Mercurial > hg > CbC > CbC_gcc
comparison gcc/tree-nrv.c @ 0:a06113de4d67
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author | kent <kent@cr.ie.u-ryukyu.ac.jp> |
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date | Fri, 17 Jul 2009 14:47:48 +0900 |
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children | 77e2b8dfacca |
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1 /* Language independent return value optimizations | |
2 Copyright (C) 2004, 2005, 2007, 2008 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 3, 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 COPYING3. If not see | |
18 <http://www.gnu.org/licenses/>. */ | |
19 | |
20 #include "config.h" | |
21 #include "system.h" | |
22 #include "coretypes.h" | |
23 #include "tm.h" | |
24 #include "tree.h" | |
25 #include "rtl.h" | |
26 #include "function.h" | |
27 #include "basic-block.h" | |
28 #include "expr.h" | |
29 #include "diagnostic.h" | |
30 #include "tree-flow.h" | |
31 #include "timevar.h" | |
32 #include "tree-dump.h" | |
33 #include "tree-pass.h" | |
34 #include "langhooks.h" | |
35 | |
36 /* This file implements return value optimizations for functions which | |
37 return aggregate types. | |
38 | |
39 Basically this pass searches the function for return statements which | |
40 return a local aggregate. When converted to RTL such statements will | |
41 generate a copy from the local aggregate to final return value destination | |
42 mandated by the target's ABI. | |
43 | |
44 That copy can often be avoided by directly constructing the return value | |
45 into the final destination mandated by the target's ABI. | |
46 | |
47 This is basically a generic equivalent to the C++ front-end's | |
48 Named Return Value optimization. */ | |
49 | |
50 struct nrv_data | |
51 { | |
52 /* This is the temporary (a VAR_DECL) which appears in all of | |
53 this function's RETURN_EXPR statements. */ | |
54 tree var; | |
55 | |
56 /* This is the function's RESULT_DECL. We will replace all occurrences | |
57 of VAR with RESULT_DECL when we apply this optimization. */ | |
58 tree result; | |
59 }; | |
60 | |
61 static tree finalize_nrv_r (tree *, int *, void *); | |
62 | |
63 /* Callback for the tree walker. | |
64 | |
65 If TP refers to a RETURN_EXPR, then set the expression being returned | |
66 to nrv_data->result. | |
67 | |
68 If TP refers to nrv_data->var, then replace nrv_data->var with | |
69 nrv_data->result. | |
70 | |
71 If we reach a node where we know all the subtrees are uninteresting, | |
72 then set *WALK_SUBTREES to zero. */ | |
73 | |
74 static tree | |
75 finalize_nrv_r (tree *tp, int *walk_subtrees, void *data) | |
76 { | |
77 struct walk_stmt_info *wi = (struct walk_stmt_info *) data; | |
78 struct nrv_data *dp = (struct nrv_data *) wi->info; | |
79 | |
80 /* No need to walk into types. */ | |
81 if (TYPE_P (*tp)) | |
82 *walk_subtrees = 0; | |
83 | |
84 /* Otherwise replace all occurrences of VAR with RESULT. */ | |
85 else if (*tp == dp->var) | |
86 *tp = dp->result; | |
87 | |
88 /* Keep iterating. */ | |
89 return NULL_TREE; | |
90 } | |
91 | |
92 /* Main entry point for return value optimizations. | |
93 | |
94 If this function always returns the same local variable, and that | |
95 local variable is an aggregate type, then replace the variable with | |
96 the function's DECL_RESULT. | |
97 | |
98 This is the equivalent of the C++ named return value optimization | |
99 applied to optimized trees in a language independent form. If we | |
100 ever encounter languages which prevent this kind of optimization, | |
101 then we could either have the languages register the optimization or | |
102 we could change the gating function to check the current language. */ | |
103 | |
104 static unsigned int | |
105 tree_nrv (void) | |
106 { | |
107 tree result = DECL_RESULT (current_function_decl); | |
108 tree result_type = TREE_TYPE (result); | |
109 tree found = NULL; | |
110 basic_block bb; | |
111 gimple_stmt_iterator gsi; | |
112 struct nrv_data data; | |
113 | |
114 /* If this function does not return an aggregate type in memory, then | |
115 there is nothing to do. */ | |
116 if (!aggregate_value_p (result, current_function_decl)) | |
117 return 0; | |
118 | |
119 /* If a GIMPLE type is returned in memory, finalize_nrv_r might create | |
120 non-GIMPLE. */ | |
121 if (is_gimple_reg_type (result_type)) | |
122 return 0; | |
123 | |
124 /* If the front end already did something like this, don't do it here. */ | |
125 if (DECL_NAME (result)) | |
126 return 0; | |
127 | |
128 /* Look through each block for assignments to the RESULT_DECL. */ | |
129 FOR_EACH_BB (bb) | |
130 { | |
131 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) | |
132 { | |
133 gimple stmt = gsi_stmt (gsi); | |
134 tree ret_val; | |
135 | |
136 if (gimple_code (stmt) == GIMPLE_RETURN) | |
137 { | |
138 /* In a function with an aggregate return value, the | |
139 gimplifier has changed all non-empty RETURN_EXPRs to | |
140 return the RESULT_DECL. */ | |
141 ret_val = gimple_return_retval (stmt); | |
142 if (ret_val) | |
143 gcc_assert (ret_val == result); | |
144 } | |
145 else if (gimple_has_lhs (stmt) | |
146 && gimple_get_lhs (stmt) == result) | |
147 { | |
148 tree rhs; | |
149 | |
150 if (!gimple_assign_copy_p (stmt)) | |
151 return 0; | |
152 | |
153 rhs = gimple_assign_rhs1 (stmt); | |
154 | |
155 /* Now verify that this return statement uses the same value | |
156 as any previously encountered return statement. */ | |
157 if (found != NULL) | |
158 { | |
159 /* If we found a return statement using a different variable | |
160 than previous return statements, then we can not perform | |
161 NRV optimizations. */ | |
162 if (found != rhs) | |
163 return 0; | |
164 } | |
165 else | |
166 found = rhs; | |
167 | |
168 /* The returned value must be a local automatic variable of the | |
169 same type and alignment as the function's result. */ | |
170 if (TREE_CODE (found) != VAR_DECL | |
171 || TREE_THIS_VOLATILE (found) | |
172 || DECL_CONTEXT (found) != current_function_decl | |
173 || TREE_STATIC (found) | |
174 || TREE_ADDRESSABLE (found) | |
175 || DECL_ALIGN (found) > DECL_ALIGN (result) | |
176 || !useless_type_conversion_p (result_type, | |
177 TREE_TYPE (found))) | |
178 return 0; | |
179 } | |
180 else if (gimple_has_lhs (stmt)) | |
181 { | |
182 tree addr = get_base_address (gimple_get_lhs (stmt)); | |
183 /* If there's any MODIFY of component of RESULT, | |
184 then bail out. */ | |
185 if (addr && addr == result) | |
186 return 0; | |
187 } | |
188 } | |
189 } | |
190 | |
191 if (!found) | |
192 return 0; | |
193 | |
194 /* If dumping details, then note once and only the NRV replacement. */ | |
195 if (dump_file && (dump_flags & TDF_DETAILS)) | |
196 { | |
197 fprintf (dump_file, "NRV Replaced: "); | |
198 print_generic_expr (dump_file, found, dump_flags); | |
199 fprintf (dump_file, " with: "); | |
200 print_generic_expr (dump_file, result, dump_flags); | |
201 fprintf (dump_file, "\n"); | |
202 } | |
203 | |
204 /* At this point we know that all the return statements return the | |
205 same local which has suitable attributes for NRV. Copy debugging | |
206 information from FOUND to RESULT if it will be useful. But don't set | |
207 DECL_ABSTRACT_ORIGIN to point at another function. */ | |
208 if (!DECL_IGNORED_P (found) | |
209 && !(DECL_ABSTRACT_ORIGIN (found) | |
210 && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl)) | |
211 { | |
212 DECL_NAME (result) = DECL_NAME (found); | |
213 DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found); | |
214 DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found); | |
215 } | |
216 | |
217 TREE_ADDRESSABLE (result) = TREE_ADDRESSABLE (found); | |
218 | |
219 /* Now walk through the function changing all references to VAR to be | |
220 RESULT. */ | |
221 data.var = found; | |
222 data.result = result; | |
223 FOR_EACH_BB (bb) | |
224 { | |
225 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); ) | |
226 { | |
227 gimple stmt = gsi_stmt (gsi); | |
228 /* If this is a copy from VAR to RESULT, remove it. */ | |
229 if (gimple_assign_copy_p (stmt) | |
230 && gimple_assign_lhs (stmt) == result | |
231 && gimple_assign_rhs1 (stmt) == found) | |
232 gsi_remove (&gsi, true); | |
233 else | |
234 { | |
235 struct walk_stmt_info wi; | |
236 memset (&wi, 0, sizeof (wi)); | |
237 wi.info = &data; | |
238 walk_gimple_op (stmt, finalize_nrv_r, &wi); | |
239 gsi_next (&gsi); | |
240 } | |
241 } | |
242 } | |
243 | |
244 /* FOUND is no longer used. Ensure it gets removed. */ | |
245 var_ann (found)->used = 0; | |
246 return 0; | |
247 } | |
248 | |
249 static bool | |
250 gate_pass_return_slot (void) | |
251 { | |
252 return optimize > 0; | |
253 } | |
254 | |
255 struct gimple_opt_pass pass_nrv = | |
256 { | |
257 { | |
258 GIMPLE_PASS, | |
259 "nrv", /* name */ | |
260 gate_pass_return_slot, /* gate */ | |
261 tree_nrv, /* execute */ | |
262 NULL, /* sub */ | |
263 NULL, /* next */ | |
264 0, /* static_pass_number */ | |
265 TV_TREE_NRV, /* tv_id */ | |
266 PROP_cfg, /* properties_required */ | |
267 0, /* properties_provided */ | |
268 0, /* properties_destroyed */ | |
269 0, /* todo_flags_start */ | |
270 TODO_dump_func | TODO_ggc_collect /* todo_flags_finish */ | |
271 } | |
272 }; | |
273 | |
274 /* Determine (pessimistically) whether DEST is available for NRV | |
275 optimization, where DEST is expected to be the LHS of a modify | |
276 expression where the RHS is a function returning an aggregate. | |
277 | |
278 We search for a base VAR_DECL and look to see if it is call clobbered. | |
279 Note that we could do better, for example, by | |
280 attempting to doing points-to analysis on INDIRECT_REFs. */ | |
281 | |
282 static bool | |
283 dest_safe_for_nrv_p (tree dest) | |
284 { | |
285 while (handled_component_p (dest)) | |
286 dest = TREE_OPERAND (dest, 0); | |
287 | |
288 if (! SSA_VAR_P (dest)) | |
289 return false; | |
290 | |
291 if (TREE_CODE (dest) == SSA_NAME) | |
292 dest = SSA_NAME_VAR (dest); | |
293 | |
294 if (is_call_used (dest)) | |
295 return false; | |
296 | |
297 return true; | |
298 } | |
299 | |
300 /* Walk through the function looking for GIMPLE_ASSIGNs with calls that | |
301 return in memory on the RHS. For each of these, determine whether it is | |
302 safe to pass the address of the LHS as the return slot, and mark the | |
303 call appropriately if so. | |
304 | |
305 The NRV shares the return slot with a local variable in the callee; this | |
306 optimization shares the return slot with the target of the call within | |
307 the caller. If the NRV is performed (which we can't know in general), | |
308 this optimization is safe if the address of the target has not | |
309 escaped prior to the call. If it has, modifications to the local | |
310 variable will produce visible changes elsewhere, as in PR c++/19317. */ | |
311 | |
312 static unsigned int | |
313 execute_return_slot_opt (void) | |
314 { | |
315 basic_block bb; | |
316 | |
317 FOR_EACH_BB (bb) | |
318 { | |
319 gimple_stmt_iterator gsi; | |
320 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) | |
321 { | |
322 gimple stmt = gsi_stmt (gsi); | |
323 bool slot_opt_p; | |
324 | |
325 if (is_gimple_call (stmt) | |
326 && gimple_call_lhs (stmt) | |
327 && !gimple_call_return_slot_opt_p (stmt) | |
328 && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)), | |
329 gimple_call_fndecl (stmt)) | |
330 ) | |
331 { | |
332 /* Check if the location being assigned to is | |
333 call-clobbered. */ | |
334 slot_opt_p = dest_safe_for_nrv_p (gimple_call_lhs (stmt)); | |
335 gimple_call_set_return_slot_opt (stmt, slot_opt_p); | |
336 } | |
337 } | |
338 } | |
339 return 0; | |
340 } | |
341 | |
342 struct gimple_opt_pass pass_return_slot = | |
343 { | |
344 { | |
345 GIMPLE_PASS, | |
346 "retslot", /* name */ | |
347 NULL, /* gate */ | |
348 execute_return_slot_opt, /* execute */ | |
349 NULL, /* sub */ | |
350 NULL, /* next */ | |
351 0, /* static_pass_number */ | |
352 0, /* tv_id */ | |
353 PROP_ssa | PROP_alias, /* properties_required */ | |
354 0, /* properties_provided */ | |
355 0, /* properties_destroyed */ | |
356 0, /* todo_flags_start */ | |
357 0 /* todo_flags_finish */ | |
358 } | |
359 }; |