unroll.c

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     can precondition the loop so as to make the exit tests unnecessary.
     Just like variable splitting, this is not safe if the loop is entered
     via a jump to the bottom.  Also, can not do this if no strength
     reduce info, because precondition_loop_p uses this info.  */

  /* Must copy the loop body for preconditioning before the following
     find_splittable_regs call since that will emit insns which need to
     be after the preconditioned loop copies, but immediately before the
     unrolled loop copies.  */

  /* Also, it is not safe to split induction variables for the preconditioned
     copies of the loop body.  If we split induction variables, then the code
     assumes that each induction variable can be represented as a function
     of its initial value and the loop iteration number.  This is not true
     in this case, because the last preconditioned copy of the loop body
     could be any iteration from the first up to the `unroll_number-1'th,
     depending on the initial value of the iteration variable.  Therefore
     we can not split induction variables here, because we can not calculate
     their value.  Hence, this code must occur before find_splittable_regs
     is called.  */

  if (unroll_type == UNROLL_NAIVE && ! splitting_not_safe && strength_reduce_p)
    {
      rtx initial_value, final_value, increment;

      if (precondition_loop_p (&initial_value, &final_value, &increment,
			       loop_start, loop_end))
	{
	  register rtx diff, temp;
	  enum machine_mode mode;
	  rtx *labels;
	  int abs_inc, neg_inc;

	  map->reg_map = (rtx *) alloca (maxregnum * sizeof (rtx));

	  map->const_equiv_map = (rtx *) alloca (maxregnum * sizeof (rtx));
	  map->const_age_map = (unsigned *) alloca (maxregnum
						    * sizeof (unsigned));
	  map->const_equiv_map_size = maxregnum;
	  global_const_equiv_map = map->const_equiv_map;
	  global_const_equiv_map_size = maxregnum;

	  init_reg_map (map, maxregnum);

	  /* Limit loop unrolling to 4, since this will make 7 copies of
	     the loop body.  */
	  if (unroll_number > 4)
	    unroll_number = 4;

	  /* Save the absolute value of the increment, and also whether or
	     not it is negative.  */
	  neg_inc = 0;
	  abs_inc = INTVAL (increment);
	  if (abs_inc < 0)
	    {
	      abs_inc = - abs_inc;
	      neg_inc = 1;
	    }

	  start_sequence ();

	  /* Decide what mode to do these calculations in.  Choose the larger
	     of final_value's mode and initial_value's mode, or a full-word if
	     both are constants.  */
	  mode = GET_MODE (final_value);
	  if (mode == VOIDmode)
	    {
	      mode = GET_MODE (initial_value);
	      if (mode == VOIDmode)
		mode = word_mode;
	    }
	  else if (mode != GET_MODE (initial_value)
		   && (GET_MODE_SIZE (mode)
		       < GET_MODE_SIZE (GET_MODE (initial_value))))
	    mode = GET_MODE (initial_value);

	  /* Calculate the difference between the final and initial values.
	     Final value may be a (plus (reg x) (const_int 1)) rtx.
	     Let the following cse pass simplify this if initial value is
	     a constant. 

	     We must copy the final and initial values here to avoid
	     improperly shared rtl.  */

	  diff = expand_binop (mode, sub_optab, copy_rtx (final_value),
			       copy_rtx (initial_value), NULL_RTX, 0,
			       OPTAB_LIB_WIDEN);

	  /* Now calculate (diff % (unroll * abs (increment))) by using an
	     and instruction.  */
	  diff = expand_binop (GET_MODE (diff), and_optab, diff,
			       GEN_INT (unroll_number * abs_inc - 1),
			       NULL_RTX, 0, OPTAB_LIB_WIDEN);

	  /* Now emit a sequence of branches to jump to the proper precond
	     loop entry point.  */

	  labels = (rtx *) alloca (sizeof (rtx) * unroll_number);
	  for (i = 0; i < unroll_number; i++)
	    labels[i] = gen_label_rtx ();

	  /* Check for the case where the initial value is greater than or equal
	     to the final value.  In that case, we want to execute exactly
	     one loop iteration.  The code below will fail for this case.  */

	  emit_cmp_insn (initial_value, final_value, neg_inc ? LE : GE,
			 NULL_RTX, mode, 0, 0);
	  if (neg_inc)
	    emit_jump_insn (gen_ble (labels[1]));
	  else
	    emit_jump_insn (gen_bge (labels[1]));
	  JUMP_LABEL (get_last_insn ()) = labels[1];
	  LABEL_NUSES (labels[1])++;

	  /* Assuming the unroll_number is 4, and the increment is 2, then
	     for a negative increment:	for a positive increment:
	     diff = 0,1   precond 0	diff = 0,7   precond 0
	     diff = 2,3   precond 3     diff = 1,2   precond 1
	     diff = 4,5   precond 2     diff = 3,4   precond 2
	     diff = 6,7   precond 1     diff = 5,6   precond 3  */

	  /* We only need to emit (unroll_number - 1) branches here, the
	     last case just falls through to the following code.  */

	  /* ??? This would give better code if we emitted a tree of branches
	     instead of the current linear list of branches.  */

	  for (i = 0; i < unroll_number - 1; i++)
	    {
	      int cmp_const;
	      enum rtx_code cmp_code;

	      /* For negative increments, must invert the constant compared
		 against, except when comparing against zero.  */
	      if (i == 0)
		{
		  cmp_const = 0;
		  cmp_code = EQ;
		}
	      else if (neg_inc)
		{
		  cmp_const = unroll_number - i;
		  cmp_code = GE;
		}
	      else
		{
		  cmp_const = i;
		  cmp_code = LE;
		}

	      emit_cmp_insn (diff, GEN_INT (abs_inc * cmp_const),
			     cmp_code, NULL_RTX, mode, 0, 0);

	      if (i == 0)
		emit_jump_insn (gen_beq (labels[i]));
	      else if (neg_inc)
		emit_jump_insn (gen_bge (labels[i]));
	      else
		emit_jump_insn (gen_ble (labels[i]));
	      JUMP_LABEL (get_last_insn ()) = labels[i];
	      LABEL_NUSES (labels[i])++;
	    }

	  /* If the increment is greater than one, then we need another branch,
	     to handle other cases equivalent to 0.  */

	  /* ??? This should be merged into the code above somehow to help
	     simplify the code here, and reduce the number of branches emitted.
	     For the negative increment case, the branch here could easily
	     be merged with the `0' case branch above.  For the positive
	     increment case, it is not clear how this can be simplified.  */
	     
	  if (abs_inc != 1)
	    {
	      int cmp_const;
	      enum rtx_code cmp_code;

	      if (neg_inc)
		{
		  cmp_const = abs_inc - 1;
		  cmp_code = LE;
		}
	      else
		{
		  cmp_const = abs_inc * (unroll_number - 1) + 1;
		  cmp_code = GE;
		}

	      emit_cmp_insn (diff, GEN_INT (cmp_const), cmp_code, NULL_RTX,
			     mode, 0, 0);

	      if (neg_inc)
		emit_jump_insn (gen_ble (labels[0]));
	      else
		emit_jump_insn (gen_bge (labels[0]));
	      JUMP_LABEL (get_last_insn ()) = labels[0];
	      LABEL_NUSES (labels[0])++;
	    }

	  sequence = gen_sequence ();
	  end_sequence ();
	  emit_insn_before (sequence, loop_start);
	  
	  /* Only the last copy of the loop body here needs the exit
	     test, so set copy_end to exclude the compare/branch here,
	     and then reset it inside the loop when get to the last
	     copy.  */

	  if (GET_CODE (last_loop_insn) == BARRIER)
	    copy_end = PREV_INSN (PREV_INSN (last_loop_insn));
	  else if (GET_CODE (last_loop_insn) == JUMP_INSN)
	    {
#ifdef HAVE_cc0
	      /* The immediately preceding insn is a compare which we do not
		 want to copy.  */
	      copy_end = PREV_INSN (PREV_INSN (last_loop_insn));
#else
	      /* The immediately preceding insn may not be a compare, so we
		 must copy it.  */
	      copy_end = PREV_INSN (last_loop_insn);
#endif
	    }
	  else
	    abort ();

	  for (i = 1; i < unroll_number; i++)
	    {
	      emit_label_after (labels[unroll_number - i],
				PREV_INSN (loop_start));

	      bzero ((char *) map->insn_map, max_insnno * sizeof (rtx));
	      bzero ((char *) map->const_equiv_map, maxregnum * sizeof (rtx));
	      bzero ((char *) map->const_age_map,
		     maxregnum * sizeof (unsigned));
	      map->const_age = 0;

	      for (j = 0; j < max_labelno; j++)
		if (local_label[j])
		  map->label_map[j] = gen_label_rtx ();

	      for (j = FIRST_PSEUDO_REGISTER; j < max_reg_before_loop; j++)
		if (local_regno[j])
		  map->reg_map[j] = gen_reg_rtx (GET_MODE (regno_reg_rtx[j]));

	      /* The last copy needs the compare/branch insns at the end,
		 so reset copy_end here if the loop ends with a conditional
		 branch.  */

	      if (i == unroll_number - 1)
		{
		  if (GET_CODE (last_loop_insn) == BARRIER)
		    copy_end = PREV_INSN (PREV_INSN (last_loop_insn));
		  else
		    copy_end = last_loop_insn;
		}

	      /* None of the copies are the `last_iteration', so just
		 pass zero for that parameter.  */
	      copy_loop_body (copy_start, copy_end, map, exit_label, 0,
			      unroll_type, start_label, loop_end,
			      loop_start, copy_end);
	    }
	  emit_label_after (labels[0], PREV_INSN (loop_start));

	  if (GET_CODE (last_loop_insn) == BARRIER)
	    {
	      insert_before = PREV_INSN (last_loop_insn);
	      copy_end = PREV_INSN (insert_before);
	    }
	  else
	    {
#ifdef HAVE_cc0
	      /* The immediately preceding insn is a compare which we do not
		 want to copy.  */
	      insert_before = PREV_INSN (last_loop_insn);
	      copy_end = PREV_INSN (insert_before);
#else
	      /* The immediately preceding insn may not be a compare, so we
		 must copy it.  */
	      insert_before = last_loop_insn;
	      copy_end = PREV_INSN (last_loop_insn);
#endif
	    }

	  /* Set unroll type to MODULO now.  */
	  unroll_type = UNROLL_MODULO;
	  loop_preconditioned = 1;
	}
    }

  /* If reach here, and the loop type is UNROLL_NAIVE, then don't unroll
     the loop unless all loops are being unrolled.  */
  if (unroll_type == UNROLL_NAIVE && ! flag_unroll_all_loops)
    {
      if (loop_dump_stream)
	fprintf (loop_dump_stream, "Unrolling failure: Naive unrolling not being done.\n");
      return;
    }

  /* At this point, we are guaranteed to unroll the loop.  */

  /* For each biv and giv, determine whether it can be safely split into
     a different variable for each unrolled copy of the loop body.
     We precalculate and save this info here, since computing it is
     expensive.

     Do this before deleting any instructions from the loop, so that
     back_branch_in_range_p will work correctly.  */

  if (splitting_not_safe)
    temp = 0;
  else
    temp = find_splittable_regs (unroll_type, loop_start, loop_end,
				end_insert_before, unroll_number);

  /* find_splittable_regs may have created some new registers, so must
     reallocate the reg_map with the new larger size, and must realloc
     the constant maps also.  */

  maxregnum = max_reg_num ();
  map->reg_map = (rtx *) alloca (maxregnum * sizeof (rtx));

  init_reg_map (map, maxregnum);

  /* Space is needed in some of the map for new registers, so new_maxregnum
     is an (over)estimate of how many registers will exist at the end.  */
  new_maxregnum = maxregnum + (temp * unroll_number * 2);

  /* Must realloc space for the constant maps, because the number of registers
     may have changed.  */

  map->const_equiv_map = (rtx *) alloca (new_maxregnum * sizeof (rtx));
  map->const_age_map = (unsigned *) alloca (new_maxregnum * sizeof (unsigned));

  map->const_equiv_map_size = new_maxregnum;
  global_const_equiv_map = map->const_equiv_map;
  global_const_equiv_map_size = new_maxregnum;

  /* Search the list of bivs and givs to find ones which need to be remapped
     when split, and set their reg_map entry appropriately.  */

  for (bl = loop_iv_list; bl; bl = bl->next)
    {
      if (REGNO (bl->biv->src_reg) != bl->regno)
	map->reg_map[bl->regno] = bl->biv->src_reg;
#if 0
      /* Currently, non-reduced/final-value givs are never split.  */
      for (v = bl->giv; v; v = v->next_iv)
	if (REGNO (v->src_reg) != bl->regno)
	  map->reg_map[REGNO (v->dest_reg)] = v->src_reg;
#endif
    }

  /* If the loop is being partially unrolled, and the iteration variables
     are being split, and are being renamed for the split, then must fix up
     the compare/jump instruction at the end of the loop to refer to the new
     registers.  This compare isn't copied, so the registers used in it
     will never be replaced if it isn't done here.  */

  if (unroll_type == UNROLL_MODULO)
    {
      insn = NEXT_INSN (copy_end);
      if (GET_CODE (insn) == INSN || GET_CODE (insn) == JUMP_INSN)
	PATTERN (insn) = remap_split_bivs (PATTERN (insn));
    }

  /* For unroll_number - 1 times, make a copy of each instruction
     between copy_start and copy_end, and insert these new instructions
     before the end of the loop.  */

  for (i = 0; i < unroll_number; i++)
    {
      bzero ((char *) map->insn_map, max_insnno * sizeof (rtx));
      bzero ((char *) map->const_equiv_map, new_maxregnum * sizeof (rtx));
      bzero ((char *) map->const_age_map, new_maxregnum * sizeof (unsigned));
      map->const_age = 0;