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6de9cd9a DN |
1 | /* Language independent return value optimizations |
2 | Copyright (C) 2004 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 | |
7 | it under the terms of the GNU General Public License as published by | |
8 | the Free Software Foundation; either version 2, or (at your option) | |
9 | any later version. | |
10 | ||
11 | GCC is distributed in the hope that it will be useful, | |
12 | but WITHOUT ANY WARRANTY; without even the implied warranty of | |
13 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
14 | GNU General Public License for more details. | |
15 | ||
16 | You should have received a copy of the GNU General Public License | |
17 | along with GCC; see the file COPYING. If not, write to | |
18 | the Free Software Foundation, 59 Temple Place - Suite 330, | |
19 | Boston, MA 02111-1307, USA. */ | |
20 | ||
21 | #include "config.h" | |
22 | #include "system.h" | |
23 | #include "coretypes.h" | |
24 | #include "tm.h" | |
25 | #include "tree.h" | |
26 | #include "rtl.h" | |
27 | #include "function.h" | |
28 | #include "basic-block.h" | |
29 | #include "expr.h" | |
30 | #include "diagnostic.h" | |
31 | #include "tree-flow.h" | |
32 | #include "timevar.h" | |
33 | #include "tree-dump.h" | |
34 | #include "tree-pass.h" | |
35 | #include "langhooks.h" | |
36 | ||
37 | /* This file implements return value optimizations for functions which | |
38 | return aggregate types. | |
39 | ||
40 | Basically this pass searches the function for return statements which | |
41 | return a local aggregate. When converted to RTL such statements will | |
42 | generate a copy from the local aggregate to final return value destination | |
43 | mandated by the target's ABI. | |
44 | ||
45 | That copy can often be avoided by directly constructing the return value | |
46 | into the final destination mandated by the target's ABI. | |
47 | ||
48 | This is basically a generic equivalent to the C++ front-end's | |
49 | Named Return Value optimization. */ | |
50 | ||
51 | struct nrv_data | |
52 | { | |
53 | /* This is the temporary (a VAR_DECL) which appears in all of | |
54 | this function's RETURN_EXPR statements. */ | |
55 | tree var; | |
56 | ||
1ea7e6ad | 57 | /* This is the function's RESULT_DECL. We will replace all occurrences |
6de9cd9a DN |
58 | of VAR with RESULT_DECL when we apply this optimization. */ |
59 | tree result; | |
60 | }; | |
61 | ||
62 | static tree finalize_nrv_r (tree *, int *, void *); | |
63 | ||
64 | /* Callback for the tree walker. | |
65 | ||
66 | If TP refers to a RETURN_EXPR, then set the expression being returned | |
67 | to nrv_data->result. | |
68 | ||
69 | If TP refers to nrv_data->var, then replace nrv_data->var with | |
70 | nrv_data->result. | |
71 | ||
72 | If we reach a node where we know all the subtrees are uninteresting, | |
73 | then set *WALK_SUBTREES to zero. */ | |
74 | ||
75 | static tree | |
76 | finalize_nrv_r (tree *tp, int *walk_subtrees, void *data) | |
77 | { | |
78 | struct nrv_data *dp = (struct nrv_data *)data; | |
79 | ||
80 | /* No need to walk into types. */ | |
81 | if (TYPE_P (*tp)) | |
82 | *walk_subtrees = 0; | |
500b9b49 RK |
83 | |
84 | /* If this is a RETURN_EXPR, set the expression being returned to RESULT. */ | |
6de9cd9a DN |
85 | else if (TREE_CODE (*tp) == RETURN_EXPR) |
86 | TREE_OPERAND (*tp, 0) = dp->result; | |
500b9b49 | 87 | |
61ada8ae | 88 | /* Otherwise replace all occurrences of VAR with RESULT. */ |
6de9cd9a DN |
89 | else if (*tp == dp->var) |
90 | *tp = dp->result; | |
91 | ||
92 | /* Keep iterating. */ | |
93 | return NULL_TREE; | |
94 | } | |
95 | ||
96 | /* Main entry point for return value optimizations. | |
97 | ||
98 | If this function always returns the same local variable, and that | |
99 | local variable is an aggregate type, then replace the variable with | |
100 | the function's DECL_RESULT. | |
101 | ||
102 | This is the equivalent of the C++ named return value optimization | |
103 | applied to optimized trees in a language independent form. If we | |
104 | ever encounter languages which prevent this kind of optimization, | |
105 | then we could either have the languages register the optimization or | |
106 | we could change the gating function to check the current language. */ | |
107 | ||
108 | static void | |
109 | tree_nrv (void) | |
110 | { | |
111 | tree result = DECL_RESULT (current_function_decl); | |
112 | tree result_type = TREE_TYPE (result); | |
113 | tree found = NULL; | |
114 | basic_block bb; | |
115 | struct nrv_data data; | |
116 | ||
117 | /* If this function does not return an aggregate type in memory, then | |
118 | there is nothing to do. */ | |
119 | if (!aggregate_value_p (result, current_function_decl)) | |
120 | return; | |
121 | ||
122 | /* Look through each block for suitable return expressions. RETURN_EXPRs | |
123 | end basic blocks, so we only have to look at the last statement in | |
124 | each block. That makes this very fast. */ | |
125 | FOR_EACH_BB (bb) | |
126 | { | |
127 | tree stmt = last_stmt (bb); | |
128 | ||
129 | if (stmt && TREE_CODE (stmt) == RETURN_EXPR) | |
130 | { | |
131 | tree ret_expr = TREE_OPERAND (stmt, 0); | |
132 | ||
133 | /* This probably should not happen, but just to be safe do | |
134 | not perform NRV optimizations if only some of the return | |
135 | statement return a value. */ | |
136 | if (!ret_expr | |
137 | || TREE_CODE (ret_expr) != MODIFY_EXPR | |
138 | || TREE_CODE (TREE_OPERAND (ret_expr, 0)) != RESULT_DECL) | |
139 | return; | |
140 | ||
141 | /* Now verify that this return statement uses the same value | |
142 | as any previously encountered return statement. */ | |
143 | if (found != NULL) | |
144 | { | |
145 | /* If we found a return statement using a different variable | |
146 | than previous return statements, then we can not perform | |
147 | NRV optimizations. */ | |
148 | if (found != TREE_OPERAND (ret_expr, 1)) | |
149 | return; | |
150 | } | |
151 | else | |
152 | found = TREE_OPERAND (ret_expr, 1); | |
153 | ||
154 | /* The returned value must be a local automatic variable of the | |
155 | same type and alignment as the function's result. */ | |
156 | if (TREE_CODE (found) != VAR_DECL | |
e79b60a7 | 157 | || TREE_THIS_VOLATILE (found) |
6de9cd9a DN |
158 | || DECL_CONTEXT (found) != current_function_decl |
159 | || TREE_STATIC (found) | |
160 | || TREE_ADDRESSABLE (found) | |
161 | || DECL_ALIGN (found) > DECL_ALIGN (result) | |
162 | || !lang_hooks.types_compatible_p (TREE_TYPE (found), | |
163 | result_type)) | |
164 | return; | |
165 | } | |
166 | } | |
167 | ||
168 | if (!found) | |
169 | return; | |
170 | ||
171 | /* If dumping details, then note once and only the NRV replacement. */ | |
172 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
173 | { | |
174 | fprintf (dump_file, "NRV Replaced: "); | |
175 | print_generic_expr (dump_file, found, dump_flags); | |
176 | fprintf (dump_file, " with: "); | |
177 | print_generic_expr (dump_file, result, dump_flags); | |
178 | fprintf (dump_file, "\n"); | |
179 | } | |
180 | ||
181 | /* At this point we know that all the return statements return the | |
182 | same local which has suitable attributes for NRV. Copy debugging | |
183 | information from FOUND to RESULT. */ | |
184 | DECL_NAME (result) = DECL_NAME (found); | |
185 | DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found); | |
186 | DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found); | |
187 | TREE_ADDRESSABLE (result) = TREE_ADDRESSABLE (found); | |
188 | ||
189 | /* Now walk through the function changing all references to VAR to be | |
190 | RESULT. */ | |
191 | data.var = found; | |
192 | data.result = result; | |
193 | FOR_EACH_BB (bb) | |
194 | { | |
195 | block_stmt_iterator bsi; | |
196 | ||
197 | for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi)) | |
198 | walk_tree (bsi_stmt_ptr (bsi), finalize_nrv_r, &data, 0); | |
199 | } | |
200 | ||
201 | /* FOUND is no longer used. Ensure it gets removed. */ | |
202 | var_ann (found)->used = 0; | |
203 | } | |
204 | ||
205 | struct tree_opt_pass pass_nrv = | |
206 | { | |
207 | "nrv", /* name */ | |
208 | NULL, /* gate */ | |
209 | tree_nrv, /* execute */ | |
210 | NULL, /* sub */ | |
211 | NULL, /* next */ | |
212 | 0, /* static_pass_number */ | |
213 | TV_TREE_NRV, /* tv_id */ | |
214 | PROP_cfg, /* properties_required */ | |
215 | 0, /* properties_provided */ | |
216 | 0, /* properties_destroyed */ | |
217 | 0, /* todo_flags_start */ | |
9f8628ba PB |
218 | TODO_dump_func | TODO_ggc_collect, /* todo_flags_finish */ |
219 | 0 /* letter */ | |
6de9cd9a | 220 | }; |