1 /* Definitions for the branch prediction routines in the GNU compiler.
2 Copyright (C) 2001-2018 Free Software Foundation, Inc.
4 This file is part of GCC.
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
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
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/>. */
20 /* Before including this file, you should define a macro:
22 DEF_PREDICTOR (ENUM, NAME, HITRATE)
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.
30 NAME is used in the debugging output to determine predictor type.
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). */
37 /* A value used as final outcome of all heuristics. */
38 DEF_PREDICTOR (PRED_COMBINED, "combined", PROB_ALWAYS, 0)
40 /* An outcome estimated by Dempster-Shaffer theory. */
41 DEF_PREDICTOR (PRED_DS_THEORY, "DS theory", PROB_ALWAYS, 0)
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)
47 /* Heuristic applying when no heuristic below applies. */
48 DEF_PREDICTOR (PRED_NO_PREDICTION, "no prediction", PROB_ALWAYS, 0)
50 /* Mark unconditional jump as taken. */
51 DEF_PREDICTOR (PRED_UNCONDITIONAL, "unconditional jump", PROB_ALWAYS,
52 PRED_FLAG_FIRST_MATCH)
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)
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)
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)
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)
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)
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)
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)
89 /* Branch containing goto is probably not taken. */
90 DEF_PREDICTOR (PRED_CONTINUE, "continue", HITRATE (67), 0)
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)
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)
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)
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)
108 /* Edge causing loop to terminate by computing value used by later
110 DEF_PREDICTOR (PRED_LOOP_EXTRA_EXIT, "extra loop exit", HITRATE (67),
111 PRED_FLAG_FIRST_MATCH)
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)
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)",
123 DEF_PREDICTOR (PRED_TREE_OPCODE_NONEQUAL, "opcode values nonequal (on trees)",
125 DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)", HITRATE (90), 0)
127 /* Branch guarding call is probably taken. */
128 DEF_PREDICTOR (PRED_CALL, "call", HITRATE (67), 0)
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)
136 /* Branch causing function to terminate is probably not taken. */
137 DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (66),
140 /* Branch containing goto is probably not taken. */
141 DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (66), 0)
143 /* Branch ending with return constant is probably not taken. */
144 DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (65), 0)
146 /* Branch ending with return negative constant is probably not taken. */
147 DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (98), 0)
149 /* Branch ending with return; is probably not taken */
150 DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (71), 0)
152 /* Branches to compare induction variable to a loop bound is
154 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE_GUESS, "guess loop iv compare",
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)
162 /* In the following code
167 guess that cond is unlikely. */
168 DEF_PREDICTOR (PRED_LOOP_GUARD, "loop guard", HITRATE (73), 0)
170 /* Same but for loops containing recursion. */
171 DEF_PREDICTOR (PRED_LOOP_GUARD_WITH_RECURSION, "loop guard with recursion",
174 /* Branches to hot labels are likely. */
175 DEF_PREDICTOR (PRED_HOT_LABEL, "hot label", HITRATE (85), 0)
177 /* Branches to cold labels are extremely unlikely. */
178 DEF_PREDICTOR (PRED_COLD_LABEL, "cold label", PROB_VERY_LIKELY,
179 PRED_FLAG_FIRST_MATCH)
181 /* The following predictors are used in Fortran. */
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)
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",
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",
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)
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)
209 /* Branch belonging to a zero-sized array. */
210 DEF_PREDICTOR (PRED_FORTRAN_SIZE_ZERO, "Fortran zero-sized array", \
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", \
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", \
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", \