gcc/predict.def

   1 /* Definitions for the branch prediction routines in the GNU compiler.
   2    Copyright (C) 2001-2024 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 /* 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_UNINITIALIZED,
  58                PRED_FLAG_FIRST_MATCH)
  59
  60 /* Hints provided by user via __builtin_expect_with_probability.  */
  61 DEF_PREDICTOR (PRED_BUILTIN_EXPECT_WITH_PROBABILITY,
  62                "__builtin_expect_with_probability", PROB_UNINITIALIZED,
  63                PRED_FLAG_FIRST_MATCH)
  64
  65 /* Assume that any given atomic operation has low contention,
  66    and thus the compare-and-swap operation succeeds. */
  67 DEF_PREDICTOR (PRED_COMPARE_AND_SWAP, "compare and swap", PROB_VERY_LIKELY,
  68                PRED_FLAG_FIRST_MATCH)
  69
  70 /* Hints dropped by user via __builtin_expect feature.  Note: the
  71    probability of PROB_VERY_LIKELY is now overwritten by param
  72    builtin_expect_probability with a default value of HITRATE(90).
  73    Refer to param.def for details.  */
  74 DEF_PREDICTOR (PRED_BUILTIN_EXPECT, "__builtin_expect", PROB_VERY_LIKELY,
  75                PRED_FLAG_FIRST_MATCH)
  76
  77 /* Branches to hot labels are likely.  */
  78 DEF_PREDICTOR (PRED_HOT_LABEL, "hot label", HITRATE (90),
  79                PRED_FLAG_FIRST_MATCH)
  80
  81 /* Branches to cold labels are extremely unlikely.  */
  82 DEF_PREDICTOR (PRED_COLD_LABEL, "cold label", HITRATE (90),
  83                PRED_FLAG_FIRST_MATCH)
  84
  85 /* Return value of malloc function is almost always non-null.  */
  86 DEF_PREDICTOR (PRED_MALLOC_NONNULL, "malloc returned non-NULL", \
  87                PROB_VERY_LIKELY, PRED_FLAG_FIRST_MATCH)
  88
  89 /* Use number of loop iterations guessed by the contents of the loop.  */
  90 DEF_PREDICTOR (PRED_LOOP_ITERATIONS_GUESSED, "guessed loop iterations",
  91                PROB_UNINITIALIZED, PRED_FLAG_FIRST_MATCH)
  92
  93 /* Use number of loop iterations guessed by the contents of the loop.  */
  94 DEF_PREDICTOR (PRED_LOOP_ITERATIONS_MAX, "guessed loop iterations",
  95                PROB_UNINITIALIZED, PRED_FLAG_FIRST_MATCH)
  96
  97 /* Branch containing goto is probably not taken.  */
  98 DEF_PREDICTOR (PRED_CONTINUE, "continue", HITRATE (67), 0)
  99
 100 /* Branch to basic block containing call marked by noreturn attribute.  */
 101 DEF_PREDICTOR (PRED_NORETURN, "noreturn call", PROB_VERY_LIKELY,
 102                PRED_FLAG_FIRST_MATCH)
 103
 104 /* Branch to basic block containing call marked by cold function attribute.  */
 105 DEF_PREDICTOR (PRED_COLD_FUNCTION, "cold function call", PROB_VERY_LIKELY,
 106                PRED_FLAG_FIRST_MATCH)
 107
 108 /* Edge causing loop to terminate is probably not taken.  */
 109 DEF_PREDICTOR (PRED_LOOP_EXIT, "loop exit", HITRATE (89),
 110                PRED_FLAG_FIRST_MATCH)
 111
 112 /* Same as LOOP_EXIT but for loops containing recursive call.  */
 113 DEF_PREDICTOR (PRED_LOOP_EXIT_WITH_RECURSION, "loop exit with recursion",
 114                HITRATE (78), PRED_FLAG_FIRST_MATCH)
 115
 116 /* Edge causing loop to terminate by computing value used by later
 117    conditional.  */
 118 DEF_PREDICTOR (PRED_LOOP_EXTRA_EXIT, "extra loop exit", HITRATE (67),
 119                PRED_FLAG_FIRST_MATCH)
 120
 121 /* Pointers are usually not NULL.  */
 122 DEF_PREDICTOR (PRED_POINTER, "pointer", HITRATE (70), 0)
 123 DEF_PREDICTOR (PRED_TREE_POINTER, "pointer (on trees)", HITRATE (70), 0)
 124
 125 /* NE is probable, EQ not etc...  */
 126 DEF_PREDICTOR (PRED_OPCODE_POSITIVE, "opcode values positive", HITRATE (59), 0)
 127 DEF_PREDICTOR (PRED_OPCODE_NONEQUAL, "opcode values nonequal", HITRATE (66), 0)
 128 DEF_PREDICTOR (PRED_FPOPCODE, "fp_opcode", HITRATE (90), 0)
 129 DEF_PREDICTOR (PRED_TREE_OPCODE_POSITIVE, "opcode values positive (on trees)",
 130                HITRATE (59), 0)
 131 DEF_PREDICTOR (PRED_TREE_OPCODE_NONEQUAL, "opcode values nonequal (on trees)",
 132                HITRATE (66), 0)
 133 DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)", HITRATE (90), 0)
 134
 135 /* Branch guarding call is probably taken.  */
 136 DEF_PREDICTOR (PRED_CALL, "call", HITRATE (67), 0)
 137
 138 /* Call predictors are for now ignored, lets leave the predictor
 139    to measure its benefit.  */
 140 DEF_PREDICTOR (PRED_INDIR_CALL, "indirect call", PROB_EVEN, 0)
 141 DEF_PREDICTOR (PRED_POLYMORPHIC_CALL, "polymorphic call", PROB_EVEN, 0)
 142 DEF_PREDICTOR (PRED_RECURSIVE_CALL, "recursive call", PROB_EVEN, 0)
 143
 144 /* Branch causing function to terminate is probably not taken.  */
 145 DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (66),
 146                0)
 147
 148 /* Branch containing goto is probably not taken.  */
 149 DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (66), 0)
 150
 151 /* Branch ending with return constant is probably not taken.  */
 152 DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (65), 0)
 153
 154 /* Branch ending with return negative constant is probably not taken.  */
 155 DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (98), 0)
 156
 157 /* Branch ending with return; is probably not taken */
 158 DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (71), 0)
 159
 160 /* Branches to compare induction variable to a loop bound is
 161    extremely likely.  */
 162 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE_GUESS, "guess loop iv compare",
 163                HITRATE (64), 0)
 164
 165 /* Use number of loop iterations determined by # of iterations analysis
 166    to set probability of branches that compares IV to loop bound variable.  */
 167 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE, "loop iv compare", PROB_UNINITIALIZED,
 168                PRED_FLAG_FIRST_MATCH)
 169
 170 /* In the following code
 171    for (loop1)
 172      if (cond)
 173        for (loop2)
 174          body;
 175    guess that cond is unlikely.  */
 176 DEF_PREDICTOR (PRED_LOOP_GUARD, "loop guard", HITRATE (73), 0)
 177
 178 /* Same but for loops containing recursion.  */
 179 DEF_PREDICTOR (PRED_LOOP_GUARD_WITH_RECURSION, "loop guard with recursion",
 180                HITRATE (85), 0)
 181
 182 /* The following predictors are used in Fortran. */
 183
 184 /* Branch leading to an integer overflow are extremely unlikely.  */
 185 DEF_PREDICTOR (PRED_FORTRAN_OVERFLOW, "Fortran overflow", PROB_ALWAYS,
 186                PRED_FLAG_FIRST_MATCH)
 187
 188 /* Branch leading to a failure status are unlikely.  This can occur for out
 189    of memory.  This predictor only occurs when the user explicitly asked
 190    for a return status.  By default, the code aborts,
 191    which is handled via PRED_NORETURN.  */
 192 DEF_PREDICTOR (PRED_FORTRAN_FAIL_ALLOC, "Fortran fail alloc",
 193                PROB_VERY_LIKELY, 0)
 194
 195 /* Predictor is used for an allocation of an already allocated memory or
 196    deallocating an already deallocated allocatable.  */
 197 DEF_PREDICTOR (PRED_FORTRAN_REALLOC, "Fortran repeated allocation/deallocation",
 198                PROB_LIKELY, 0)
 199
 200 /* Branch leading to an I/O failure status are unlikely.  This predictor is
 201    used for I/O failures such as for invalid unit numbers.  This predictor
 202    only occurs when the user explicitly asked for a return status.  By default,
 203    the code aborts, which is handled via PRED_NORETURN.  */
 204 DEF_PREDICTOR (PRED_FORTRAN_FAIL_IO, "Fortran fail IO", HITRATE (85), 0)
 205
 206 /* Branch leading to a run-time warning message which is printed only once
 207    are unlikely.  The print-warning branch itself can be likely or unlikely.  */
 208 DEF_PREDICTOR (PRED_FORTRAN_WARN_ONCE, "Fortran warn once", HITRATE (75), 0)
 209
 210 /* Branch belonging to a zero-sized array.  */
 211 DEF_PREDICTOR (PRED_FORTRAN_SIZE_ZERO, "Fortran zero-sized array", \
 212                HITRATE (99), 0)
 213
 214 /* Branch belonging to an invalid bound index, in a context where it is
 215    standard conform and well defined but rather pointless and, hence, rather
 216    unlikely to occur.  */
 217 DEF_PREDICTOR (PRED_FORTRAN_INVALID_BOUND, "Fortran invalid bound", \
 218                HITRATE (90), 0)
 219
 220 /* Branch belonging to the handling of absent optional arguments.  This
 221    predictor is used when an optional dummy argument, associated with an
 222    absent argument, is passed on as actual argument to another procedure,
 223    which in turn has an optional argument.  */
 224 DEF_PREDICTOR (PRED_FORTRAN_ABSENT_DUMMY, "Fortran absent dummy", \
 225                HITRATE (60), 0)
 226
 227 /* Fortran DO statement generates a pre-header guard:
 228    empty = (step > 0 ? to < from : to > from), which can be predicted
 229    to be very likely.  */
 230 DEF_PREDICTOR (PRED_FORTRAN_LOOP_PREHEADER, "Fortran loop preheader", \
 231                HITRATE (99), 0)
 232
 233 /* Fortran assumed size arrays can be non-contiguous, so they need
 234    to be repacked.  */
 235
 236 DEF_PREDICTOR (PRED_FORTRAN_CONTIGUOUS, "Fortran contiguous", \
 237                HITRATE (75), 0)
 238