gcc/predict.def

   1 /* Definitions for the branch prediction routines in the GNU compiler.
   2    Copyright (C) 2001-2016 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 3, 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 COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 /* Before including this file, you should define a macro:
  21
  22      DEF_PREDICTOR (ENUM, NAME, HITRATE)
  23
  24    This macro will be called once for each predictor.  The ENUM will
  25    be of type `enum predictor', and will enumerate all supported
  26    predictors.  The order of DEF_PREDICTOR calls is important, as
  27    in the first match combining heuristics, the predictor appearing
  28    first in this file will win.
  29
  30    NAME is used in the debugging output to determine predictor type.
  31
  32    HITRATE is the probability that edge predicted by predictor as taken
  33    will be really taken (so it should be always above
  34    REG_BR_PROB_BASE / 2).  */
  35
  36
  37 /* A value used as final outcome of all heuristics.  */
  38 DEF_PREDICTOR (PRED_COMBINED, "combined", PROB_ALWAYS, 0)
  39
  40 /* An outcome estimated by Dempster-Shaffer theory.  */
  41 DEF_PREDICTOR (PRED_DS_THEORY, "DS theory", PROB_ALWAYS, 0)
  42
  43 /* A combined heuristics using probability determined by first
  44    matching heuristics from this list.  */
  45 DEF_PREDICTOR (PRED_FIRST_MATCH, "first match", PROB_ALWAYS, 0)
  46
  47 /* Heuristic applying when no heuristic below applies.  */
  48 DEF_PREDICTOR (PRED_NO_PREDICTION, "no prediction", PROB_ALWAYS, 0)
  49
  50 /* Mark unconditional jump as taken.  */
  51 DEF_PREDICTOR (PRED_UNCONDITIONAL, "unconditional jump", PROB_ALWAYS,
  52                PRED_FLAG_FIRST_MATCH)
  53
  54 /* Use number of loop iterations determined by # of iterations
  55    analysis to set probability.  We don't want to use Dempster-Shaffer
  56    theory here, as the predictions is exact.  */
  57 DEF_PREDICTOR (PRED_LOOP_ITERATIONS, "loop iterations", PROB_ALWAYS,
  58                PRED_FLAG_FIRST_MATCH)
  59
  60 /* Assume that any given atomic operation has low contention,
  61    and thus the compare-and-swap operation succeeds. */
  62 DEF_PREDICTOR (PRED_COMPARE_AND_SWAP, "compare and swap", PROB_VERY_LIKELY,
  63                PRED_FLAG_FIRST_MATCH)
  64
  65 /* Hints dropped by user via __builtin_expect feature.  Note: the
  66    probability of PROB_VERY_LIKELY is now overwritten by param
  67    builtin_expect_probability with a default value of HITRATE(90).
  68    Refer to param.def for details.  */
  69 DEF_PREDICTOR (PRED_BUILTIN_EXPECT, "__builtin_expect", PROB_VERY_LIKELY,
  70                PRED_FLAG_FIRST_MATCH)
  71
  72 /* Use number of loop iterations guessed by the contents of the loop.  */
  73 DEF_PREDICTOR (PRED_LOOP_ITERATIONS_GUESSED, "guessed loop iterations",
  74                PROB_ALWAYS, PRED_FLAG_FIRST_MATCH)
  75
  76 /* Use number of loop iterations guessed by the contents of the loop.  */
  77 DEF_PREDICTOR (PRED_LOOP_ITERATIONS_MAX, "guessed loop iterations",
  78                PROB_ALWAYS, PRED_FLAG_FIRST_MATCH)
  79
  80 /* Branch containing goto is probably not taken.  */
  81 DEF_PREDICTOR (PRED_CONTINUE, "continue", HITRATE (67), 0)
  82
  83 /* Branch to basic block containing call marked by noreturn attribute.  */
  84 DEF_PREDICTOR (PRED_NORETURN, "noreturn call", PROB_VERY_LIKELY,
  85                PRED_FLAG_FIRST_MATCH)
  86
  87 /* Branch to basic block containing call marked by cold function attribute.  */
  88 DEF_PREDICTOR (PRED_COLD_FUNCTION, "cold function call", PROB_VERY_LIKELY,
  89                PRED_FLAG_FIRST_MATCH)
  90
  91 /* Edge causing loop to terminate is probably not taken.  */
  92 DEF_PREDICTOR (PRED_LOOP_EXIT, "loop exit", HITRATE (85),
  93                PRED_FLAG_FIRST_MATCH)
  94
  95 /* Same as LOOP_EXIT but for loops containing recursive call.  */
  96 DEF_PREDICTOR (PRED_LOOP_EXIT_WITH_RECURSION, "loop exit with recursion",
  97                HITRATE (72), PRED_FLAG_FIRST_MATCH)
  98
  99 /* Edge causing loop to terminate by computing value used by later
 100    conditional.  */
 101 DEF_PREDICTOR (PRED_LOOP_EXTRA_EXIT, "extra loop exit", HITRATE (83),
 102                PRED_FLAG_FIRST_MATCH)
 103
 104 /* Pointers are usually not NULL.  */
 105 DEF_PREDICTOR (PRED_POINTER, "pointer", HITRATE (70), 0)
 106 DEF_PREDICTOR (PRED_TREE_POINTER, "pointer (on trees)", HITRATE (70), 0)
 107
 108 /* NE is probable, EQ not etc...  */
 109 DEF_PREDICTOR (PRED_OPCODE_POSITIVE, "opcode values positive", HITRATE (64), 0)
 110 DEF_PREDICTOR (PRED_OPCODE_NONEQUAL, "opcode values nonequal", HITRATE (66), 0)
 111 DEF_PREDICTOR (PRED_FPOPCODE, "fp_opcode", HITRATE (90), 0)
 112 DEF_PREDICTOR (PRED_TREE_OPCODE_POSITIVE, "opcode values positive (on trees)",
 113                HITRATE (64), 0)
 114 DEF_PREDICTOR (PRED_TREE_OPCODE_NONEQUAL, "opcode values nonequal (on trees)",
 115                HITRATE (66), 0)
 116 DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)", HITRATE (90), 0)
 117
 118 /* Branch guarding call is probably taken.  */
 119 DEF_PREDICTOR (PRED_CALL, "call", HITRATE (67), 0)
 120
 121 /* Recursive calls are usually not taken or the function will recurse
 122    indefinitely.  */
 123 DEF_PREDICTOR (PRED_RECURSIVE_CALL, "recursive call", HITRATE (75), 0)
 124
 125 /* Branch causing function to terminate is probably not taken.
 126    FIXME: early return currently predicts code:
 127    int foo (int a)
 128    {
 129       if (a)
 130         bar();
 131       else
 132         bar2();
 133    }
 134    even though there is no return statement involved.  We probably want to track
 135    this from FE or retire the predictor.  */
 136 DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (54), 0)
 137
 138 /* Branch containing goto is probably not taken.
 139    FIXME: Currently not used.  */
 140 DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (70), 0)
 141
 142 /* Branch ending with return constant is probably not taken.  */
 143 DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (69), 0)
 144
 145 /* Branch ending with return negative constant is probably not taken.  */
 146 DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (98), 0)
 147
 148 /* Branch ending with return; is probably not taken */
 149 DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (91), 0)
 150
 151 /* Branches to compare induction variable to a loop bound is
 152    extremely likely.  */
 153 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE_GUESS, "guess loop iv compare",
 154                HITRATE (98), 0)
 155
 156 /* Use number of loop iterations determined by # of iterations analysis
 157    to set probability of branches that compares IV to loop bound variable.  */
 158 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE, "loop iv compare", PROB_VERY_LIKELY,
 159                PRED_FLAG_FIRST_MATCH)
 160
 161 /* In the following code
 162    for (loop1)
 163      if (cond)
 164        for (loop2)
 165          body;
 166    guess that cond is unlikely.  */
 167 DEF_PREDICTOR (PRED_LOOP_GUARD, "loop guard", HITRATE (66), 0)
 168
 169 /* Same but for loops containing recursion.  */
 170 DEF_PREDICTOR (PRED_LOOP_GUARD_WITH_RECURSION, "loop guard with recursion",
 171                HITRATE (85), 0)
 172
 173 /* Branches to hot labels are likely.  */
 174 DEF_PREDICTOR (PRED_HOT_LABEL, "hot label", HITRATE (85), 0)
 175
 176 /* Branches to cold labels are extremely unlikely.  */
 177 DEF_PREDICTOR (PRED_COLD_LABEL, "cold label", PROB_VERY_LIKELY,
 178                PRED_FLAG_FIRST_MATCH)
 179
 180
 181 /* The following predictors are used in Fortran. */
 182
 183 /* Branch leading to an integer overflow are extremely unlikely.  */
 184 DEF_PREDICTOR (PRED_FORTRAN_OVERFLOW, "Fortran overflow", PROB_ALWAYS,
 185                PRED_FLAG_FIRST_MATCH)
 186
 187 /* Branch leading to a failure status are unlikely.  This can occur for out
 188    of memory.  This predictor only occurs when the user explicitly asked
 189    for a return status.  By default, the code aborts,
 190    which is handled via PRED_NORETURN.  */
 191 DEF_PREDICTOR (PRED_FORTRAN_FAIL_ALLOC, "Fortran fail alloc", PROB_VERY_LIKELY, 0)
 192
 193 /* Predictor is used for an allocation of an already allocated memory or
 194    deallocating an already deallocated allocatable.  */
 195 DEF_PREDICTOR (PRED_FORTRAN_REALLOC, \
 196                "Fortran repeated allocation/deallocation", PROB_LIKELY, 0)
 197
 198 /* Branch leading to an I/O failure status are unlikely.  This predictor is
 199    used for I/O failures such as for invalid unit numbers.  This predictor
 200    only occurs when the user explicitly asked for a return status.  By default,
 201    the code aborts, which is handled via PRED_NORETURN.  */
 202 DEF_PREDICTOR (PRED_FORTRAN_FAIL_IO, "Fortran fail IO", HITRATE (85), 0)
 203
 204 /* Branch leading to a run-time warning message which is printed only once
 205    are unlikely.  The print-warning branch itself can be likely or unlikely.  */
 206 DEF_PREDICTOR (PRED_FORTRAN_WARN_ONCE, "Fortran warn once", HITRATE (75), 0)
 207
 208 /* Branch belonging to a zero-sized array.  */
 209 DEF_PREDICTOR (PRED_FORTRAN_SIZE_ZERO, "Fortran zero-sized array", \
 210                HITRATE (99), 0)
 211
 212 /* Branch belonging to an invalid bound index, in a context where it is
 213    standard conform and well defined but rather pointless and, hence, rather
 214    unlikely to occur.  */
 215 DEF_PREDICTOR (PRED_FORTRAN_INVALID_BOUND, "Fortran invalid bound", \
 216                HITRATE (90), 0)
 217
 218 /* Branch belonging to the handling of absent optional arguments.  This
 219    predictor is used when an optional dummy argument, associated with an
 220    absent argument, is passed on as actual argument to another procedure,
 221    which in turn has an optional argument.  */
 222 DEF_PREDICTOR (PRED_FORTRAN_ABSENT_DUMMY, "Fortran absent dummy", \
 223                HITRATE (60), 0)
 224
 225 /* Fortran DO statement generates a pre-header guard:
 226    empty = (step > 0 ? to < from : to > from), which can be predicted
 227    to be very likely.  */
 228 DEF_PREDICTOR (PRED_FORTRAN_LOOP_PREHEADER, "Fortran loop preheader", \
 229                HITRATE (99), 0)