gcc/predict.c

   1 /* Branch prediction routines for the GNU compiler.
   2    Copyright (C) 2000 Free Software Foundation, Inc.
   3
   4    This file is part of GNU CC.
   5
   6    GNU CC is free software; you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 2, or (at your option)
   9    any later version.
  10
  11    GNU CC is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with GNU CC; see the file COPYING.  If not, write to
  18    the Free Software Foundation, 59 Temple Place - Suite 330,
  19    Boston, MA 02111-1307, USA.  */
  20
  21 /* References:
  22
  23    [1] "Branch Prediction for Free"
  24        Ball and Larus; PLDI '93.
  25    [2] "Static Branch Frequency and Program Profile Analysis"
  26        Wu and Larus; MICRO-27.
  27    [3] "Corpus-based Static Branch Prediction"
  28        Calder, Grunwald, Lindsay, Martin, Mozer, and Zorn; PLDI '95.
  29
  30 */
  31
  32
  33 #include "config.h"
  34 #include "system.h"
  35 #include "tree.h"
  36 #include "rtl.h"
  37 #include "tm_p.h"
  38 #include "hard-reg-set.h"
  39 #include "basic-block.h"
  40 #include "insn-config.h"
  41 #include "regs.h"
  42 #include "flags.h"
  43 #include "output.h"
  44 #include "function.h"
  45 #include "except.h"
  46 #include "toplev.h"
  47 #include "recog.h"
  48 #include "insn-flags.h"
  49 #include "expr.h"
  50
  51
  52 /* Random guesstimation given names.  */
  53 #define PROB_NEVER              (0)
  54 #define PROB_VERY_UNLIKELY      (REG_BR_PROB_BASE / 10 - 1)
  55 #define PROB_UNLIKELY           (REG_BR_PROB_BASE * 4 / 10 - 1)
  56 #define PROB_EVEN               (REG_BR_PROB_BASE / 2)
  57 #define PROB_LIKELY             (REG_BR_PROB_BASE - PROB_UNLIKELY)
  58 #define PROB_VERY_LIKELY        (REG_BR_PROB_BASE - PROB_VERY_UNLIKELY)
  59 #define PROB_ALWAYS             (REG_BR_PROB_BASE)
  60
  61 /* Statically estimate the probability that a branch will be taken.
  62    ??? In the next revision there will be a number of other predictors added
  63    from the above references. Further, each heuristic will be factored out
  64    into its own function for clarity (and to facilitate the combination of
  65    predictions).  */
  66
  67 void
  68 estimate_probability (loops_info)
  69      struct loops *loops_info;
  70 {
  71   int i;
  72
  73   /* Try to predict out blocks in a loop that are not part of a
  74      natural loop.  */
  75   for (i = 0; i < loops_info->num; i++)
  76     {
  77       int j;
  78
  79       for (j = loops_info->array[i].first->index;
  80            j <= loops_info->array[i].last->index;
  81            ++j)
  82         {
  83           edge e;
  84
  85           if (! TEST_BIT (loops_info->array[i].nodes, j))
  86             for (e = BASIC_BLOCK(j)->pred; e; e = e->pred_next)
  87               if (TEST_BIT (loops_info->array[i].nodes, e->src->index))
  88                 {
  89                   rtx last_insn = BLOCK_END (e->src->index);
  90                   rtx cond, earliest;
  91
  92                   if (GET_CODE (last_insn) != JUMP_INSN
  93                       || ! condjump_p (last_insn) || simplejump_p (last_insn))
  94                     continue;
  95                   cond = get_condition (last_insn, &earliest);
  96                   if (! cond)
  97                     continue;
  98                   if (! find_reg_note (last_insn, REG_BR_PROB, 0))
  99                     REG_NOTES (last_insn)
 100                       = gen_rtx_EXPR_LIST (REG_BR_PROB,
 101                                            GEN_INT (PROB_VERY_LIKELY),
 102                                            REG_NOTES (last_insn));
 103                 }
 104         }
 105     }
 106
 107   /* Attempt to predict conditional jumps using a number of heuristics.
 108      For each conditional jump, we try each heuristic in a fixed order.
 109      If more than one heuristic applies to a particular branch, the first
 110      is used as the prediction for the branch.  */
 111   for (i = 0; i < n_basic_blocks - 1; i++)
 112     {
 113       rtx last_insn = BLOCK_END (i);
 114       rtx cond, earliest;
 115       int prob;
 116       edge e;
 117
 118       if (GET_CODE (last_insn) != JUMP_INSN
 119           || ! condjump_p (last_insn) || simplejump_p (last_insn))
 120         continue;
 121
 122       if (find_reg_note (last_insn, REG_BR_PROB, 0))
 123         continue;
 124
 125       cond = get_condition (last_insn, &earliest);
 126       if (! cond)
 127         continue;
 128
 129       /* If one of the successor blocks has no successors, predict
 130          that side not taken.  */
 131       /* ??? Ought to do the same for any subgraph with no exit.  */
 132       for (e = BASIC_BLOCK (i)->succ; e; e = e->succ_next)
 133         if (e->dest->succ == NULL)
 134           {
 135             if (e->flags & EDGE_FALLTHRU)
 136               prob = PROB_ALWAYS;
 137             else
 138               prob = PROB_NEVER;
 139             goto emitnote;
 140           }
 141
 142       /* Try "pointer heuristic."
 143          A comparison ptr == 0 is predicted as false.
 144          Similarly, a comparison ptr1 == ptr2 is predicted as false.  */
 145       switch (GET_CODE (cond))
 146         {
 147         case EQ:
 148           if (GET_CODE (XEXP (cond, 0)) == REG
 149               && REGNO_POINTER_FLAG (REGNO (XEXP (cond, 0)))
 150               && (XEXP (cond, 1) == const0_rtx
 151                   || (GET_CODE (XEXP (cond, 1)) == REG
 152                       && REGNO_POINTER_FLAG (REGNO (XEXP (cond, 1))))))
 153             {
 154               prob = PROB_UNLIKELY;
 155               goto emitnote;
 156             }
 157           break;
 158         case NE:
 159           if (GET_CODE (XEXP (cond, 0)) == REG
 160               && REGNO_POINTER_FLAG (REGNO (XEXP (cond, 0)))
 161               && (XEXP (cond, 1) == const0_rtx
 162                   || (GET_CODE (XEXP (cond, 1)) == REG
 163                       && REGNO_POINTER_FLAG (REGNO (XEXP (cond, 1))))))
 164             {
 165               prob = PROB_LIKELY;
 166               goto emitnote;
 167             }
 168           break;
 169
 170         default:
 171           break;
 172         }
 173
 174       /* Try "opcode heuristic."
 175          EQ tests are usually false and NE tests are usually true. Also,
 176          most quantities are positive, so we can make the appropriate guesses
 177          about signed comparisons against zero.  */
 178       switch (GET_CODE (cond))
 179         {
 180         case CONST_INT:
 181           /* Unconditional branch.  */
 182           prob = (cond == const0_rtx ? PROB_NEVER : PROB_ALWAYS);
 183           goto emitnote;
 184
 185         case EQ:
 186           prob = PROB_UNLIKELY;
 187           goto emitnote;
 188         case NE:
 189           prob = PROB_LIKELY;
 190           goto emitnote;
 191         case LE:
 192         case LT:
 193           if (XEXP (cond, 1) == const0_rtx)
 194             {
 195               prob = PROB_UNLIKELY;
 196               goto emitnote;
 197             }
 198           break;
 199         case GE:
 200         case GT:
 201           if (XEXP (cond, 1) == const0_rtx
 202               || (GET_CODE (XEXP (cond, 1)) == CONST_INT
 203                   && INTVAL (XEXP (cond, 1)) == -1))
 204             {
 205               prob = PROB_LIKELY;
 206               goto emitnote;
 207             }
 208           break;
 209
 210         default:
 211           break;
 212         }
 213
 214       /* If we havn't chosen something by now, predict 50-50.  */
 215       prob = PROB_EVEN;
 216
 217     emitnote:
 218       REG_NOTES (last_insn)
 219         = gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob),
 220                              REG_NOTES (last_insn));
 221     }
 222 }
 223 \f
 224 /* __builtin_expect dropped tokens into the insn stream describing
 225    expected values of registers.  Generate branch probabilities
 226    based off these values.  */
 227
 228 void
 229 expected_value_to_br_prob ()
 230 {
 231   rtx insn, cond, ev = NULL_RTX, ev_reg = NULL_RTX;
 232
 233   for (insn = get_insns (); insn ; insn = NEXT_INSN (insn))
 234     {
 235       switch (GET_CODE (insn))
 236         {
 237         case NOTE:
 238           /* Look for expected value notes.  */
 239           if (NOTE_LINE_NUMBER (insn) == NOTE_INSN_EXPECTED_VALUE)
 240             {
 241               ev = NOTE_EXPECTED_VALUE (insn);
 242               ev_reg = XEXP (ev, 0);
 243             }
 244           continue;
 245
 246         case CODE_LABEL:
 247           /* Never propagate across labels.  */
 248           ev = NULL_RTX;
 249           continue;
 250
 251         default:
 252           /* Look for insns that clobber the EV register.  */
 253           if (ev && reg_set_p (ev_reg, insn))
 254             ev = NULL_RTX;
 255           continue;
 256
 257         case JUMP_INSN:
 258           /* Look for simple conditional branches.  If we havn't got an
 259              expected value yet, no point going further.  */
 260           if (GET_CODE (insn) != JUMP_INSN || ev == NULL_RTX)
 261             continue;
 262           if (! condjump_p (insn) || simplejump_p (insn))
 263             continue;
 264           break;
 265         }
 266
 267       /* Collect the branch condition, hopefully relative to EV_REG.  */
 268       /* ???  At present we'll miss things like
 269                 (expected_value (eq r70 0))
 270                 (set r71 -1)
 271                 (set r80 (lt r70 r71))
 272                 (set pc (if_then_else (ne r80 0) ...))
 273          as canonicalize_condition will render this to us as
 274                 (lt r70, r71)
 275          Could use cselib to try and reduce this further.  */
 276       cond = XEXP (SET_SRC (PATTERN (insn)), 0);
 277       cond = canonicalize_condition (insn, cond, 0, NULL, ev_reg);
 278       if (! cond
 279           || XEXP (cond, 0) != ev_reg
 280           || GET_CODE (XEXP (cond, 1)) != CONST_INT)
 281         continue;
 282
 283       /* Substitute and simplify.  Given that the expression we're
 284          building involves two constants, we should wind up with either
 285          true or false.  */
 286       cond = gen_rtx_fmt_ee (GET_CODE (cond), VOIDmode,
 287                              XEXP (ev, 1), XEXP (cond, 1));
 288       cond = simplify_rtx (cond);
 289
 290       /* Turn the condition into a scaled branch probability.  */
 291       if (cond == const1_rtx)
 292         cond = GEN_INT (PROB_VERY_LIKELY);
 293       else if (cond == const0_rtx)
 294         cond = GEN_INT (PROB_VERY_UNLIKELY);
 295       else
 296         abort ();
 297       REG_NOTES (insn) = alloc_EXPR_LIST (REG_BR_PROB, cond, REG_NOTES (insn));
 298     }
 299 }