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 | /* Prediction which is an outcome of combining multiple value predictions. */ |
98 | DEF_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). */ |
104 | DEF_PREDICTOR (PRED_COMBINED_VALUE_PREDICTIONS_PHI, |
105 | "combined value predictions" , PROB_UNINITIALIZED, 0) |
106 | |
107 | /* Branch containing goto is probably not taken. */ |
108 | DEF_PREDICTOR (PRED_CONTINUE, "continue" , HITRATE (67), 0) |
109 | |
110 | /* Branch to basic block containing call marked by noreturn attribute. */ |
111 | DEF_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. */ |
115 | DEF_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. */ |
119 | DEF_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. */ |
123 | DEF_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. */ |
128 | DEF_PREDICTOR (, "extra loop exit" , HITRATE (67), |
129 | PRED_FLAG_FIRST_MATCH) |
130 | |
131 | /* Pointers are usually not NULL. */ |
132 | DEF_PREDICTOR (PRED_POINTER, "pointer" , HITRATE (70), 0) |
133 | DEF_PREDICTOR (PRED_TREE_POINTER, "pointer (on trees)" , HITRATE (70), 0) |
134 | |
135 | /* NE is probable, EQ not etc... */ |
136 | DEF_PREDICTOR (PRED_OPCODE_POSITIVE, "opcode values positive" , HITRATE (59), 0) |
137 | DEF_PREDICTOR (PRED_OPCODE_NONEQUAL, "opcode values nonequal" , HITRATE (66), 0) |
138 | DEF_PREDICTOR (PRED_FPOPCODE, "fp_opcode" , HITRATE (90), 0) |
139 | DEF_PREDICTOR (PRED_TREE_OPCODE_POSITIVE, "opcode values positive (on trees)" , |
140 | HITRATE (59), 0) |
141 | DEF_PREDICTOR (PRED_TREE_OPCODE_NONEQUAL, "opcode values nonequal (on trees)" , |
142 | HITRATE (66), 0) |
143 | DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)" , HITRATE (90), 0) |
144 | |
145 | /* Branch guarding call is probably taken. */ |
146 | DEF_PREDICTOR (PRED_CALL, "call" , HITRATE (67), 0) |
147 | |
148 | /* Call predictors are for now ignored, lets leave the predictor |
149 | to measure its benefit. */ |
150 | DEF_PREDICTOR (PRED_INDIR_CALL, "indirect call" , PROB_EVEN, 0) |
151 | DEF_PREDICTOR (PRED_POLYMORPHIC_CALL, "polymorphic call" , PROB_EVEN, 0) |
152 | DEF_PREDICTOR (PRED_RECURSIVE_CALL, "recursive call" , PROB_EVEN, 0) |
153 | |
154 | /* Branch causing function to terminate is probably not taken. */ |
155 | DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)" , HITRATE (66), |
156 | 0) |
157 | |
158 | /* Branch containing goto is probably not taken. */ |
159 | DEF_PREDICTOR (PRED_GOTO, "goto" , HITRATE (66), 0) |
160 | |
161 | /* Branch ending with return constant is probably not taken. */ |
162 | DEF_PREDICTOR (PRED_CONST_RETURN, "const return" , HITRATE (65), 0) |
163 | |
164 | /* Branch ending with return negative constant is probably not taken. */ |
165 | DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return" , HITRATE (98), 0) |
166 | |
167 | /* Branch ending with return; is probably not taken */ |
168 | DEF_PREDICTOR (PRED_NULL_RETURN, "null return" , HITRATE (71), 0) |
169 | |
170 | /* Branches to compare induction variable to a loop bound is |
171 | extremely likely. */ |
172 | DEF_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. */ |
177 | DEF_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. */ |
186 | DEF_PREDICTOR (PRED_LOOP_GUARD, "loop guard" , HITRATE (73), 0) |
187 | |
188 | /* Same but for loops containing recursion. */ |
189 | DEF_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. */ |
195 | DEF_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. */ |
202 | DEF_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. */ |
207 | DEF_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. */ |
214 | DEF_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. */ |
218 | DEF_PREDICTOR (PRED_FORTRAN_WARN_ONCE, "Fortran warn once" , HITRATE (75), 0) |
219 | |
220 | /* Branch belonging to a zero-sized array. */ |
221 | DEF_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. */ |
227 | DEF_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. */ |
234 | DEF_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. */ |
240 | DEF_PREDICTOR (, "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 | |
246 | DEF_PREDICTOR (PRED_FORTRAN_CONTIGUOUS, "Fortran contiguous" , \ |
247 | HITRATE (75), 0) |
248 | |
249 | |