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    FIXME: early return currently predicts code:
 133    int foo (int a)
 134    {
 135       if (a)
 136         bar();
 137       else
 138         bar2();
 139    }
 140    even though there is no return statement involved.  We probably want to track
 141    this from FE or retire the predictor.  */
 142 DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (54), 0)
 143
 144 /* Branch containing goto is probably not taken.
 145    FIXME: Currently not used.  */
 146 DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (70), 0)
 147
 148 /* Branch ending with return constant is probably not taken.  */
 149 DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (69), 0)
 150
 151 /* Branch ending with return negative constant is probably not taken.  */
 152 DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (98), 0)
 153
 154 /* Branch ending with return; is probably not taken */
 155 DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (91), 0)
 156
 157 /* Branches to compare induction variable to a loop bound is
 158    extremely likely.  */
 159 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE_GUESS, "guess loop iv compare",
 160                HITRATE (98), 0)
 161
 162 /* Use number of loop iterations determined by # of iterations analysis
 163    to set probability of branches that compares IV to loop bound variable.  */
 164 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE, "loop iv compare", PROB_VERY_LIKELY,
 165                PRED_FLAG_FIRST_MATCH)
 166
 167 /* In the following code
 168    for (loop1)
 169      if (cond)
 170        for (loop2)
 171          body;
 172    guess that cond is unlikely.  */
 173 DEF_PREDICTOR (PRED_LOOP_GUARD, "loop guard", HITRATE (66), 0)
 174
 175 /* Same but for loops containing recursion.  */
 176 DEF_PREDICTOR (PRED_LOOP_GUARD_WITH_RECURSION, "loop guard with recursion",
 177                HITRATE (85), 0)
 178
 179 /* Branches to hot labels are likely.  */
 180 DEF_PREDICTOR (PRED_HOT_LABEL, "hot label", HITRATE (85), 0)
 181
 182 /* Branches to cold labels are extremely unlikely.  */
 183 DEF_PREDICTOR (PRED_COLD_LABEL, "cold label", PROB_VERY_LIKELY,
 184                PRED_FLAG_FIRST_MATCH)
 185
 186
 187 /* The following predictors are used in Fortran. */
 188
 189 /* Branch leading to an integer overflow are extremely unlikely.  */
 190 DEF_PREDICTOR (PRED_FORTRAN_OVERFLOW, "Fortran overflow", PROB_ALWAYS,
 191                PRED_FLAG_FIRST_MATCH)
 192
 193 /* Branch leading to a failure status are unlikely.  This can occur for out
 194    of memory.  This predictor only occurs when the user explicitly asked
 195    for a return status.  By default, the code aborts,
 196    which is handled via PRED_NORETURN.  */
 197 DEF_PREDICTOR (PRED_FORTRAN_FAIL_ALLOC, "Fortran fail alloc", PROB_VERY_LIKELY, 0)
 198
 199 /* Predictor is used for an allocation of an already allocated memory or
 200    deallocating an already deallocated allocatable.  */
 201 DEF_PREDICTOR (PRED_FORTRAN_REALLOC, \
 202                "Fortran repeated allocation/deallocation", PROB_LIKELY, 0)
 203
 204 /* Branch leading to an I/O failure status are unlikely.  This predictor is
 205    used for I/O failures such as for invalid unit numbers.  This predictor
 206    only occurs when the user explicitly asked for a return status.  By default,
 207    the code aborts, which is handled via PRED_NORETURN.  */
 208 DEF_PREDICTOR (PRED_FORTRAN_FAIL_IO, "Fortran fail IO", HITRATE (85), 0)
 209
 210 /* Branch leading to a run-time warning message which is printed only once
 211    are unlikely.  The print-warning branch itself can be likely or unlikely.  */
 212 DEF_PREDICTOR (PRED_FORTRAN_WARN_ONCE, "Fortran warn once", HITRATE (75), 0)
 213
 214 /* Branch belonging to a zero-sized array.  */
 215 DEF_PREDICTOR (PRED_FORTRAN_SIZE_ZERO, "Fortran zero-sized array", \
 216                HITRATE (99), 0)
 217
 218 /* Branch belonging to an invalid bound index, in a context where it is
 219    standard conform and well defined but rather pointless and, hence, rather
 220    unlikely to occur.  */
 221 DEF_PREDICTOR (PRED_FORTRAN_INVALID_BOUND, "Fortran invalid bound", \
 222                HITRATE (90), 0)
 223
 224 /* Branch belonging to the handling of absent optional arguments.  This
 225    predictor is used when an optional dummy argument, associated with an
 226    absent argument, is passed on as actual argument to another procedure,
 227    which in turn has an optional argument.  */
 228 DEF_PREDICTOR (PRED_FORTRAN_ABSENT_DUMMY, "Fortran absent dummy", \
 229                HITRATE (60), 0)
 230
 231 /* Fortran DO statement generates a pre-header guard:
 232    empty = (step > 0 ? to < from : to > from), which can be predicted
 233    to be very likely.  */
 234 DEF_PREDICTOR (PRED_FORTRAN_LOOP_PREHEADER, "Fortran loop preheader", \
 235                HITRATE (99), 0)