1/* Definitions for the branch prediction routines in the GNU compiler.
2 Copyright (C) 2001-2024 Free Software Foundation, Inc.
3
4This file is part of GCC.
5
6GCC is free software; you can redistribute it and/or modify it under
7the terms of the GNU General Public License as published by the Free
8Software Foundation; either version 3, or (at your option) any later
9version.
10
11GCC is distributed in the hope that it will be useful, but WITHOUT ANY
12WARRANTY; without even the implied warranty of MERCHANTABILITY or
13FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14for more details.
15
16You should have received a copy of the GNU General Public License
17along 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. */
38DEF_PREDICTOR (PRED_COMBINED, "combined", PROB_ALWAYS, 0)
39
40/* An outcome estimated by Dempster-Shaffer theory. */
41DEF_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. */
45DEF_PREDICTOR (PRED_FIRST_MATCH, "first match", PROB_ALWAYS, 0)
46
47/* Heuristic applying when no heuristic below applies. */
48DEF_PREDICTOR (PRED_NO_PREDICTION, "no prediction", PROB_ALWAYS, 0)
49
50/* Mark unconditional jump as taken. */
51DEF_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. */
57DEF_PREDICTOR (PRED_LOOP_ITERATIONS, "loop iterations", PROB_UNINITIALIZED,
58 PRED_FLAG_FIRST_MATCH)
59
60/* Hints provided by user via __builtin_expect_with_probability. */
61DEF_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. */
67DEF_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. */
74DEF_PREDICTOR (PRED_BUILTIN_EXPECT, "__builtin_expect", PROB_VERY_LIKELY,
75 PRED_FLAG_FIRST_MATCH)
76
77/* Branches to hot labels are likely. */
78DEF_PREDICTOR (PRED_HOT_LABEL, "hot label", HITRATE (90),
79 PRED_FLAG_FIRST_MATCH)
80
81/* Branches to cold labels are extremely unlikely. */
82DEF_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. */
86DEF_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. */
90DEF_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. */
94DEF_PREDICTOR (PRED_LOOP_ITERATIONS_MAX, "guessed loop iterations",
95 PROB_UNINITIALIZED, PRED_FLAG_FIRST_MATCH)
96
97/* Prediction which is an outcome of combining multiple value predictions. */
98DEF_PREDICTOR (PRED_COMBINED_VALUE_PREDICTIONS,
99 "combined value predictions", PROB_UNINITIALIZED, 0)
100
101/* Prediction which is an outcome of combining multiple value predictions
102 on PHI statement (this is less accurate since we do not know reverse
103 edge probabilities at that time). */
104DEF_PREDICTOR (PRED_COMBINED_VALUE_PREDICTIONS_PHI,
105 "combined value predictions", PROB_UNINITIALIZED, 0)
106
107/* Branch containing goto is probably not taken. */
108DEF_PREDICTOR (PRED_CONTINUE, "continue", HITRATE (67), 0)
109
110/* Branch to basic block containing call marked by noreturn attribute. */
111DEF_PREDICTOR (PRED_NORETURN, "noreturn call", PROB_VERY_LIKELY,
112 PRED_FLAG_FIRST_MATCH)
113
114/* Branch to basic block containing call marked by cold function attribute. */
115DEF_PREDICTOR (PRED_COLD_FUNCTION, "cold function call", PROB_VERY_LIKELY,
116 PRED_FLAG_FIRST_MATCH)
117
118/* Edge causing loop to terminate is probably not taken. */
119DEF_PREDICTOR (PRED_LOOP_EXIT, "loop exit", HITRATE (89),
120 PRED_FLAG_FIRST_MATCH)
121
122/* Same as LOOP_EXIT but for loops containing recursive call. */
123DEF_PREDICTOR (PRED_LOOP_EXIT_WITH_RECURSION, "loop exit with recursion",
124 HITRATE (78), PRED_FLAG_FIRST_MATCH)
125
126/* Edge causing loop to terminate by computing value used by later
127 conditional. */
128DEF_PREDICTOR (PRED_LOOP_EXTRA_EXIT, "extra loop exit", HITRATE (67),
129 PRED_FLAG_FIRST_MATCH)
130
131/* Pointers are usually not NULL. */
132DEF_PREDICTOR (PRED_POINTER, "pointer", HITRATE (70), 0)
133DEF_PREDICTOR (PRED_TREE_POINTER, "pointer (on trees)", HITRATE (70), 0)
134
135/* NE is probable, EQ not etc... */
136DEF_PREDICTOR (PRED_OPCODE_POSITIVE, "opcode values positive", HITRATE (59), 0)
137DEF_PREDICTOR (PRED_OPCODE_NONEQUAL, "opcode values nonequal", HITRATE (66), 0)
138DEF_PREDICTOR (PRED_FPOPCODE, "fp_opcode", HITRATE (90), 0)
139DEF_PREDICTOR (PRED_TREE_OPCODE_POSITIVE, "opcode values positive (on trees)",
140 HITRATE (59), 0)
141DEF_PREDICTOR (PRED_TREE_OPCODE_NONEQUAL, "opcode values nonequal (on trees)",
142 HITRATE (66), 0)
143DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)", HITRATE (90), 0)
144
145/* Branch guarding call is probably taken. */
146DEF_PREDICTOR (PRED_CALL, "call", HITRATE (67), 0)
147
148/* Call predictors are for now ignored, lets leave the predictor
149 to measure its benefit. */
150DEF_PREDICTOR (PRED_INDIR_CALL, "indirect call", PROB_EVEN, 0)
151DEF_PREDICTOR (PRED_POLYMORPHIC_CALL, "polymorphic call", PROB_EVEN, 0)
152DEF_PREDICTOR (PRED_RECURSIVE_CALL, "recursive call", PROB_EVEN, 0)
153
154/* Branch causing function to terminate is probably not taken. */
155DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (66),
156 0)
157
158/* Branch containing goto is probably not taken. */
159DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (66), 0)
160
161/* Branch ending with return constant is probably not taken. */
162DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (65), 0)
163
164/* Branch ending with return negative constant is probably not taken. */
165DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (98), 0)
166
167/* Branch ending with return; is probably not taken */
168DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (71), 0)
169
170/* Branches to compare induction variable to a loop bound is
171 extremely likely. */
172DEF_PREDICTOR (PRED_LOOP_IV_COMPARE_GUESS, "guess loop iv compare",
173 HITRATE (64), 0)
174
175/* Use number of loop iterations determined by # of iterations analysis
176 to set probability of branches that compares IV to loop bound variable. */
177DEF_PREDICTOR (PRED_LOOP_IV_COMPARE, "loop iv compare", PROB_UNINITIALIZED,
178 PRED_FLAG_FIRST_MATCH)
179
180/* In the following code
181 for (loop1)
182 if (cond)
183 for (loop2)
184 body;
185 guess that cond is unlikely. */
186DEF_PREDICTOR (PRED_LOOP_GUARD, "loop guard", HITRATE (73), 0)
187
188/* Same but for loops containing recursion. */
189DEF_PREDICTOR (PRED_LOOP_GUARD_WITH_RECURSION, "loop guard with recursion",
190 HITRATE (85), 0)
191
192/* The following predictors are used in Fortran. */
193
194/* Branch leading to an integer overflow are extremely unlikely. */
195DEF_PREDICTOR (PRED_FORTRAN_OVERFLOW, "Fortran overflow", PROB_ALWAYS,
196 PRED_FLAG_FIRST_MATCH)
197
198/* Branch leading to a failure status are unlikely. This can occur for out
199 of memory. This predictor only occurs when the user explicitly asked
200 for a return status. By default, the code aborts,
201 which is handled via PRED_NORETURN. */
202DEF_PREDICTOR (PRED_FORTRAN_FAIL_ALLOC, "Fortran fail alloc",
203 PROB_VERY_LIKELY, 0)
204
205/* Predictor is used for an allocation of an already allocated memory or
206 deallocating an already deallocated allocatable. */
207DEF_PREDICTOR (PRED_FORTRAN_REALLOC, "Fortran repeated allocation/deallocation",
208 PROB_LIKELY, 0)
209
210/* Branch leading to an I/O failure status are unlikely. This predictor is
211 used for I/O failures such as for invalid unit numbers. This predictor
212 only occurs when the user explicitly asked for a return status. By default,
213 the code aborts, which is handled via PRED_NORETURN. */
214DEF_PREDICTOR (PRED_FORTRAN_FAIL_IO, "Fortran fail IO", HITRATE (85), 0)
215
216/* Branch leading to a run-time warning message which is printed only once
217 are unlikely. The print-warning branch itself can be likely or unlikely. */
218DEF_PREDICTOR (PRED_FORTRAN_WARN_ONCE, "Fortran warn once", HITRATE (75), 0)
219
220/* Branch belonging to a zero-sized array. */
221DEF_PREDICTOR (PRED_FORTRAN_SIZE_ZERO, "Fortran zero-sized array", \
222 HITRATE (99), 0)
223
224/* Branch belonging to an invalid bound index, in a context where it is
225 standard conform and well defined but rather pointless and, hence, rather
226 unlikely to occur. */
227DEF_PREDICTOR (PRED_FORTRAN_INVALID_BOUND, "Fortran invalid bound", \
228 HITRATE (90), 0)
229
230/* Branch belonging to the handling of absent optional arguments. This
231 predictor is used when an optional dummy argument, associated with an
232 absent argument, is passed on as actual argument to another procedure,
233 which in turn has an optional argument. */
234DEF_PREDICTOR (PRED_FORTRAN_ABSENT_DUMMY, "Fortran absent dummy", \
235 HITRATE (60), 0)
236
237/* Fortran DO statement generates a pre-header guard:
238 empty = (step > 0 ? to < from : to > from), which can be predicted
239 to be very likely. */
240DEF_PREDICTOR (PRED_FORTRAN_LOOP_PREHEADER, "Fortran loop preheader", \
241 HITRATE (99), 0)
242
243/* Fortran assumed size arrays can be non-contiguous, so they need
244 to be repacked. */
245
246DEF_PREDICTOR (PRED_FORTRAN_CONTIGUOUS, "Fortran contiguous", \
247 HITRATE (75), 0)
248
249

source code of gcc/predict.def