gcc/predict.def

   1 /* Definitions for the branch prediction routines in the GNU compiler.
   2    Copyright (C) 2001, 2003, 2004 Free Software Foundation, Inc.
   3
   4 This file is part of GCC.
   5
   6 GCC is free software; you can redistribute it and/or modify it under
   7 the terms of the GNU General Public License as published by the Free
   8 Software Foundation; either version 2, or (at your option) any later
   9 version.
  10
  11 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  12 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14 for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING.  If not, write to the Free
  18 Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
  19 02110-1301, USA.  */
  20
  21 /* Before including this file, you should define a macro:
  22
  23      DEF_PREDICTOR (ENUM, NAME, HITRATE)
  24
  25    This macro will be called once for each predictor.  The ENUM will
  26    be of type `enum predictor', and will enumerate all supported
  27    predictors.  The order of DEF_PREDICTOR calls is important, as
  28    in the first match combining heuristics, the predictor appearing
  29    first in this file will win.
  30
  31    NAME is used in the debugging output to determine predictor type.
  32
  33    HITRATE is the probability that edge predicted by predictor as taken
  34    will be really taken (so it should be always above
  35    REG_BR_PROB_BASE / 2).  */
  36
  37
  38 /* A value used as final outcome of all heuristics.  */
  39 DEF_PREDICTOR (PRED_COMBINED, "combined", PROB_ALWAYS, 0)
  40
  41 /* An outcome estimated by Dempster-Shaffer theory.  */
  42 DEF_PREDICTOR (PRED_DS_THEORY, "DS theory", PROB_ALWAYS, 0)
  43
  44 /* A combined heuristics using probability determined by first
  45    matching heuristics from this list.  */
  46 DEF_PREDICTOR (PRED_FIRST_MATCH, "first match", PROB_ALWAYS, 0)
  47
  48 /* Heuristic applying when no heuristic below applies.  */
  49 DEF_PREDICTOR (PRED_NO_PREDICTION, "no prediction", PROB_ALWAYS, 0)
  50
  51 /* Mark unconditional jump as taken.  */
  52 DEF_PREDICTOR (PRED_UNCONDITIONAL, "unconditional jump", PROB_ALWAYS,
  53                PRED_FLAG_FIRST_MATCH)
  54
  55 /* Use number of loop iterations determined by loop unroller to set
  56    probability.  We don't want to use Dempster-Shaffer theory here,
  57    as the predictions is exact.  */
  58 DEF_PREDICTOR (PRED_LOOP_ITERATIONS, "loop iterations", PROB_ALWAYS,
  59                PRED_FLAG_FIRST_MATCH)
  60
  61 /* For guessed profiles, the loops having unknown number of iterations
  62    are predicted to iterate relatively few (10) times at average.
  63    For functions containing one loop with large known number of iterations
  64    and other loops having unbounded loops we would end up predicting all
  65    the other loops cold that is not usually the case.  So we need to artifically
  66    flatten the profile.
  67
  68    We need to cut the maximal predicted iterations to large enought iterations
  69    so the loop appears important, but safely within HOT_BB_COUNT_FRACTION
  70    range.  */
  71 #define MAX_PRED_LOOP_ITERATIONS 100
  72
  73 /* Hints dropped by user via __builtin_expect feature.  */
  74 DEF_PREDICTOR (PRED_BUILTIN_EXPECT, "__builtin_expect", PROB_VERY_LIKELY,
  75                PRED_FLAG_FIRST_MATCH)
  76
  77 /* Branch containing goto is probably not taken.  */
  78 DEF_PREDICTOR (PRED_CONTINUE, "continue", HITRATE (56), 0)
  79
  80 /* Branch to basic block containing call marked by noreturn attribute.  */
  81 DEF_PREDICTOR (PRED_NORETURN, "noreturn call", HITRATE (99),
  82                PRED_FLAG_FIRST_MATCH)
  83
  84 /* Loopback edge is taken.  */
  85 DEF_PREDICTOR (PRED_LOOP_BRANCH, "loop branch", HITRATE (89),
  86                PRED_FLAG_FIRST_MATCH)
  87
  88 /* Edge causing loop to terminate is probably not taken.  */
  89 DEF_PREDICTOR (PRED_LOOP_EXIT, "loop exit", HITRATE (90),
  90                PRED_FLAG_FIRST_MATCH)
  91
  92 /* Condition emitted by preconditiong code to ensure that variable
  93    setting number of iterations is greater than initial value of iterator.  */
  94 DEF_PREDICTOR (PRED_LOOP_CONDITION, "loop condition", PROB_VERY_LIKELY, 0)
  95
  96 /* Preconditioning makes linear list of branches.  */
  97 DEF_PREDICTOR (PRED_LOOP_PRECONDITIONING, "loop preconditioning", PROB_VERY_LIKELY, 0)
  98
  99 /* Copied condition for the first iteration of loop is probably true.  */
 100 DEF_PREDICTOR (PRED_LOOP_HEADER, "loop header", HITRATE (64), 0)
 101
 102 /* Pointers are usually not NULL.  */
 103 DEF_PREDICTOR (PRED_POINTER, "pointer", HITRATE (81), 0)
 104 DEF_PREDICTOR (PRED_TREE_POINTER, "pointer (on trees)", HITRATE (81), 0)
 105
 106 /* NE is probable, EQ not etc...  */
 107 DEF_PREDICTOR (PRED_OPCODE_POSITIVE, "opcode values positive", HITRATE (79), 0)
 108 DEF_PREDICTOR (PRED_OPCODE_NONEQUAL, "opcode values nonequal", HITRATE (71), 0)
 109 DEF_PREDICTOR (PRED_FPOPCODE, "fp_opcode", HITRATE (90), 0)
 110 DEF_PREDICTOR (PRED_TREE_OPCODE_POSITIVE, "opcode values positive (on trees)", HITRATE (79), 0)
 111 DEF_PREDICTOR (PRED_TREE_OPCODE_NONEQUAL, "opcode values nonequal (on trees)", HITRATE (71), 0)
 112 DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)", HITRATE (90), 0)
 113
 114 /* Branch guarding call is probably taken.  */
 115 DEF_PREDICTOR (PRED_CALL, "call", HITRATE (70), 0)
 116
 117 /* Branch causing function to terminate is probably not taken.  */
 118 DEF_PREDICTOR (PRED_EARLY_RETURN, "early return", HITRATE (67), 0)
 119 DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (67), 0)
 120
 121 /* Branch containing goto is probably not taken.  */
 122 DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (70), 0)
 123
 124 /* Branch ending with return constant is probably not taken.  */
 125 DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (95), 0)
 126
 127 /* Branch ending with return negative constant is probably not taken.  */
 128 DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (96), 0)
 129
 130 /* Branch ending with return; is probably not taken */
 131 DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (90), 0)
 132
 133 /* Branches to a mudflap bounds check are extremely unlikely.  */
 134 DEF_PREDICTOR (PRED_MUDFLAP, "mudflap check", HITRATE (99), 0)