gcc/predict.def

   1 /* Definitions for the branch prediction routines in the GNU compiler.
   2    Copyright (C) 2001-2018 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 br_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 /* Return value of malloc function is almost always non-null.  */
  55 DEF_PREDICTOR (PRED_MALLOC_NONNULL, "malloc returned non-NULL", \
  56                PROB_VERY_LIKELY, PRED_FLAG_FIRST_MATCH)
  57
  58 /* Use number of loop iterations determined by # of iterations
  59    analysis to set probability.  We don't want to use Dempster-Shaffer
  60    theory here, as the predictions is exact.  */
  61 DEF_PREDICTOR (PRED_LOOP_ITERATIONS, "loop iterations", PROB_UNINITIALIZED,
  62                PRED_FLAG_FIRST_MATCH)
  63
  64 /* Assume that any given atomic operation has low contention,
  65    and thus the compare-and-swap operation succeeds. */
  66 DEF_PREDICTOR (PRED_COMPARE_AND_SWAP, "compare and swap", PROB_VERY_LIKELY,
  67                PRED_FLAG_FIRST_MATCH)
  68
  69 /* Hints dropped by user via __builtin_expect feature.  Note: the
  70    probability of PROB_VERY_LIKELY is now overwritten by param
  71    builtin_expect_probability with a default value of HITRATE(90).
  72    Refer to param.def for details.  */
  73 DEF_PREDICTOR (PRED_BUILTIN_EXPECT, "__builtin_expect", PROB_VERY_LIKELY,
  74                PRED_FLAG_FIRST_MATCH)
  75
  76 /* Hints provided by user via __builtin_expect_with_probability.  */
  77 DEF_PREDICTOR (PRED_BUILTIN_EXPECT_WITH_PROBABILITY,
  78                "__builtin_expect_with_probability", PROB_UNINITIALIZED,
  79                PRED_FLAG_FIRST_MATCH)
  80
  81 /* Use number of loop iterations guessed by the contents of the loop.  */
  82 DEF_PREDICTOR (PRED_LOOP_ITERATIONS_GUESSED, "guessed loop iterations",
  83                PROB_UNINITIALIZED, PRED_FLAG_FIRST_MATCH)
  84
  85 /* Use number of loop iterations guessed by the contents of the loop.  */
  86 DEF_PREDICTOR (PRED_LOOP_ITERATIONS_MAX, "guessed loop iterations",
  87                PROB_UNINITIALIZED, PRED_FLAG_FIRST_MATCH)
  88
  89 /* Branch containing goto is probably not taken.  */
  90 DEF_PREDICTOR (PRED_CONTINUE, "continue", HITRATE (67), 0)
  91
  92 /* Branch to basic block containing call marked by noreturn attribute.  */
  93 DEF_PREDICTOR (PRED_NORETURN, "noreturn call", PROB_VERY_LIKELY,
  94                PRED_FLAG_FIRST_MATCH)
  95
  96 /* Branch to basic block containing call marked by cold function attribute.  */
  97 DEF_PREDICTOR (PRED_COLD_FUNCTION, "cold function call", PROB_VERY_LIKELY,
  98                PRED_FLAG_FIRST_MATCH)
  99
 100 /* Edge causing loop to terminate is probably not taken.  */
 101 DEF_PREDICTOR (PRED_LOOP_EXIT, "loop exit", HITRATE (89),
 102                PRED_FLAG_FIRST_MATCH)
 103
 104 /* Same as LOOP_EXIT but for loops containing recursive call.  */
 105 DEF_PREDICTOR (PRED_LOOP_EXIT_WITH_RECURSION, "loop exit with recursion",
 106                HITRATE (78), PRED_FLAG_FIRST_MATCH)
 107
 108 /* Edge causing loop to terminate by computing value used by later
 109    conditional.  */
 110 DEF_PREDICTOR (PRED_LOOP_EXTRA_EXIT, "extra loop exit", HITRATE (67),
 111                PRED_FLAG_FIRST_MATCH)
 112
 113 /* Pointers are usually not NULL.  */
 114 DEF_PREDICTOR (PRED_POINTER, "pointer", HITRATE (70), 0)
 115 DEF_PREDICTOR (PRED_TREE_POINTER, "pointer (on trees)", HITRATE (70), 0)
 116
 117 /* NE is probable, EQ not etc...  */
 118 DEF_PREDICTOR (PRED_OPCODE_POSITIVE, "opcode values positive", HITRATE (59), 0)
 119 DEF_PREDICTOR (PRED_OPCODE_NONEQUAL, "opcode values nonequal", HITRATE (66), 0)
 120 DEF_PREDICTOR (PRED_FPOPCODE, "fp_opcode", HITRATE (90), 0)
 121 DEF_PREDICTOR (PRED_TREE_OPCODE_POSITIVE, "opcode values positive (on trees)",
 122                HITRATE (59), 0)
 123 DEF_PREDICTOR (PRED_TREE_OPCODE_NONEQUAL, "opcode values nonequal (on trees)",
 124                HITRATE (66), 0)
 125 DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)", HITRATE (90), 0)
 126
 127 /* Branch guarding call is probably taken.  */
 128 DEF_PREDICTOR (PRED_CALL, "call", HITRATE (67), 0)
 129
 130 /* Call predictors are for now ignored, lets leave the predictor
 131    to measure its benefit.  */
 132 DEF_PREDICTOR (PRED_INDIR_CALL, "indirect call", PROB_EVEN, 0)
 133 DEF_PREDICTOR (PRED_POLYMORPHIC_CALL, "polymorphic call", PROB_EVEN, 0)
 134 DEF_PREDICTOR (PRED_RECURSIVE_CALL, "recursive call", PROB_EVEN, 0)
 135
 136 /* Branch causing function to terminate is probably not taken.  */
 137 DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (66),
 138                0)
 139
 140 /* Branch containing goto is probably not taken.  */
 141 DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (66), 0)
 142
 143 /* Branch ending with return constant is probably not taken.  */
 144 DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (65), 0)
 145
 146 /* Branch ending with return negative constant is probably not taken.  */
 147 DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (98), 0)
 148
 149 /* Branch ending with return; is probably not taken */
 150 DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (71), 0)
 151
 152 /* Branches to compare induction variable to a loop bound is
 153    extremely likely.  */
 154 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE_GUESS, "guess loop iv compare",
 155                HITRATE (64), 0)
 156
 157 /* Use number of loop iterations determined by # of iterations analysis
 158    to set probability of branches that compares IV to loop bound variable.  */
 159 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE, "loop iv compare", PROB_UNINITIALIZED,
 160                PRED_FLAG_FIRST_MATCH)
 161
 162 /* In the following code
 163    for (loop1)
 164      if (cond)
 165        for (loop2)
 166          body;
 167    guess that cond is unlikely.  */
 168 DEF_PREDICTOR (PRED_LOOP_GUARD, "loop guard", HITRATE (73), 0)
 169
 170 /* Same but for loops containing recursion.  */
 171 DEF_PREDICTOR (PRED_LOOP_GUARD_WITH_RECURSION, "loop guard with recursion",
 172                HITRATE (85), 0)
 173
 174 /* Branches to hot labels are likely.  */
 175 DEF_PREDICTOR (PRED_HOT_LABEL, "hot label", HITRATE (85), 0)
 176
 177 /* Branches to cold labels are extremely unlikely.  */
 178 DEF_PREDICTOR (PRED_COLD_LABEL, "cold label", PROB_VERY_LIKELY,
 179                PRED_FLAG_FIRST_MATCH)
 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",
 192                PROB_VERY_LIKELY, 0)
 193
 194 /* Predictor is used for an allocation of an already allocated memory or
 195    deallocating an already deallocated allocatable.  */
 196 DEF_PREDICTOR (PRED_FORTRAN_REALLOC, "Fortran repeated allocation/deallocation",
 197                PROB_LIKELY, 0)
 198
 199 /* Branch leading to an I/O failure status are unlikely.  This predictor is
 200    used for I/O failures such as for invalid unit numbers.  This predictor
 201    only occurs when the user explicitly asked for a return status.  By default,
 202    the code aborts, which is handled via PRED_NORETURN.  */
 203 DEF_PREDICTOR (PRED_FORTRAN_FAIL_IO, "Fortran fail IO", HITRATE (85), 0)
 204
 205 /* Branch leading to a run-time warning message which is printed only once
 206    are unlikely.  The print-warning branch itself can be likely or unlikely.  */
 207 DEF_PREDICTOR (PRED_FORTRAN_WARN_ONCE, "Fortran warn once", HITRATE (75), 0)
 208
 209 /* Branch belonging to a zero-sized array.  */
 210 DEF_PREDICTOR (PRED_FORTRAN_SIZE_ZERO, "Fortran zero-sized array", \
 211                HITRATE (99), 0)
 212
 213 /* Branch belonging to an invalid bound index, in a context where it is
 214    standard conform and well defined but rather pointless and, hence, rather
 215    unlikely to occur.  */
 216 DEF_PREDICTOR (PRED_FORTRAN_INVALID_BOUND, "Fortran invalid bound", \
 217                HITRATE (90), 0)
 218
 219 /* Branch belonging to the handling of absent optional arguments.  This
 220    predictor is used when an optional dummy argument, associated with an
 221    absent argument, is passed on as actual argument to another procedure,
 222    which in turn has an optional argument.  */
 223 DEF_PREDICTOR (PRED_FORTRAN_ABSENT_DUMMY, "Fortran absent dummy", \
 224                HITRATE (60), 0)
 225
 226 /* Fortran DO statement generates a pre-header guard:
 227    empty = (step > 0 ? to < from : to > from), which can be predicted
 228    to be very likely.  */
 229 DEF_PREDICTOR (PRED_FORTRAN_LOOP_PREHEADER, "Fortran loop preheader", \
 230                HITRATE (99), 0)