cfgloopanal.c

/* Natural loop analysis code for GNU compiler.
   Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007 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 "rtl.h"
#include "hard-reg-set.h"
#include "obstack.h"
#include "basic-block.h"
#include "cfgloop.h"
#include "expr.h"
#include "output.h"
#include "graphds.h"

/* Checks whether BB is executed exactly once in each LOOP iteration.  */

bool
just_once_each_iteration_p (const struct loop *loop, const_basic_block bb)
{
  /* It must be executed at least once each iteration.  */
  if (!dominated_by_p (CDI_DOMINATORS, loop->latch, bb))
    return false;

  /* And just once.  */
  if (bb->loop_father != loop)
    return false;

  /* But this was not enough.  We might have some irreducible loop here.  */
  if (bb->flags & BB_IRREDUCIBLE_LOOP)
    return false;

  return true;
}

/* Marks the edge E in graph G irreducible if it connects two vertices in the
   same scc.  */

static void
check_irred (struct graph *g, struct graph_edge *e)
{
  edge real = (edge) e->data;

  /* All edges should lead from a component with higher number to the
     one with lower one.  */
  gcc_assert (g->vertices[e->src].component >= g->vertices[e->dest].component);

  if (g->vertices[e->src].component != g->vertices[e->dest].component)
    return;

  real->flags |= EDGE_IRREDUCIBLE_LOOP;
  if (flow_bb_inside_loop_p (real->src->loop_father, real->dest))
    real->src->flags |= BB_IRREDUCIBLE_LOOP;
}

/* Marks blocks and edges that are part of non-recognized loops; i.e. we
   throw away all latch edges and mark blocks inside any remaining cycle.
   Everything is a bit complicated due to fact we do not want to do this
   for parts of cycles that only "pass" through some loop -- i.e. for
   each cycle, we want to mark blocks that belong directly to innermost
   loop containing the whole cycle.

   LOOPS is the loop tree.  */

#define LOOP_REPR(LOOP) ((LOOP)->num + last_basic_block)
#define BB_REPR(BB) ((BB)->index + 1)

void
mark_irreducible_loops (void)
{
  basic_block act;
  edge e;
  edge_iterator ei;
  int src, dest;
  unsigned depth;
  struct graph *g;
  int num = number_of_loops ();
  struct loop *cloop;

  gcc_assert (current_loops != NULL);

  /* Reset the flags.  */
  FOR_BB_BETWEEN (act, ENTRY_BLOCK_PTR, EXIT_BLOCK_PTR, next_bb)
    {
      act->flags &= ~BB_IRREDUCIBLE_LOOP;
      FOR_EACH_EDGE (e, ei, act->succs)
	e->flags &= ~EDGE_IRREDUCIBLE_LOOP;
    }

  /* Create the edge lists.  */
  g = new_graph (last_basic_block + num);

  FOR_BB_BETWEEN (act, ENTRY_BLOCK_PTR, EXIT_BLOCK_PTR, next_bb)
    FOR_EACH_EDGE (e, ei, act->succs)
      {
	/* Ignore edges to exit.  */
	if (e->dest == EXIT_BLOCK_PTR)
	  continue;

	src = BB_REPR (act);
	dest = BB_REPR (e->dest);

	/* Ignore latch edges.  */
	if (e->dest->loop_father->header == e->dest
	    && e->dest->loop_father->latch == act)
	  continue;

	/* Edges inside a single loop should be left where they are.  Edges
	   to subloop headers should lead to representative of the subloop,
	   but from the same place.

	   Edges exiting loops should lead from representative
	   of the son of nearest common ancestor of the loops in that
	   act lays.  */

	if (e->dest->loop_father->header == e->dest)
	  dest = LOOP_REPR (e->dest->loop_father);

	if (!flow_bb_inside_loop_p (act->loop_father, e->dest))
	  {
	    depth = 1 + loop_depth (find_common_loop (act->loop_father,
						      e->dest->loop_father));
	    if (depth == loop_depth (act->loop_father))
	      cloop = act->loop_father;
	    else
	      cloop = VEC_index (loop_p, act->loop_father->superloops, depth);

	    src = LOOP_REPR (cloop);
	  }

	add_edge (g, src, dest)->data = e;
      }

  /* Find the strongly connected components.  */
  graphds_scc (g, NULL);

  /* Mark the irreducible loops.  */
  for_each_edge (g, check_irred);

  free_graph (g);

  loops_state_set (LOOPS_HAVE_MARKED_IRREDUCIBLE_REGIONS);
}

/* Counts number of insns inside LOOP.  */
int
num_loop_insns (const struct loop *loop)
{
  basic_block *bbs, bb;
  unsigned i, ninsns = 0;
  rtx insn;

  bbs = get_loop_body (loop);
  for (i = 0; i < loop->num_nodes; i++)
    {
      bb = bbs[i];
      ninsns++;
      for (insn = BB_HEAD (bb); insn != BB_END (bb); insn = NEXT_INSN (insn))
	if (INSN_P (insn))
	  ninsns++;
    }
  free(bbs);

  return ninsns;
}

/* Counts number of insns executed on average per iteration LOOP.  */
int
average_num_loop_insns (const struct loop *loop)
{
  basic_block *bbs, bb;
  unsigned i, binsns, ninsns, ratio;
  rtx insn;

  ninsns = 0;
  bbs = get_loop_body (loop);
  for (i = 0; i < loop->num_nodes; i++)
    {
      bb = bbs[i];

      binsns = 1;
      for (insn = BB_HEAD (bb); insn != BB_END (bb); insn = NEXT_INSN (insn))
	if (INSN_P (insn))
	  binsns++;

      ratio = loop->header->frequency == 0
	      ? BB_FREQ_MAX
	      : (bb->frequency * BB_FREQ_MAX) / loop->header->frequency;
      ninsns += binsns * ratio;
    }
  free(bbs);

  ninsns /= BB_FREQ_MAX;
  if (!ninsns)
    ninsns = 1; /* To avoid division by zero.  */

  return ninsns;
}

/* Returns expected number of iterations of LOOP, according to
   measured or guessed profile.  No bounding is done on the
   value.  */

gcov_type
expected_loop_iterations_unbounded (const struct loop *loop)
{
  edge e;
  edge_iterator ei;

  if (loop->latch->count || loop->header->count)
    {
      gcov_type count_in, count_latch, expected;

      count_in = 0;
      count_latch = 0;

      FOR_EACH_EDGE (e, ei, loop->header->preds)
	if (e->src == loop->latch)
	  count_latch = e->count;
	else
	  count_in += e->count;

      if (count_in == 0)
	expected = count_latch * 2;
      else
	expected = (count_latch + count_in - 1) / count_in;

      return expected;
    }
  else
    {
      int freq_in, freq_latch;

      freq_in = 0;
      freq_latch = 0;

      FOR_EACH_EDGE (e, ei, loop->header->preds)
	if (e->src == loop->latch)
	  freq_latch = EDGE_FREQUENCY (e);
	else
	  freq_in += EDGE_FREQUENCY (e);

      if (freq_in == 0)
	return freq_latch * 2;

      return (freq_latch + freq_in - 1) / freq_in;
    }
}

/* Returns expected number of LOOP iterations.  The returned value is bounded
   by REG_BR_PROB_BASE.  */

unsigned
expected_loop_iterations (const struct loop *loop)
{
  gcov_type expected = expected_loop_iterations_unbounded (loop);
  return (expected > REG_BR_PROB_BASE ? REG_BR_PROB_BASE : expected);
}

/* Returns the maximum level of nesting of subloops of LOOP.  */

unsigned
get_loop_level (const struct loop *loop)
{
  const struct loop *ploop;
  unsigned mx = 0, l;

  for (ploop = loop->inner; ploop; ploop = ploop->next)
    {
      l = get_loop_level (ploop);
      if (l >= mx)
	mx = l + 1;
    }
  return mx;
}

/* Returns estimate on cost of computing SEQ.  */

static unsigned
seq_cost (const_rtx seq)
{
  unsigned cost = 0;
  rtx set;

  for (; seq; seq = NEXT_INSN (seq))
    {
      set = single_set (seq);
      if (set)
	cost += rtx_cost (set, SET);
      else
	cost++;
    }

  return cost;
}

/* The properties of the target.  */

unsigned target_avail_regs;	/* Number of available registers.  */
unsigned target_res_regs;	/* Number of registers reserved for temporary
				   expressions.  */
unsigned target_reg_cost;	/* The cost for register when there still
				   is some reserve, but we are approaching
				   the number of available registers.  */
unsigned target_spill_cost;	/* The cost for register when we need
				   to spill.  */

/* Initialize the constants for computing set costs.  */

void
init_set_costs (void)
{
  rtx seq;
  rtx reg1 = gen_raw_REG (SImode, FIRST_PSEUDO_REGISTER);
  rtx reg2 = gen_raw_REG (SImode, FIRST_PSEUDO_REGISTER + 1);
  rtx addr = gen_raw_REG (Pmode, FIRST_PSEUDO_REGISTER + 2);
  rtx mem = validize_mem (gen_rtx_MEM (SImode, addr));
  unsigned i;

  target_avail_regs = 0;
  for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
    if (TEST_HARD_REG_BIT (reg_class_contents[GENERAL_REGS], i)
	&& !fixed_regs[i])
      target_avail_regs++;

  target_res_regs = 3;

  /* Set up the costs for using extra registers:

     1) If not many free registers remain, we should prefer having an
	additional move to decreasing the number of available registers.
	(TARGET_REG_COST).
     2) If no registers are available, we need to spill, which may require
	storing the old value to memory and loading it back
	(TARGET_SPILL_COST).  */

  start_sequence ();
  emit_move_insn (reg1, reg2);
  seq = get_insns ();
  end_sequence ();
  target_reg_cost = seq_cost (seq);

  start_sequence ();
  emit_move_insn (mem, reg1);
  emit_move_insn (reg2, mem);
  seq = get_insns ();
  end_sequence ();
  target_spill_cost = seq_cost (seq);
}

/* Estimates cost of increased register pressure caused by making N_NEW new
   registers live around the loop.  N_OLD is the number of registers live
   around the loop.  */

unsigned
estimate_reg_pressure_cost (unsigned n_new, unsigned n_old)
{
  unsigned regs_needed = n_new + n_old;

  /* If we have enough registers, we should use them and not restrict
     the transformations unnecessarily.  */
  if (regs_needed + target_res_regs <= target_avail_regs)
    return 0;

  /* If we are close to running out of registers, try to preserve them.  */
  if (regs_needed <= target_avail_regs)
    return target_reg_cost * n_new;
  
  /* If we run out of registers, it is very expensive to add another one.  */
  return target_spill_cost * n_new;
}

/* Sets EDGE_LOOP_EXIT flag for all loop exits.  */

void
mark_loop_exit_edges (void)
{
  basic_block bb;
  edge e;

  if (number_of_loops () <= 1)
    return;

  FOR_EACH_BB (bb)
    {
      edge_iterator ei;

      FOR_EACH_EDGE (e, ei, bb->succs)
	{
	  if (loop_outer (bb->loop_father)
	      && loop_exit_edge_p (bb->loop_father, e))
	    e->flags |= EDGE_LOOP_EXIT;
	  else
	    e->flags &= ~EDGE_LOOP_EXIT;
	}
    }
}