loop-ivcanon.c   [plain text]

/* Induction variable canonicalization.
   Copyright (C) 2004, 2005 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 2, 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 COPYING.  If not, write to the Free
Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301, USA.  */

/* This pass detects the loops that iterate a constant number of times,
   adds a canonical induction variable (step -1, tested against 0) 
   and replaces the exit test.  This enables the less powerful rtl
   level analysis to use this information.

   This might spoil the code in some cases (by increasing register pressure).
   Note that in the case the new variable is not needed, ivopts will get rid
   of it, so it might only be a problem when there are no other linear induction
   variables.  In that case the created optimization possibilities are likely
   to pay up.

   Additionally in case we detect that it is beneficial to unroll the
   loop completely, we do it right here to expose the optimization
   possibilities to the following passes.  */

#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 "diagnostic.h"
#include "tree-flow.h"
#include "tree-dump.h"
#include "cfgloop.h"
#include "tree-pass.h"
#include "ggc.h"
#include "tree-chrec.h"
#include "tree-scalar-evolution.h"
#include "params.h"
#include "flags.h"
#include "tree-inline.h"

/* Specifies types of loops that may be unrolled.  */

enum unroll_level
{
  UL_SINGLE_ITER,	/* Only loops that exit immediately in the first
			   iteration.  */
  UL_NO_GROWTH,		/* Only loops whose unrolling will not cause increase
			   of code size.  */
  UL_ALL		/* All suitable loops.  */
};

/* Adds a canonical induction variable to LOOP iterating NITER times.  EXIT
   is the exit edge whose condition is replaced.  */

static void
create_canonical_iv (struct loop *loop, edge exit, tree niter)
{
  edge in;
  tree cond, type, var;
  block_stmt_iterator incr_at;
  enum tree_code cmp;

  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "Added canonical iv to loop %d, ", loop->num);
      print_generic_expr (dump_file, niter, TDF_SLIM);
      fprintf (dump_file, " iterations.\n");
    }

  cond = last_stmt (exit->src);
  in = EDGE_SUCC (exit->src, 0);
  if (in == exit)
    in = EDGE_SUCC (exit->src, 1);

  /* Note that we do not need to worry about overflows, since
     type of niter is always unsigned and all comparisons are
     just for equality/nonequality -- i.e. everything works
     with a modulo arithmetics.  */

  type = TREE_TYPE (niter);
  niter = fold_build2 (PLUS_EXPR, type,
		       niter,
		       build_int_cst (type, 1));
  incr_at = bsi_last (in->src);
  create_iv (niter,
	     build_int_cst (type, -1),
	     NULL_TREE, loop,
	     &incr_at, false, NULL, &var);

  cmp = (exit->flags & EDGE_TRUE_VALUE) ? EQ_EXPR : NE_EXPR;
  COND_EXPR_COND (cond) = build2 (cmp, boolean_type_node,
				  var,
				  build_int_cst (type, 0));
  update_stmt (cond);
}

/* Computes an estimated number of insns in LOOP.  */

unsigned
tree_num_loop_insns (struct loop *loop)
{
  basic_block *body = get_loop_body (loop);
  block_stmt_iterator bsi;
  unsigned size = 1, i;

  for (i = 0; i < loop->num_nodes; i++)
    for (bsi = bsi_start (body[i]); !bsi_end_p (bsi); bsi_next (&bsi))
      size += estimate_num_insns (bsi_stmt (bsi));
  free (body);

  return size;
}

/* Estimate number of insns of completely unrolled loop.  We assume
   that the size of the unrolled loop is decreased in the
   following way (the numbers of insns are based on what
   estimate_num_insns returns for appropriate statements):

   1) exit condition gets removed (2 insns)
   2) increment of the control variable gets removed (2 insns)
   3) All remaining statements are likely to get simplified
      due to constant propagation.  Hard to estimate; just
      as a heuristics we decrease the rest by 1/3.

   NINSNS is the number of insns in the loop before unrolling.
   NUNROLL is the number of times the loop is unrolled.  */

static unsigned HOST_WIDE_INT
estimated_unrolled_size (unsigned HOST_WIDE_INT ninsns,
			 unsigned HOST_WIDE_INT nunroll)
{
  HOST_WIDE_INT unr_insns = 2 * ((HOST_WIDE_INT) ninsns - 4) / 3;
  if (unr_insns <= 0)
    unr_insns = 1;
  unr_insns *= (nunroll + 1);

  return unr_insns;
}

/* Tries to unroll LOOP completely, i.e. NITER times.  LOOPS is the
   loop tree.  UL determines which loops we are allowed to unroll. 
   EXIT is the exit of the loop that should be eliminated.  */

static bool
try_unroll_loop_completely (struct loops *loops ATTRIBUTE_UNUSED,
			    struct loop *loop,
			    edge exit, tree niter,
			    enum unroll_level ul)
{
  unsigned HOST_WIDE_INT n_unroll, ninsns, max_unroll, unr_insns;
  tree old_cond, cond, dont_exit, do_exit;

  if (loop->inner)
    return false;

  if (!host_integerp (niter, 1))
    return false;
  n_unroll = tree_low_cst (niter, 1);

  max_unroll = PARAM_VALUE (PARAM_MAX_COMPLETELY_PEEL_TIMES);
  if (n_unroll > max_unroll)
    return false;

  if (n_unroll)
    {
      if (ul == UL_SINGLE_ITER)
	return false;

      ninsns = tree_num_loop_insns (loop);

      if (n_unroll * ninsns
	  > (unsigned) PARAM_VALUE (PARAM_MAX_COMPLETELY_PEELED_INSNS))
	return false;

      if (ul == UL_NO_GROWTH)
	{
	  unr_insns = estimated_unrolled_size (ninsns, n_unroll);
	  
	  if (dump_file && (dump_flags & TDF_DETAILS))
	    {
	      fprintf (dump_file, "  Loop size: %d\n", (int) ninsns);
	      fprintf (dump_file, "  Estimated size after unrolling: %d\n",
		       (int) unr_insns);
	    }
	  
	  if (unr_insns > ninsns)
	    {
	      if (dump_file && (dump_flags & TDF_DETAILS))
		fprintf (dump_file, "Not unrolling loop %d:\n", loop->num);
	      return false;
	    }
	}
    }

  if (exit->flags & EDGE_TRUE_VALUE)
    {
      dont_exit = boolean_false_node;
      do_exit = boolean_true_node;
    }
  else
    {
      dont_exit = boolean_true_node;
      do_exit = boolean_false_node;
    }
  cond = last_stmt (exit->src);
    
  if (n_unroll)
    {
      sbitmap wont_exit;
      edge *edges_to_remove = XNEWVEC (edge, n_unroll);
      unsigned int n_to_remove = 0;

      old_cond = COND_EXPR_COND (cond);
      COND_EXPR_COND (cond) = dont_exit;
      update_stmt (cond);
      initialize_original_copy_tables ();

      wont_exit = sbitmap_alloc (n_unroll + 1);
      sbitmap_ones (wont_exit);
      RESET_BIT (wont_exit, 0);

      if (!tree_duplicate_loop_to_header_edge (loop, loop_preheader_edge (loop),
					       loops, n_unroll, wont_exit,
					       exit, edges_to_remove,
					       &n_to_remove,
					       DLTHE_FLAG_UPDATE_FREQ
					       | DLTHE_FLAG_COMPLETTE_PEEL))
	{
	  COND_EXPR_COND (cond) = old_cond;
	  update_stmt (cond);
          free_original_copy_tables ();
	  free (wont_exit);
	  free (edges_to_remove);
	  return false;
	}
      free (wont_exit);
      free (edges_to_remove);
      free_original_copy_tables ();
    }
  
  COND_EXPR_COND (cond) = do_exit;
  update_stmt (cond);

  update_ssa (TODO_update_ssa);

  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "Unrolled loop %d completely.\n", loop->num);

  return true;
}

/* Adds a canonical induction variable to LOOP if suitable.  LOOPS is the loops
   tree.  CREATE_IV is true if we may create a new iv.  UL determines 
   which loops we are allowed to completely unroll.  If TRY_EVAL is true, we try
   to determine the number of iterations of a loop by direct evaluation. 
   Returns true if cfg is changed.  */

static bool
canonicalize_loop_induction_variables (struct loops *loops, struct loop *loop,
				       bool create_iv, enum unroll_level ul,
				       bool try_eval)
{
  edge exit = NULL;
  tree niter;

  niter = number_of_iterations_in_loop (loop);
  if (TREE_CODE (niter) == INTEGER_CST)
    {
      exit = loop->single_exit;
      if (!just_once_each_iteration_p (loop, exit->src))
	return false;

      /* The result of number_of_iterations_in_loop is by one higher than
	 we expect (i.e. it returns number of executions of the exit
	 condition, not of the loop latch edge).  */
      niter = fold_build2 (MINUS_EXPR, TREE_TYPE (niter), niter,
			   build_int_cst (TREE_TYPE (niter), 1));
    }
  else
    {
      /* If the loop has more than one exit, try checking all of them
	 for # of iterations determinable through scev.  */
      if (!loop->single_exit)
	niter = find_loop_niter (loop, &exit);

      /* Finally if everything else fails, try brute force evaluation.  */
      if (try_eval
	  && (chrec_contains_undetermined (niter)
	      || TREE_CODE (niter) != INTEGER_CST))
	niter = find_loop_niter_by_eval (loop, &exit);

      if (chrec_contains_undetermined (niter)
	  || TREE_CODE (niter) != INTEGER_CST)
	return false;
    }

  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "Loop %d iterates ", loop->num);
      print_generic_expr (dump_file, niter, TDF_SLIM);
      fprintf (dump_file, " times.\n");
    }

  if (try_unroll_loop_completely (loops, loop, exit, niter, ul))
    return true;

  if (create_iv)
    create_canonical_iv (loop, exit, niter);

  return false;
}

/* The main entry point of the pass.  Adds canonical induction variables
   to the suitable LOOPS.  */

unsigned int
canonicalize_induction_variables (struct loops *loops)
{
  unsigned i;
  struct loop *loop;
  bool changed = false;
  
  for (i = 1; i < loops->num; i++)
    {
      loop = loops->parray[i];

      if (loop)
	changed |= canonicalize_loop_induction_variables (loops, loop,
							  true, UL_SINGLE_ITER,
							  true);
    }

  /* Clean up the information about numbers of iterations, since brute force
     evaluation could reveal new information.  */
  scev_reset ();

  if (changed)
    return TODO_cleanup_cfg;
  return 0;
}

/* Unroll LOOPS completely if they iterate just few times.  Unless
   MAY_INCREASE_SIZE is true, perform the unrolling only if the
   size of the code does not increase.  */

unsigned int
tree_unroll_loops_completely (struct loops *loops, bool may_increase_size)
{
  unsigned i;
  struct loop *loop;
  bool changed = false;
  enum unroll_level ul;

  for (i = 1; i < loops->num; i++)
    {
      loop = loops->parray[i];

      if (!loop)
	continue;

      if (may_increase_size && maybe_hot_bb_p (loop->header))
	ul = UL_ALL;
      else
	ul = UL_NO_GROWTH;
      changed |= canonicalize_loop_induction_variables (loops, loop,
							false, ul,
							!flag_tree_loop_ivcanon);
    }

  /* Clean up the information about numbers of iterations, since complete
     unrolling might have invalidated it.  */
  scev_reset ();

  if (changed)
    return TODO_cleanup_cfg;
  return 0;
}

/* Checks whether LOOP is empty.  */

static bool
empty_loop_p (struct loop *loop)
{
  edge exit;
  struct tree_niter_desc niter;
  tree phi, def;
  basic_block *body;
  block_stmt_iterator bsi;
  unsigned i;
  tree stmt;

  /* If the loop has multiple exits, it is too hard for us to handle.
     Similarly, if the exit is not dominating, we cannot determine
     whether the loop is not infinite.  */
  exit = single_dom_exit (loop);
  if (!exit)
    return false;

  /* The loop must be finite.  */
  if (!number_of_iterations_exit (loop, exit, &niter, false))
    return false;

  /* Values of all loop exit phi nodes must be invariants.  */
  for (phi = phi_nodes (exit->dest); phi; phi = PHI_CHAIN (phi))
    {
      if (!is_gimple_reg (PHI_RESULT (phi)))
	continue;

      def = PHI_ARG_DEF_FROM_EDGE (phi, exit);

      if (!expr_invariant_in_loop_p (loop, def))
	return false;
    }

  /* And there should be no memory modifying or from other reasons
     unremovable statements.  */
  body = get_loop_body (loop);
  for (i = 0; i < loop->num_nodes; i++)
    {
      /* Irreducible region might be infinite.  */
      if (body[i]->flags & BB_IRREDUCIBLE_LOOP)
	{
	  free (body);
	  return false;
	}
	
      for (bsi = bsi_start (body[i]); !bsi_end_p (bsi); bsi_next (&bsi))
	{
	  stmt = bsi_stmt (bsi);
	  if (!ZERO_SSA_OPERANDS (stmt, SSA_OP_VIRTUAL_DEFS)
	      || stmt_ann (stmt)->has_volatile_ops)
	    {
	      free (body);
	      return false;
	    }

	  /* Also, asm statements and calls may have side effects and we
	     cannot change the number of times they are executed.  */
	  switch (TREE_CODE (stmt))
	    {
	    case RETURN_EXPR:
	    case MODIFY_EXPR:
	      stmt = get_call_expr_in (stmt);
	      if (!stmt)
		break;

	    case CALL_EXPR:
	      if (TREE_SIDE_EFFECTS (stmt))
		{
		  free (body);
		  return false;
		}
	      break;

	    case ASM_EXPR:
	      /* We cannot remove volatile assembler.  */
	      if (ASM_VOLATILE_P (stmt))
		{
		  free (body);
		  return false;
		}
	      break;

	    default:
	      break;
	    }
	}
      }
  free (body);

  return true;
}

/* Remove LOOP by making it exit in the first iteration.  */

static void
remove_empty_loop (struct loop *loop)
{
  edge exit = single_dom_exit (loop), non_exit;
  tree cond_stmt = last_stmt (exit->src);
  tree do_exit;
  basic_block *body;
  unsigned n_before, freq_in, freq_h;
  gcov_type exit_count = exit->count;

  non_exit = EDGE_SUCC (exit->src, 0);
  if (non_exit == exit)
    non_exit = EDGE_SUCC (exit->src, 1);

  if (exit->flags & EDGE_TRUE_VALUE)
    do_exit = boolean_true_node;
  else
    do_exit = boolean_false_node;

  COND_EXPR_COND (cond_stmt) = do_exit;
  update_stmt (cond_stmt);

  /* Let us set the probabilities of the edges coming from the exit block.  */
  exit->probability = REG_BR_PROB_BASE;
  non_exit->probability = 0;
  non_exit->count = 0;

  /* Update frequencies and counts.  Everything before
     the exit needs to be scaled FREQ_IN/FREQ_H times,
     where FREQ_IN is the frequency of the entry edge
     and FREQ_H is the frequency of the loop header.
     Everything after the exit has zero frequency.  */
  freq_h = loop->header->frequency;
  freq_in = EDGE_FREQUENCY (loop_preheader_edge (loop));
  if (freq_h != 0)
    {
      body = get_loop_body_in_dom_order (loop);
      for (n_before = 1; n_before <= loop->num_nodes; n_before++)
	if (body[n_before - 1] == exit->src)
	  break;
      scale_bbs_frequencies_int (body, n_before, freq_in, freq_h);
      scale_bbs_frequencies_int (body + n_before, loop->num_nodes - n_before,
				 0, 1);
      free (body);
    }

  /* Number of executions of exit is not changed, thus we need to restore
     the original value.  */
  exit->count = exit_count;
}

/* Removes LOOP if it is empty.  Returns true if LOOP is removed.  CHANGED
   is set to true if LOOP or any of its subloops is removed.  */

static bool
try_remove_empty_loop (struct loop *loop, bool *changed)
{
  bool nonempty_subloop = false;
  struct loop *sub;

  /* First, all subloops must be removed.  */
  for (sub = loop->inner; sub; sub = sub->next)
    nonempty_subloop |= !try_remove_empty_loop (sub, changed);

  if (nonempty_subloop || !empty_loop_p (loop))
    return false;

  remove_empty_loop (loop);
  *changed = true;
  return true;
}

/* Remove the empty LOOPS.  */

unsigned int
remove_empty_loops (struct loops *loops)
{
  bool changed = false;
  struct loop *loop;

  for (loop = loops->tree_root->inner; loop; loop = loop->next)
    try_remove_empty_loop (loop, &changed);

  if (changed)
    {
      scev_reset ();
      return TODO_cleanup_cfg;
    }
  return 0;
}