1/* Definitions for the branch prediction routines in the GNU compiler.
2 Copyright (C) 2001-2023 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/* Branch containing goto is probably not taken. */
98DEF_PREDICTOR (PRED_CONTINUE, "continue", HITRATE (67), 0)
99
100/* Branch to basic block containing call marked by noreturn attribute. */
101DEF_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. */
105DEF_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. */
109DEF_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. */
113DEF_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. */
118DEF_PREDICTOR (PRED_LOOP_EXTRA_EXIT, "extra loop exit", HITRATE (67),
119 PRED_FLAG_FIRST_MATCH)
120
121/* Pointers are usually not NULL. */
122DEF_PREDICTOR (PRED_POINTER, "pointer", HITRATE (70), 0)
123DEF_PREDICTOR (PRED_TREE_POINTER, "pointer (on trees)", HITRATE (70), 0)
124
125/* NE is probable, EQ not etc... */
126DEF_PREDICTOR (PRED_OPCODE_POSITIVE, "opcode values positive", HITRATE (59), 0)
127DEF_PREDICTOR (PRED_OPCODE_NONEQUAL, "opcode values nonequal", HITRATE (66), 0)
128DEF_PREDICTOR (PRED_FPOPCODE, "fp_opcode", HITRATE (90), 0)
129DEF_PREDICTOR (PRED_TREE_OPCODE_POSITIVE, "opcode values positive (on trees)",
130 HITRATE (59), 0)
131DEF_PREDICTOR (PRED_TREE_OPCODE_NONEQUAL, "opcode values nonequal (on trees)",
132 HITRATE (66), 0)
133DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)", HITRATE (90), 0)
134
135/* Branch guarding call is probably taken. */
136DEF_PREDICTOR (PRED_CALL, "call", HITRATE (67), 0)
137
138/* Call predictors are for now ignored, lets leave the predictor
139 to measure its benefit. */
140DEF_PREDICTOR (PRED_INDIR_CALL, "indirect call", PROB_EVEN, 0)
141DEF_PREDICTOR (PRED_POLYMORPHIC_CALL, "polymorphic call", PROB_EVEN, 0)
142DEF_PREDICTOR (PRED_RECURSIVE_CALL, "recursive call", PROB_EVEN, 0)
143
144/* Branch causing function to terminate is probably not taken. */
145DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (66),
146 0)
147
148/* Branch containing goto is probably not taken. */
149DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (66), 0)
150
151/* Branch ending with return constant is probably not taken. */
152DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (65), 0)
153
154/* Branch ending with return negative constant is probably not taken. */
155DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (98), 0)
156
157/* Branch ending with return; is probably not taken */
158DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (71), 0)
159
160/* Branches to compare induction variable to a loop bound is
161 extremely likely. */
162DEF_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. */
167DEF_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. */
176DEF_PREDICTOR (PRED_LOOP_GUARD, "loop guard", HITRATE (73), 0)
177
178/* Same but for loops containing recursion. */
179DEF_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. */
185DEF_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. */
192DEF_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. */
197DEF_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. */
204DEF_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. */
208DEF_PREDICTOR (PRED_FORTRAN_WARN_ONCE, "Fortran warn once", HITRATE (75), 0)
209
210/* Branch belonging to a zero-sized array. */
211DEF_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. */
217DEF_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. */
224DEF_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. */
230DEF_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
236DEF_PREDICTOR (PRED_FORTRAN_CONTIGUOUS, "Fortran contiguous", \
237 HITRATE (75), 0)
238
239

source code of gcc/predict.def