8sa1-gcc/gcc/predict.c

/* Branch prediction routines for the GNU compiler.
   Copyright (C) 2000 Free Software Foundation, Inc.

   This file is part of GNU CC.

   GNU CC 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.

   GNU CC 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 GNU CC; see the file COPYING.  If not, write to
   the Free Software Foundation, 59 Temple Place - Suite 330,
   Boston, MA 02111-1307, USA.  */

/* References:

   [1] "Branch Prediction for Free"
       Ball and Larus; PLDI '93.
   [2] "Static Branch Frequency and Program Profile Analysis"
       Wu and Larus; MICRO-27.
   [3] "Corpus-based Static Branch Prediction"
       Calder, Grunwald, Lindsay, Martin, Mozer, and Zorn; PLDI '95.
*/


#include "config.h"
#include "system.h"
#include "tree.h"
#include "rtl.h"
#include "tm_p.h"
#include "basic-block.h"
#include "insn-config.h"
#include "regs.h"
#include "hard-reg-set.h"
#include "flags.h"
#include "output.h"
#include "function.h"
#include "except.h"
#include "toplev.h"
#include "recog.h"
#include "insn-flags.h"
#include "expr.h"



/* Statically estimate the probability that a branch will be taken.
   ??? In the next revision there will be a number of other predictors added
   from the above references. Further, each heuristic will be factored out
   into its own function for clarity (and to facilitate the combination of
   predictions).  */

void
estimate_probability (loops_info)
     struct loops *loops_info;
{
  int i;

  /* Try to predict out blocks in a loop that are not part of a
     natural loop.  */
  for (i = 0; i < loops_info->num; i++)
    {
      int j;

      for (j = loops_info->array[i].first->index;
	   j <= loops_info->array[i].last->index;
	   ++j)
	{
	  edge e;
	  
	  if (! TEST_BIT (loops_info->array[i].nodes, j))
	    for (e = BASIC_BLOCK(j)->pred; e; e = e->pred_next)
	      if (TEST_BIT (loops_info->array[i].nodes, e->src->index))
		{
		  rtx last_insn = BLOCK_END (e->src->index);
		  rtx cond, earliest;

		  if (GET_CODE (last_insn) != JUMP_INSN
		      || ! condjump_p (last_insn) || simplejump_p (last_insn))
		    continue;
		  cond = get_condition (last_insn, &earliest);
		  if (! cond)
		    continue;
		  if (! find_reg_note (last_insn, REG_BR_PROB, 0))
		    REG_NOTES (last_insn)
		      = gen_rtx_EXPR_LIST (REG_BR_PROB,
					   GEN_INT (REG_BR_PROB_BASE),
					   REG_NOTES (last_insn));
		}
	}
    }

  /* Try to predict condjumps using same algorithm as mostly_true_jump.  */
  for (i = 0; i < n_basic_blocks - 1; i++)
    {
      rtx last_insn = BLOCK_END (i);
      rtx cond, earliest;
      int prob = 0;

      if (GET_CODE (last_insn) != JUMP_INSN
	  || ! condjump_p (last_insn) || simplejump_p (last_insn))
	continue;
      cond = get_condition (last_insn, &earliest);
      if (! cond)
	continue;
      /* EQ tests are usually false and NE tests are usually true.  Also,
	 most quantities are positive, so we can make the appropriate guesses
	 about signed comparisons against zero.  */
      switch (GET_CODE (cond))
	{
	case CONST_INT:
	  /* Unconditional branch.  */
	  prob = REG_BR_PROB_BASE / 2;
	  break;
	case EQ:
	  prob = REG_BR_PROB_BASE / 10;
	  break;
	case NE:
	  prob = REG_BR_PROB_BASE / 2;
	  break;
	case LE:
	case LT:
	  if (XEXP (cond, 1) == const0_rtx)
	    prob = REG_BR_PROB_BASE / 10;
	  break;
	case GE:
	case GT:
	  if (XEXP (cond, 1) == const0_rtx
	      || (GET_CODE (XEXP (cond, 1)) == CONST_INT
		  && INTVAL (XEXP (cond, 1)) == -1))
	    prob = REG_BR_PROB_BASE / 2;
	  break;

	default:
	  prob = 0;
	}
      if (! find_reg_note (last_insn, REG_BR_PROB, 0))
	REG_NOTES (last_insn)
	  = gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob),
			       REG_NOTES (last_insn));
    }
}