gcc/predict.def

   1 /* Definitions for the branch prediction routines in the GNU compiler.
   2    Copyright (C) 2001-2017 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 /* PRED_CALL is not very reliable predictor and it turns out to be even
 122    less reliable for indirect calls and polymorphic calls.  For spec2k6
 123    the predictio nis slightly in the direction of taking the call.  */
 124 DEF_PREDICTOR (PRED_INDIR_CALL, "indirect call", HITRATE (86), 0)
 125 DEF_PREDICTOR (PRED_POLYMORPHIC_CALL, "polymorphic call", HITRATE (59), 0)
 126
 127 /* Recursive calls are usually not taken or the function will recurse
 128    indefinitely.  */
 129 DEF_PREDICTOR (PRED_RECURSIVE_CALL, "recursive call", HITRATE (75), 0)
 130
 131 /* Branch causing function to terminate is probably not taken.  */
 132 DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (66),
 133                0)
 134
 135 /* Branch containing goto is probably not taken.
 136    FIXME: Currently not used.  */
 137 DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (70), 0)
 138
 139 /* Branch ending with return constant is probably not taken.  */
 140 DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (69), 0)
 141
 142 /* Branch ending with return negative constant is probably not taken.  */
 143 DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (98), 0)
 144
 145 /* Branch ending with return; is probably not taken */
 146 DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (91), 0)
 147
 148 /* Branches to compare induction variable to a loop bound is
 149    extremely likely.  */
 150 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE_GUESS, "guess loop iv compare",
 151                HITRATE (98), 0)
 152
 153 /* Use number of loop iterations determined by # of iterations analysis
 154    to set probability of branches that compares IV to loop bound variable.  */
 155 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE, "loop iv compare", PROB_VERY_LIKELY,
 156                PRED_FLAG_FIRST_MATCH)
 157
 158 /* In the following code
 159    for (loop1)
 160      if (cond)
 161        for (loop2)
 162          body;
 163    guess that cond is unlikely.  */
 164 DEF_PREDICTOR (PRED_LOOP_GUARD, "loop guard", HITRATE (66), 0)
 165
 166 /* Same but for loops containing recursion.  */
 167 DEF_PREDICTOR (PRED_LOOP_GUARD_WITH_RECURSION, "loop guard with recursion",
 168                HITRATE (85), 0)
 169
 170 /* Branches to hot labels are likely.  */
 171 DEF_PREDICTOR (PRED_HOT_LABEL, "hot label", HITRATE (85), 0)
 172
 173 /* Branches to cold labels are extremely unlikely.  */
 174 DEF_PREDICTOR (PRED_COLD_LABEL, "cold label", PROB_VERY_LIKELY,
 175                PRED_FLAG_FIRST_MATCH)
 176
 177
 178 /* The following predictors are used in Fortran. */
 179
 180 /* Branch leading to an integer overflow are extremely unlikely.  */
 181 DEF_PREDICTOR (PRED_FORTRAN_OVERFLOW, "Fortran overflow", PROB_ALWAYS,
 182                PRED_FLAG_FIRST_MATCH)
 183
 184 /* Branch leading to a failure status are unlikely.  This can occur for out
 185    of memory.  This predictor only occurs when the user explicitly asked
 186    for a return status.  By default, the code aborts,
 187    which is handled via PRED_NORETURN.  */
 188 DEF_PREDICTOR (PRED_FORTRAN_FAIL_ALLOC, "Fortran fail alloc",
 189                PROB_VERY_LIKELY, 0)
 190
 191 /* Predictor is used for an allocation of an already allocated memory or
 192    deallocating an already deallocated allocatable.  */
 193 DEF_PREDICTOR (PRED_FORTRAN_REALLOC, "Fortran repeated allocation/deallocation",
 194                PROB_LIKELY, 0)
 195
 196 /* Branch leading to an I/O failure status are unlikely.  This predictor is
 197    used for I/O failures such as for invalid unit numbers.  This predictor
 198    only occurs when the user explicitly asked for a return status.  By default,
 199    the code aborts, which is handled via PRED_NORETURN.  */
 200 DEF_PREDICTOR (PRED_FORTRAN_FAIL_IO, "Fortran fail IO", HITRATE (85), 0)
 201
 202 /* Branch leading to a run-time warning message which is printed only once
 203    are unlikely.  The print-warning branch itself can be likely or unlikely.  */
 204 DEF_PREDICTOR (PRED_FORTRAN_WARN_ONCE, "Fortran warn once", HITRATE (75), 0)
 205
 206 /* Branch belonging to a zero-sized array.  */
 207 DEF_PREDICTOR (PRED_FORTRAN_SIZE_ZERO, "Fortran zero-sized array", \
 208                HITRATE (99), 0)
 209
 210 /* Branch belonging to an invalid bound index, in a context where it is
 211    standard conform and well defined but rather pointless and, hence, rather
 212    unlikely to occur.  */
 213 DEF_PREDICTOR (PRED_FORTRAN_INVALID_BOUND, "Fortran invalid bound", \
 214                HITRATE (90), 0)
 215
 216 /* Branch belonging to the handling of absent optional arguments.  This
 217    predictor is used when an optional dummy argument, associated with an
 218    absent argument, is passed on as actual argument to another procedure,
 219    which in turn has an optional argument.  */
 220 DEF_PREDICTOR (PRED_FORTRAN_ABSENT_DUMMY, "Fortran absent dummy", \
 221                HITRATE (60), 0)
 222
 223 /* Fortran DO statement generates a pre-header guard:
 224    empty = (step > 0 ? to < from : to > from), which can be predicted
 225    to be very likely.  */
 226 DEF_PREDICTOR (PRED_FORTRAN_LOOP_PREHEADER, "Fortran loop preheader", \
 227                HITRATE (99), 0)