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