llvm-gcc-4.0/gcc/tree-ssa-loop-prefetch.c

/* APPLE LOCAL file lno */
/* Array prefetching.
   Copyright (C) 2004 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, 59 Temple Place - Suite 330, Boston, MA
02111-1307, USA.  */

#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 "timevar.h"
#include "cfgloop.h"
#include "varray.h"
#include "expr.h"
#include "tree-pass.h"
#include "ggc.h"
#include "insn-config.h"
#include "recog.h"
#include "hashtab.h"
#include "tree-chrec.h"
#include "tree-scalar-evolution.h"

/* This pass inserts prefetch instructions to optimize cache usage during
   accesses to arrays in loops.  It processes loops sequentially and:

   1) Gathers all memory references in the single loop.
   2) For each of the references it decides when it is profitable to prefetch
      it.  To do it, we evaluate the reuse among the accesses, and determines
      two values: PREFETCH_BEFORE (meaning that it only makes sense to do
      prefetching in the first PREFETCH_BEFORE iterations of the loop) and
      PREFETCH_MOD (meaning that it only makes sense to prefetch in the
      iterations of the loop that are zero modulo PREFETCH_MOD).  For example
      (assuming cache line size is 64 bytes, char has size 1 byte and there
      is no hardware sequential prefetch):

      char *a;
      for (i = 0; i < max; i++)
	{
	  a[255] = ...;		(0)
	  a[i] = ...;		(1)
	  a[i + 64] = ...;	(2)
	  a[16*i] = ...;	(3)
	  a[187*i] = ...;	(4)
	  a[187*i + 50] = ...;	(5)
	}

       (0) obviously has PREFETCH_BEFORE 1
       (1) has PREFETCH_BEFORE 64, since (2) accesses the same memory
           location 64 iterations before it, and PREFETCH_MOD 64 (since
	   it hits the same cache line otherwise).
       (2) has PREFETCH_MOD 64
       (3) has PREFETCH_MOD 4
       (4) has PREFETCH_MOD 1.  We do not set PREFETCH_BEFORE here, since
           the cache line accessed by (4) is the same with probability only
	   7/32.
       (5) has PREFETCH_MOD 1 as well.

   3) We determine how much ahead we need to prefetch.  The number of
      iterations needed is time to fetch / time spent in one iteration of
      the loop.  The problem is that we do not know either of these values,
      so we just make a heuristic guess based on a magic (possibly)
      target-specific constant and size of the loop.

   4) Determine which of the references we prefetch.  We take into account
      that there is a maximum number of simultaneous prefetches (provided
      by machine description).  We prefetch as many prefetches as possible
      while still within this bound (starting with those with lowest
      prefetch_mod, since they are responsible for most of the cache
      misses).
      
   5) We unroll and peel loops so that we are able to satisfy PREFETCH_MOD
      and PREFETCH_BEFORE requirements (within some bounds), and to avoid
      prefetching nonaccessed memory.
      TODO -- actually implement this.
      
   6) We actually emit the prefetch instructions.  ??? Perhaps emit the
      prefetch instructions with guards in cases where 5) was not sufficient
      to satisfy the constraints?

   Some other TODO:
      -- write and use more general reuse analysis (that could be also used
	 in other cache aimed loop optimizations)
      -- make it behave sanely together with the prefetches given by user
	 (now we just ignore them; at the very least we should avoid
	 optimizing loops in that user put his own prefetches)
      -- we assume cache line size allignment of arrays; this could be
	 improved.  */

/* Magic constants follow.  These should be replaced by machine specific
   numbers.  */

/* A number that should rouhgly correspond to the number of instructions
   executed before the prefetch is completed.  */

#ifndef PREFETCH_LATENCY
#define PREFETCH_LATENCY 50
#endif

/* Number of prefetches that can run at the same time.  */

#ifndef SIMULTANEOUS_PREFETCHES
#define SIMULTANEOUS_PREFETCHES 3
#endif

/* True if write can be prefetched by a read prefetch.  */

#ifndef WRITE_CAN_USE_READ_PREFETCH
#define WRITE_CAN_USE_READ_PREFETCH 1
#endif

/* True if read can be prefetched by a write prefetch. */

#ifndef READ_CAN_USE_WRITE_PREFETCH
#define READ_CAN_USE_WRITE_PREFETCH 0
#endif

/* Cache line size.  Assumed to be a power of two.  */

#ifndef PREFETCH_BLOCK
#define PREFETCH_BLOCK 32
#endif

/* Do we have a forward hardware sequential prefetching?  */

#ifndef HAVE_FORWARD_PREFETCH
#define HAVE_FORWARD_PREFETCH 0
#endif

/* Do we have a backward hardware sequential prefetching?  */

#ifndef HAVE_BACKWARD_PREFETCH
#define HAVE_BACKWARD_PREFETCH 0
#endif

/* In some cases we are only able to determine that there is a certain
   probability that the two accesses hit the same cache line.  In this
   case, we issue the prefetches for both of them if this probability
   is less then (1000 - ACCEPTABLE_MISS_RATE) promile.  */

#ifndef ACCEPTABLE_MISS_RATE
#define ACCEPTABLE_MISS_RATE 50
#endif

#ifndef HAVE_prefetch
#define HAVE_prefetch 0
#endif

/* The group of references between that reuse may occur.  */

struct mem_ref_group
{
  tree base;			/* Base of the reference.  */
  HOST_WIDE_INT step;		/* Step of the reference.  */
  tree group_iv;		/* Induction variable for the group.  */
  bool issue_prefetch_p;	/* Is there any prefetch issued in the
				   group?  */
  struct mem_ref *refs;		/* References in the group.  */
  struct mem_ref_group *next;	/* Next group of references.  */
};

/* Assigned to PREFETCH_BEFORE when all iterations are to be prefetched.  */

#define PREFETCH_ALL		(~(unsigned HOST_WIDE_INT) 0)

/* The memory reference.  */

struct mem_ref
{
  HOST_WIDE_INT delta;		/* Constant offset of the reference.  */
  bool write_p;			/* Is it a write?  */
  struct mem_ref_group *group;	/* The group of references it belongs to.  */
  unsigned HOST_WIDE_INT prefetch_mod;
				/* Prefetch only each PREFETCH_MOD-th
				   iteration.  */
  unsigned HOST_WIDE_INT prefetch_before;
				/* Prefetch only first PREFETCH_BEFORE
				   iterations.  */
  bool issue_prefetch_p;	/* Should we really issue the prefetch?  */
  struct mem_ref *next;		/* The next reference in the group.  */
};

/* Dumps information obout reference REF to FILE.  */

static void
dump_mem_ref (FILE *file, struct mem_ref *ref)
{
  fprintf (file, "Reference %p:\n", (void *) ref);

  fprintf (file, "  group %p (base ", (void *) ref->group);
  print_generic_expr (file, ref->group->base, TDF_SLIM);
  fprintf (file, ", step ");
  fprintf (file, HOST_WIDE_INT_PRINT_DEC, ref->group->step);
  fprintf (file, ")\n");

  fprintf (dump_file, "  delta ");
  fprintf (file, HOST_WIDE_INT_PRINT_DEC, ref->delta);
  fprintf (file, "\n");

  fprintf (file, "  %s\n", ref->write_p ? "write" : "read");

  fprintf (file, "\n");
}

/* Finds a group with BASE and STEP in GROUPS, or creates one if it does not
   exist.  */

static struct mem_ref_group *
find_or_create_group (struct mem_ref_group **groups, tree base,
		      HOST_WIDE_INT step)
{
  for (; *groups; groups = &(*groups)->next)
    {
      if ((*groups)->step == step
	  && operand_equal_p ((*groups)->base, base, 0))
	return *groups;
    }

  (*groups) = xcalloc (1, sizeof (struct mem_ref_group));
  (*groups)->base = base;
  (*groups)->step = step;
  (*groups)->group_iv = NULL_TREE;
  (*groups)->refs = NULL;
  (*groups)->next = NULL;

  return *groups;
}

/* Records a memory reference in GROUP with offset DELTA and write status
   WRITE_P.  */

static void
record_ref (struct mem_ref_group *group, HOST_WIDE_INT delta,
	    bool write_p)
{
  struct mem_ref **aref;

  for (aref = &group->refs; *aref; aref = &(*aref)->next)
    {
      /* It does not have to be possible for write reference to reuse the read
	 prefetch, or vice versa.  */
      if (!WRITE_CAN_USE_READ_PREFETCH
	  && write_p
	  && !(*aref)->write_p)
	continue;
      if (!READ_CAN_USE_WRITE_PREFETCH
	  && !write_p
	  && (*aref)->write_p)
	continue;

      if ((*aref)->delta == delta)
	return;
    }

  (*aref) = xcalloc (1, sizeof (struct mem_ref));
  (*aref)->delta = delta;
  (*aref)->write_p = write_p;
  (*aref)->prefetch_before = PREFETCH_ALL;
  (*aref)->prefetch_mod = 1;
  (*aref)->issue_prefetch_p = false;
  (*aref)->group = group;
  (*aref)->next = NULL;

  if (dump_file && (dump_flags & TDF_DETAILS))
    dump_mem_ref (dump_file, *aref);
}

/* Release memory references in GROUPS.  */

static void
release_mem_refs (struct mem_ref_group *groups)
{
  struct mem_ref_group *next_g;
  struct mem_ref *ref, *next_r;

  for (; groups; groups = next_g)
    {
      next_g = groups->next;
      for (ref = groups->refs; ref; ref = next_r)
	{
	  next_r = ref->next;
	  free (ref);
	}
      free (groups);
    }
}

/* A structure used to pass arguments to idx_analyze_ref.  */

struct ar_data
{
  struct loop *loop;			/* Loop of the reference.  */
  tree stmt;				/* Statement of the reference.  */
  HOST_WIDE_INT *step;			/* Step of the memory reference.  */
  HOST_WIDE_INT *delta;			/* Offset of the memory reference.  */
};

/* Analyzes a single INDEX of a memory reference to obtain information
   described at analyze_ref.  Callback for for_each_index.  */

static bool
idx_analyze_ref (tree base, tree *index, void *data)
{
  struct ar_data *ar_data = data;
  tree ibase, step, stepsize;
  HOST_WIDE_INT istep, idelta = 0, imult = 1;

  if (!simple_iv (ar_data->loop, ar_data->stmt, *index, &ibase, &step))
    return false;

  if (zero_p (step))
    istep = 0;
  else
    {
      if (!cst_and_fits_in_hwi (step))
	return false;
      istep = int_cst_value (step);
    }

  if (TREE_CODE (ibase) == PLUS_EXPR
      && cst_and_fits_in_hwi (TREE_OPERAND (ibase, 1)))
    {
      idelta = int_cst_value (TREE_OPERAND (ibase, 1));
      ibase = TREE_OPERAND (ibase, 0);
    }
  if (cst_and_fits_in_hwi (ibase))
    {
      idelta += int_cst_value (ibase);
      ibase = fold_convert (TREE_TYPE (ibase), integer_zero_node);
    }

  if (base)
    {
      stepsize = TYPE_SIZE_UNIT (TREE_TYPE (TREE_TYPE (base)));
      if (!cst_and_fits_in_hwi (stepsize))
	return false;
      imult = int_cst_value (stepsize);

      istep *= imult;
      idelta *= imult;
    }

  *ar_data->step += istep;
  *ar_data->delta += idelta;
  *index = ibase;

  return true;
}

/* Tries to express REF in shape &BASE + STEP * iter + DELTA, where DELTA and
   STEP are integer constants and iter is number of iterations of LOOP.  The
   reference occurs in statement STMT.  */

static bool
analyze_ref (struct loop *loop, tree ref, tree *base,
	     HOST_WIDE_INT *step, HOST_WIDE_INT *delta,
	     tree stmt)
{
  struct ar_data ar_data;
  tree off;
  HOST_WIDE_INT bit_offset;

  *step = 0;
  *delta = 0;

  /* First strip off the component references.  Ignore bitfields.  */
  if (TREE_CODE (ref) == COMPONENT_REF
      && DECL_NONADDRESSABLE_P (TREE_OPERAND (ref, 1)))
    ref = TREE_OPERAND (ref, 0);

  for (; TREE_CODE (ref) == COMPONENT_REF; ref = TREE_OPERAND (ref, 0))
    {
      off = DECL_FIELD_BIT_OFFSET (TREE_OPERAND (ref, 1));
      bit_offset = TREE_INT_CST_LOW (off);

      if (bit_offset % BITS_PER_UNIT)
	abort ();
      
      *delta += bit_offset / BITS_PER_UNIT;
    }

  *base = unshare_expr (ref);
  ar_data.loop = loop;
  ar_data.stmt = stmt;
  ar_data.step = step;
  ar_data.delta = delta;
  return for_each_index (base, idx_analyze_ref, &ar_data);
}

/* Record a memory reference REF to the list REFS.  The reference occurs in
   LOOP in statement STMT and it is write if WRITE_P.  */

static void
gather_memory_references_ref (struct loop *loop, struct mem_ref_group **refs,
			      tree ref, bool write_p, tree stmt)
{
  tree base;
  HOST_WIDE_INT step, delta;
  struct mem_ref_group *agrp;

  if (!analyze_ref (loop, ref, &base, &step, &delta, stmt))
    return;

  /* Now we know that REF = &BASE + STEP * iter + DELTA, where DELTA and STEP
     are integer constants.  */
  agrp = find_or_create_group (refs, base, step);
  record_ref (agrp, delta, write_p);
}

/* Record the suitable memory references in LOOP.  */

static struct mem_ref_group *
gather_memory_references (struct loop *loop)
{
  basic_block *body = get_loop_body_in_dom_order (loop);
  basic_block bb;
  unsigned i;
  block_stmt_iterator bsi;
  tree stmt, lhs, rhs;
  struct mem_ref_group *refs = NULL;

  /* Scan the loop body in order, so that the former references precede the
     later ones.  */
  for (i = 0; i < loop->num_nodes; i++)
    {
      bb = body[i];
      if (bb->loop_father != loop)
	continue;

      for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
	{
	  stmt = bsi_stmt (bsi);
	  if (TREE_CODE (stmt) != MODIFY_EXPR)
	    continue;

	  lhs = TREE_OPERAND (stmt, 0);
	  rhs = TREE_OPERAND (stmt, 1);

	  if (TREE_CODE_CLASS (TREE_CODE (rhs)) == 'r')
	    gather_memory_references_ref (loop, &refs, rhs, false, stmt);
	  if (TREE_CODE_CLASS (TREE_CODE (lhs)) == 'r')
	    gather_memory_references_ref (loop, &refs, lhs, true, stmt);
	}
    }
  free (body);

  return refs;
}

/* Prune the prefetch candidate REF using the self-reuse.  */

static void
prune_ref_by_self_reuse (struct mem_ref *ref)
{
  HOST_WIDE_INT step = ref->group->step;
  bool backward = step < 0;

  if (step == 0)
    {
      /* Prefetch references to invariant address just once.  */
      ref->prefetch_before = 1;
      return;
    }

  if (backward)
    step = -step;

  if (step > PREFETCH_BLOCK)
    return;

  if ((backward && HAVE_BACKWARD_PREFETCH)
      || (!backward && HAVE_FORWARD_PREFETCH))
    {
      ref->prefetch_before = 1;
      return;
    }

  ref->prefetch_mod = PREFETCH_BLOCK / step;
}

/* Divides X by BY, rounding down.  */

static HOST_WIDE_INT
ddown (HOST_WIDE_INT x, unsigned HOST_WIDE_INT by)
{
  if (by <= 0)
    abort ();

  if (x >= 0)
    return x / by;
  else
    return (x + by - 1) / by;
}

/* Prune the prefetch candidate REF using the reuse with BY.
   If BY_IS_BEFORE is true, BY is before REF in the loop.  */

static void
prune_ref_by_group_reuse (struct mem_ref *ref, struct mem_ref *by,
			  bool by_is_before)
{
  HOST_WIDE_INT step = ref->group->step;
  bool backward = step < 0;
  HOST_WIDE_INT delta_r = ref->delta, delta_b = by->delta;
  HOST_WIDE_INT delta = delta_b - delta_r;
  HOST_WIDE_INT hit_from;
  unsigned HOST_WIDE_INT prefetch_before, prefetch_block;

  if (delta == 0)
    {
      /* If the references has the same address, only prefetch the
	 former.  */
      if (by_is_before)
	ref->prefetch_before = 0;
      
      return;
    }

  if (!step)
    {
      /* If the reference addresses are invariant and fall into the
	 same cache line, prefetch just the first one.  */
      if (!by_is_before)
	return;

      if (ddown (ref->delta, PREFETCH_BLOCK)
	  != ddown (by->delta, PREFETCH_BLOCK))
	return;

      ref->prefetch_before = 0;
      return;
    }

  /* Only prune the reference that is behind in the array.  */
  if (backward)
    {
      if (delta > 0)
	return;

      /* Transform the data so that we may assume that the accesses
	 are forward.  */
      delta = - delta;
      step = -step;
      delta_r = PREFETCH_BLOCK - 1 - delta_r;
      delta_b = PREFETCH_BLOCK - 1 - delta_b;
    }
  else
    {
      if (delta < 0)
	return;
    }

  /* Check whether the two references are likely to hit the same cache
     line, and how distant the iterations in that it occurs are from
     each other.  */

  if (step <= PREFETCH_BLOCK)
    {
      /* The accesses are sure to meet.  Let us check when.  */
      hit_from = ddown (delta_b, PREFETCH_BLOCK) * PREFETCH_BLOCK;
      prefetch_before = (hit_from - delta_r + step - 1) / step;

      if (prefetch_before < ref->prefetch_before)
	ref->prefetch_before = prefetch_before;

      return;
    }

  /* A more complicated case.  First let us ensure that size of cache line
     and step are coprime (here we assume that PREFETCH_BLOCK is a power
     of two.  */
  prefetch_block = PREFETCH_BLOCK;
  while ((step & 1) == 0
	 && prefetch_block > 1)
    {
      step >>= 1;
      prefetch_block >>= 1;
      delta >>= 1;
    }

  /* Now step > prefetch_block, and step and prefetch_block are coprime.
     Determine the probability that the accesses hit the same cache line.  */

  prefetch_before = delta / step;
  delta %= step;
  if ((unsigned HOST_WIDE_INT) delta
      <= (prefetch_block * ACCEPTABLE_MISS_RATE / 1000))
    {
      if (prefetch_before < ref->prefetch_before)
	ref->prefetch_before = prefetch_before;

      return;
    }

  /* Try also the following iteration.  */
  prefetch_before++;
  delta = step - delta;
  if ((unsigned HOST_WIDE_INT) delta
      <= (prefetch_block * ACCEPTABLE_MISS_RATE / 1000))
    {
      if (prefetch_before < ref->prefetch_before)
	ref->prefetch_before = prefetch_before;

      return;
    }

  /* The ref probably does not reuse by.  */
  return;
}

/* Prune the prefetch candidate REF using the reuses with other references
   in REFS.  */

static void
prune_ref_by_reuse (struct mem_ref *ref, struct mem_ref *refs)
{
  struct mem_ref *prune_by;
  bool before = true;

  prune_ref_by_self_reuse (ref);

  for (prune_by = refs; prune_by; prune_by = prune_by->next)
    {
      if (prune_by == ref)
	{
	  before = false;
	  continue;
	}

      if (!WRITE_CAN_USE_READ_PREFETCH
	  && ref->write_p
	  && !prune_by->write_p)
	continue;
      if (!READ_CAN_USE_WRITE_PREFETCH
	  && !ref->write_p
	  && prune_by->write_p)
	continue;

      prune_ref_by_group_reuse (ref, prune_by, before);
    }
}

/* Prune the prefetch candidates in GROUP using the reuse analysis.  */

static void
prune_group_by_reuse (struct mem_ref_group *group)
{
  struct mem_ref *ref_pruned;

  for (ref_pruned = group->refs; ref_pruned; ref_pruned = ref_pruned->next)
    {
      prune_ref_by_reuse (ref_pruned, group->refs);

      if (dump_file && (dump_flags & TDF_DETAILS))
	{
	  fprintf (dump_file, "Reference %p:", (void *) ref_pruned);

	  if (ref_pruned->prefetch_before == PREFETCH_ALL
	      && ref_pruned->prefetch_mod == 1)
	    fprintf (dump_file, " no restrictions");
	  else if (ref_pruned->prefetch_before == 0)
	    fprintf (dump_file, " do not prefetch");
	  else if (ref_pruned->prefetch_before <= ref_pruned->prefetch_mod)
	    fprintf (dump_file, " prefetch once");
	  else
	    {
	      if (ref_pruned->prefetch_before != PREFETCH_ALL)
		{
		  fprintf (dump_file, " prefetch before ");
		  fprintf (dump_file, HOST_WIDE_INT_PRINT_DEC,
			   ref_pruned->prefetch_before);
		}
	      if (ref_pruned->prefetch_mod != 1)
		{
		  fprintf (dump_file, " prefetch mod ");
		  fprintf (dump_file, HOST_WIDE_INT_PRINT_DEC,
			   ref_pruned->prefetch_mod);
		}
	    }
	  fprintf (dump_file, "\n");
	}
    }
}

/* Prune the list of prefetch candidates GROUPS using the reuse analysis.  */

static void
prune_by_reuse (struct mem_ref_group *groups)
{
  for (; groups; groups = groups->next)
    prune_group_by_reuse (groups);
}

/* Decide which of the prefetch candidates in GROUPS to prefetch.
   AHEAD is the number of iterations to prefetch ahead (which corresponds
   to the number of simultaneous instances of one prefetch running at a
   time).  */

static void
schedule_prefetches (struct mem_ref_group *groups, unsigned ahead)
{
  unsigned max_prefetches = SIMULTANEOUS_PREFETCHES / ahead;
  struct mem_ref *ref;

  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "Max prefetches to issue: %d.\n", max_prefetches);

  /* For now we just take memory references one by one and issue
     prefetches for as many as possible.  TODO -- select the most
     profitable prefetches.  */

  if (!max_prefetches)
    return;

  for (; groups; groups = groups->next)
    for (ref = groups->refs; ref; ref = ref->next)
      {
	/* For now do not issue prefetches for only first few of the
	   iterations.  */
	if (ref->prefetch_before != PREFETCH_ALL)
	  continue;

	ref->issue_prefetch_p = true;
	groups->issue_prefetch_p = true;
	max_prefetches--;

	if (!max_prefetches)
	  return;
      }
}

/* Issue prefetches for the referenc REF into LOOP as decided before.
   HEAD is the number of iterations to prefetch ahead.  */

static void
issue_prefetch_ref (struct loop *loop, struct mem_ref *ref, unsigned ahead)
{
  HOST_WIDE_INT delta;
  tree addr, stmts, prefetch, params, write_p;
  block_stmt_iterator bsi;

  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "Issued prefetch for %p.\n", (void *) ref);

  /* Determine the address to prefetch.  */
  delta = ahead * ref->group->step + ref->delta;
  addr = ref->group->group_iv;
  if (delta)
    addr = build (PLUS_EXPR, ptr_type_node,
		  addr, build_int_cst (ptr_type_node, delta));

  addr = force_gimple_operand (addr, &stmts, false,
			       SSA_NAME_VAR (ref->group->group_iv));

  /* Create the prefetch instruction.  */
  write_p = ref->write_p ? integer_one_node : integer_zero_node;
  params = tree_cons (NULL_TREE, addr,
		      tree_cons (NULL_TREE, write_p, NULL_TREE));
				 
  prefetch = build_function_call_expr (built_in_decls[BUILT_IN_PREFETCH],
				       params);

  /* And emit all the stuff.  We put the prefetch to the loop header, so
     that it runs early.  */
  bsi = bsi_after_labels (loop->header);
  if (stmts)
    bsi_insert_after (&bsi, stmts, BSI_CONTINUE_LINKING);
  bsi_insert_after (&bsi, prefetch, BSI_NEW_STMT);
}

/* Issue prefetches for the references in GROUPS into LOOP as decided before.
   HEAD is the number of iterations to prefetch ahead.  */

static void
issue_prefetches (struct loop *loop, struct mem_ref_group *groups,
		  unsigned ahead)
{
  struct mem_ref *ref;
  tree iv_var, base, step;
  block_stmt_iterator bsi;
  bool after;

  for (; groups; groups = groups->next)
    {
      if (!groups->issue_prefetch_p)
	continue;

      /* Create the induction variable for the group.  */
      iv_var = create_tmp_var (ptr_type_node, "prefetchtmp");
      add_referenced_tmp_var (iv_var);
      if (TREE_CODE (groups->base) == INDIRECT_REF)
	base = fold_convert (ptr_type_node, TREE_OPERAND (groups->base, 0));
      else
	base = build (ADDR_EXPR, ptr_type_node, groups->base);
      step = build_int_cst (ptr_type_node, groups->step);

      standard_iv_increment_position (loop, &bsi, &after);
      create_iv (base, step, iv_var, loop, &bsi, after, &groups->group_iv,
		 NULL);

      for (ref = groups->refs; ref; ref = ref->next)
	if (ref->issue_prefetch_p)
	  issue_prefetch_ref (loop, ref, ahead);
    }
}

/* Issue prefetch instructions for array references in LOOP.  */

static void
loop_prefetch_arrays (struct loop *loop)
{
  struct mem_ref_group *refs;
  unsigned ahead, ninsns;

  /* Step 1: gather the memory references.  */
  refs = gather_memory_references (loop);

  /* Step 2: estimate the reuse effects.  */
  prune_by_reuse (refs);

  /* Step 3: determine the ahead.  */

  /* FIXME: We should use not size of the loop, but the average number of
     instructions executed per iteration of the loop.  */
  ninsns = tree_num_loop_insns (loop);
  ahead = (PREFETCH_LATENCY + ninsns - 1) / ninsns;

  /* Step 4: what to prefetch?  */
  schedule_prefetches (refs, ahead);

  /* Step 5: unroll and peel the loops.  TODO.  */

  /* Step 6: issue the prefetches.  */
  issue_prefetches (loop, refs, ahead);
  
  release_mem_refs (refs);
}

/* Issue prefetch instructions for array references in LOOPS.  */

void
tree_ssa_prefetch_arrays (struct loops *loops)
{
  unsigned i;
  struct loop *loop;

  if (!HAVE_prefetch)
    return;

  /* We assume that size of cache line is a power of two, so verify this
     here.  */
  if (PREFETCH_BLOCK & (PREFETCH_BLOCK - 1))
    abort ();

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

      if (dump_file && (dump_flags & TDF_DETAILS))
	fprintf (dump_file, "Processing loop %d:\n", loop->num);

      if (loop)
	loop_prefetch_arrays (loop);

      if (dump_file && (dump_flags & TDF_DETAILS))
	fprintf (dump_file, "\n\n");
    }
}