gcc/predict.c


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/* Branch prediction routines for the GNU compiler.
   Copyright (C) 2000, 2001 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.  */

/* 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 "hard-reg-set.h"
#include "basic-block.h"
#include "insn-config.h"
#include "regs.h"
#include "flags.h"
#include "output.h"
#include "function.h"
#include "except.h"
#include "toplev.h"
#include "recog.h"
#include "expr.h"
#include "predict.h"

/* Random guesstimation given names.  */
#define PROB_NEVER		(0)
#define PROB_VERY_UNLIKELY	(REG_BR_PROB_BASE / 10 - 1)
#define PROB_UNLIKELY		(REG_BR_PROB_BASE * 4 / 10 - 1)
#define PROB_EVEN		(REG_BR_PROB_BASE / 2)
#define PROB_LIKELY		(REG_BR_PROB_BASE - PROB_UNLIKELY)
#define PROB_VERY_LIKELY	(REG_BR_PROB_BASE - PROB_VERY_UNLIKELY)
#define PROB_ALWAYS		(REG_BR_PROB_BASE)

static void combine_predictions_for_insn PARAMS ((rtx, basic_block));
static void dump_prediction		 PARAMS ((enum br_predictor, int,
						  basic_block, int));
static void estimate_loops_at_level	 PARAMS ((struct loop *loop));
static void propagate_freq		 PARAMS ((basic_block));
static void estimate_bb_frequencies	 PARAMS ((struct loops *));
static void counts_to_freqs		 PARAMS ((void));

/* Information we hold about each branch predictor.
   Filled using information from predict.def.  */
struct predictor_info
{
  const char *const name;	/* Name used in the debugging dumps.  */
  const int hitrate;		/* Expected hitrate used by
				   predict_insn_def call.  */
  const int flags;
};

/* Use given predictor without Dempster-Shaffer theory if it matches
   using first_match heuristics.  */
#define PRED_FLAG_FIRST_MATCH 1

/* Recompute hitrate in percent to our representation.  */

#define HITRATE(VAL) ((int)((VAL) * REG_BR_PROB_BASE + 50) / 100)

#define DEF_PREDICTOR(ENUM, NAME, HITRATE, FLAGS) {NAME, HITRATE, FLAGS},
static const struct predictor_info predictor_info[] = {
#include "predict.def"

  /* Upper bound on predictors.  */
  {NULL, 0, 0}
};
#undef DEF_PREDICTOR

void
predict_insn (insn, predictor, probability)
     rtx insn;
     int probability;
     enum br_predictor predictor;
{
  if (!any_condjump_p (insn))
    abort ();
  REG_NOTES (insn)
    = gen_rtx_EXPR_LIST (REG_BR_PRED,
			 gen_rtx_CONCAT (VOIDmode,
					 GEN_INT ((int) predictor),
					 GEN_INT ((int) probability)),
			 REG_NOTES (insn));
}

/* Predict insn by given predictor.  */
void
predict_insn_def (insn, predictor, taken)
     rtx insn;
     enum br_predictor predictor;
     enum prediction taken;
{
   int probability = predictor_info[(int) predictor].hitrate;
   if (taken != TAKEN)
     probability = REG_BR_PROB_BASE - probability;
   predict_insn (insn, predictor, probability);
}

/* Predict edge E with given probability if possible.  */
void
predict_edge (e, predictor, probability)
     edge e;
     int probability;
     enum br_predictor predictor;
{
  rtx last_insn;
  last_insn = e->src->end;

  /* We can store the branch prediction information only about
     conditional jumps.  */
  if (!any_condjump_p (last_insn))
    return;