1 /* Definitions for the branch prediction routines in the GNU compiler.
2 Copyright (C) 2001-2016 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 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 /* 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_ALWAYS,
58 PRED_FLAG_FIRST_MATCH)
60 /* Assume that any given atomic operation has low contention,
61 and thus the compare-and-swap operation succeeds. */
62 DEF_PREDICTOR (PRED_COMPARE_AND_SWAP, "compare and swap", PROB_VERY_LIKELY,
63 PRED_FLAG_FIRST_MATCH)
65 /* Hints dropped by user via __builtin_expect feature. Note: the
66 probability of PROB_VERY_LIKELY is now overwritten by param
67 builtin_expect_probability with a default value of HITRATE(90).
68 Refer to param.def for details. */
69 DEF_PREDICTOR (PRED_BUILTIN_EXPECT, "__builtin_expect", PROB_VERY_LIKELY,
70 PRED_FLAG_FIRST_MATCH)
72 /* Use number of loop iterations guessed by the contents of the loop. */
73 DEF_PREDICTOR (PRED_LOOP_ITERATIONS_GUESSED, "guessed loop iterations",
74 PROB_ALWAYS, PRED_FLAG_FIRST_MATCH)
76 /* Branch containing goto is probably not taken. */
77 DEF_PREDICTOR (PRED_CONTINUE, "continue", HITRATE (50), 0)
79 /* Branch to basic block containing call marked by noreturn attribute. */
80 DEF_PREDICTOR (PRED_NORETURN, "noreturn call", PROB_VERY_LIKELY,
81 PRED_FLAG_FIRST_MATCH)
83 /* Branch to basic block containing call marked by cold function attribute. */
84 DEF_PREDICTOR (PRED_COLD_FUNCTION, "cold function call", PROB_VERY_LIKELY,
85 PRED_FLAG_FIRST_MATCH)
87 /* Loopback edge is taken. */
88 DEF_PREDICTOR (PRED_LOOP_BRANCH, "loop branch", HITRATE (86),
89 PRED_FLAG_FIRST_MATCH)
91 /* Edge causing loop to terminate is probably not taken. */
92 DEF_PREDICTOR (PRED_LOOP_EXIT, "loop exit", HITRATE (91),
93 PRED_FLAG_FIRST_MATCH)
95 /* Edge causing loop to terminate by computing value used by later conditional.
97 DEF_PREDICTOR (PRED_LOOP_EXTRA_EXIT, "extra loop exit", HITRATE (91),
98 PRED_FLAG_FIRST_MATCH)
100 /* Pointers are usually not NULL. */
101 DEF_PREDICTOR (PRED_POINTER, "pointer", HITRATE (85), 0)
102 DEF_PREDICTOR (PRED_TREE_POINTER, "pointer (on trees)", HITRATE (85), 0)
104 /* NE is probable, EQ not etc... */
105 DEF_PREDICTOR (PRED_OPCODE_POSITIVE, "opcode values positive", HITRATE (79), 0)
106 DEF_PREDICTOR (PRED_OPCODE_NONEQUAL, "opcode values nonequal", HITRATE (71), 0)
107 DEF_PREDICTOR (PRED_FPOPCODE, "fp_opcode", HITRATE (90), 0)
108 DEF_PREDICTOR (PRED_TREE_OPCODE_POSITIVE, "opcode values positive (on trees)", HITRATE (73), 0)
109 DEF_PREDICTOR (PRED_TREE_OPCODE_NONEQUAL, "opcode values nonequal (on trees)", HITRATE (72), 0)
110 DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)", HITRATE (90), 0)
112 /* Branch guarding call is probably taken. */
113 DEF_PREDICTOR (PRED_CALL, "call", HITRATE (71), 0)
115 /* Branch causing function to terminate is probably not taken. */
116 DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (61), 0)
118 /* Branch containing goto is probably not taken. */
119 DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (70), 0)
121 /* Branch ending with return constant is probably not taken. */
122 DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (67), 0)
124 /* Branch ending with return negative constant is probably not taken. */
125 DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (96), 0)
127 /* Branch ending with return; is probably not taken */
128 DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (90), 0)
130 /* Branches to compare induction variable to a loop bound is
132 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE_GUESS, "guess loop iv compare",
135 /* Use number of loop iterations determined by # of iterations analysis
136 to set probability of branches that compares IV to loop bound variable. */
137 DEF_PREDICTOR (PRED_LOOP_IV_COMPARE, "loop iv compare", PROB_VERY_LIKELY,
138 PRED_FLAG_FIRST_MATCH)
140 /* Branches to hot labels are likely. */
141 DEF_PREDICTOR (PRED_HOT_LABEL, "hot label", HITRATE (85), 0)
143 /* Branches to cold labels are extremely unlikely. */
144 DEF_PREDICTOR (PRED_COLD_LABEL, "cold label", PROB_VERY_LIKELY,
145 PRED_FLAG_FIRST_MATCH)
148 /* The following predictors are used in Fortran. */
150 /* Branch leading to an integer overflow are extremely unlikely. */
151 DEF_PREDICTOR (PRED_FORTRAN_OVERFLOW, "overflow", PROB_ALWAYS,
152 PRED_FLAG_FIRST_MATCH)
154 /* Branch leading to a failure status are unlikely. This can occur for out
155 of memory or when trying to allocate an already allocated allocated or
156 deallocating an already deallocated allocatable. This predictor only
157 occurs when the user explicitly asked for a return status. By default,
158 the code aborts, which is handled via PRED_NORETURN. */
159 DEF_PREDICTOR (PRED_FORTRAN_FAIL_ALLOC, "fail alloc", PROB_VERY_LIKELY, 0)
161 /* Branch leading to an I/O failure status are unlikely. This predictor is
162 used for I/O failures such as for invalid unit numbers. This predictor
163 only occurs when the user explicitly asked for a return status. By default,
164 the code aborts, which is handled via PRED_NORETURN. */
165 DEF_PREDICTOR (PRED_FORTRAN_FAIL_IO, "fail alloc", HITRATE(85), 0)
167 /* Branch leading to a run-time warning message which is printed only once
168 are unlikely. The print-warning branch itself can be likely or unlikely. */
169 DEF_PREDICTOR (PRED_FORTRAN_WARN_ONCE, "warn once", HITRATE (75), 0)
171 /* Branch belonging to a zero-sized array. */
172 DEF_PREDICTOR (PRED_FORTRAN_SIZE_ZERO, "zero-sized array", HITRATE(70), 0)
174 /* Branch belonging to an invalid bound index, in a context where it is
175 standard conform and well defined but rather pointless and, hence, rather
176 unlikely to occur. */
177 DEF_PREDICTOR (PRED_FORTRAN_INVALID_BOUND, "zero-sized array", HITRATE(90), 0)
179 /* Branch belonging to the handling of absent optional arguments. This
180 predictor is used when an optional dummy argument, associated with an
181 absent argument, is passed on as actual argument to another procedure,
182 which in turn has an optional argument. */
183 DEF_PREDICTOR (PRED_FORTRAN_ABSENT_DUMMY, "absent dummy", HITRATE(60), 0)