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6de9cd9a | 1 | /* Language independent return value optimizations |
7072a650 | 2 | Copyright (C) 2004, 2005, 2007, 2008, 2009 Free Software Foundation, Inc. |
6de9cd9a DN |
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 | |
9dcd6f09 | 8 | the Free Software Foundation; either version 3, or (at your option) |
6de9cd9a DN |
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 | |
9dcd6f09 NC |
17 | along with GCC; see the file COPYING3. If not see |
18 | <http://www.gnu.org/licenses/>. */ | |
6de9cd9a DN |
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 | ||
b8698a0f | 47 | This is basically a generic equivalent to the C++ front-end's |
6de9cd9a DN |
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 | ||
1ea7e6ad | 56 | /* This is the function's RESULT_DECL. We will replace all occurrences |
6de9cd9a DN |
57 | of VAR with RESULT_DECL when we apply this optimization. */ |
58 | tree result; | |
4e3825db | 59 | int modified; |
6de9cd9a DN |
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 | { | |
726a989a RB |
78 | struct walk_stmt_info *wi = (struct walk_stmt_info *) data; |
79 | struct nrv_data *dp = (struct nrv_data *) wi->info; | |
6de9cd9a DN |
80 | |
81 | /* No need to walk into types. */ | |
82 | if (TYPE_P (*tp)) | |
83 | *walk_subtrees = 0; | |
500b9b49 | 84 | |
61ada8ae | 85 | /* Otherwise replace all occurrences of VAR with RESULT. */ |
6de9cd9a | 86 | else if (*tp == dp->var) |
4e3825db MM |
87 | { |
88 | *tp = dp->result; | |
89 | dp->modified = 1; | |
90 | } | |
6de9cd9a DN |
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. */ | |
b8698a0f | 107 | |
c2924966 | 108 | static unsigned int |
6de9cd9a DN |
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; | |
726a989a | 115 | gimple_stmt_iterator gsi; |
6de9cd9a DN |
116 | struct nrv_data data; |
117 | ||
118 | /* If this function does not return an aggregate type in memory, then | |
119 | there is nothing to do. */ | |
120 | if (!aggregate_value_p (result, current_function_decl)) | |
c2924966 | 121 | return 0; |
6de9cd9a | 122 | |
12e19e05 JJ |
123 | /* If a GIMPLE type is returned in memory, finalize_nrv_r might create |
124 | non-GIMPLE. */ | |
125 | if (is_gimple_reg_type (result_type)) | |
126 | return 0; | |
127 | ||
7716876b JM |
128 | /* If the front end already did something like this, don't do it here. */ |
129 | if (DECL_NAME (result)) | |
130 | return 0; | |
131 | ||
06208009 | 132 | /* Look through each block for assignments to the RESULT_DECL. */ |
6de9cd9a DN |
133 | FOR_EACH_BB (bb) |
134 | { | |
726a989a | 135 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) |
6de9cd9a | 136 | { |
726a989a RB |
137 | gimple stmt = gsi_stmt (gsi); |
138 | tree ret_val; | |
06208009 | 139 | |
726a989a | 140 | if (gimple_code (stmt) == GIMPLE_RETURN) |
6de9cd9a | 141 | { |
06208009 JM |
142 | /* In a function with an aggregate return value, the |
143 | gimplifier has changed all non-empty RETURN_EXPRs to | |
144 | return the RESULT_DECL. */ | |
726a989a RB |
145 | ret_val = gimple_return_retval (stmt); |
146 | if (ret_val) | |
147 | gcc_assert (ret_val == result); | |
06208009 | 148 | } |
7716876b JM |
149 | else if (gimple_has_lhs (stmt) |
150 | && gimple_get_lhs (stmt) == result) | |
06208009 | 151 | { |
726a989a RB |
152 | tree rhs; |
153 | ||
154 | if (!gimple_assign_copy_p (stmt)) | |
155 | return 0; | |
156 | ||
157 | rhs = gimple_assign_rhs1 (stmt); | |
06208009 JM |
158 | |
159 | /* Now verify that this return statement uses the same value | |
160 | as any previously encountered return statement. */ | |
161 | if (found != NULL) | |
162 | { | |
163 | /* If we found a return statement using a different variable | |
164 | than previous return statements, then we can not perform | |
165 | NRV optimizations. */ | |
726a989a | 166 | if (found != rhs) |
c2924966 | 167 | return 0; |
06208009 JM |
168 | } |
169 | else | |
726a989a | 170 | found = rhs; |
06208009 JM |
171 | |
172 | /* The returned value must be a local automatic variable of the | |
173 | same type and alignment as the function's result. */ | |
174 | if (TREE_CODE (found) != VAR_DECL | |
175 | || TREE_THIS_VOLATILE (found) | |
176 | || DECL_CONTEXT (found) != current_function_decl | |
177 | || TREE_STATIC (found) | |
178 | || TREE_ADDRESSABLE (found) | |
179 | || DECL_ALIGN (found) > DECL_ALIGN (result) | |
f4088621 RG |
180 | || !useless_type_conversion_p (result_type, |
181 | TREE_TYPE (found))) | |
c2924966 | 182 | return 0; |
6de9cd9a | 183 | } |
7716876b | 184 | else if (gimple_has_lhs (stmt)) |
e31657e8 | 185 | { |
7716876b | 186 | tree addr = get_base_address (gimple_get_lhs (stmt)); |
b8698a0f | 187 | /* If there's any MODIFY of component of RESULT, |
e31657e8 SP |
188 | then bail out. */ |
189 | if (addr && addr == result) | |
190 | return 0; | |
191 | } | |
6de9cd9a DN |
192 | } |
193 | } | |
194 | ||
195 | if (!found) | |
c2924966 | 196 | return 0; |
6de9cd9a DN |
197 | |
198 | /* If dumping details, then note once and only the NRV replacement. */ | |
199 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
200 | { | |
201 | fprintf (dump_file, "NRV Replaced: "); | |
202 | print_generic_expr (dump_file, found, dump_flags); | |
203 | fprintf (dump_file, " with: "); | |
204 | print_generic_expr (dump_file, result, dump_flags); | |
205 | fprintf (dump_file, "\n"); | |
206 | } | |
207 | ||
208 | /* At this point we know that all the return statements return the | |
209 | same local which has suitable attributes for NRV. Copy debugging | |
7716876b JM |
210 | information from FOUND to RESULT if it will be useful. But don't set |
211 | DECL_ABSTRACT_ORIGIN to point at another function. */ | |
212 | if (!DECL_IGNORED_P (found) | |
213 | && !(DECL_ABSTRACT_ORIGIN (found) | |
214 | && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl)) | |
215 | { | |
216 | DECL_NAME (result) = DECL_NAME (found); | |
217 | DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found); | |
218 | DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found); | |
219 | } | |
220 | ||
6de9cd9a DN |
221 | TREE_ADDRESSABLE (result) = TREE_ADDRESSABLE (found); |
222 | ||
223 | /* Now walk through the function changing all references to VAR to be | |
224 | RESULT. */ | |
225 | data.var = found; | |
226 | data.result = result; | |
227 | FOR_EACH_BB (bb) | |
228 | { | |
726a989a | 229 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); ) |
06208009 | 230 | { |
726a989a | 231 | gimple stmt = gsi_stmt (gsi); |
06208009 | 232 | /* If this is a copy from VAR to RESULT, remove it. */ |
726a989a RB |
233 | if (gimple_assign_copy_p (stmt) |
234 | && gimple_assign_lhs (stmt) == result | |
235 | && gimple_assign_rhs1 (stmt) == found) | |
4e3825db MM |
236 | { |
237 | unlink_stmt_vdef (stmt); | |
238 | gsi_remove (&gsi, true); | |
239 | } | |
06208009 JM |
240 | else |
241 | { | |
726a989a RB |
242 | struct walk_stmt_info wi; |
243 | memset (&wi, 0, sizeof (wi)); | |
244 | wi.info = &data; | |
4e3825db | 245 | data.modified = 0; |
726a989a | 246 | walk_gimple_op (stmt, finalize_nrv_r, &wi); |
4e3825db MM |
247 | if (data.modified) |
248 | update_stmt (stmt); | |
726a989a | 249 | gsi_next (&gsi); |
06208009 JM |
250 | } |
251 | } | |
6de9cd9a DN |
252 | } |
253 | ||
254 | /* FOUND is no longer used. Ensure it gets removed. */ | |
255 | var_ann (found)->used = 0; | |
c2924966 | 256 | return 0; |
6de9cd9a DN |
257 | } |
258 | ||
c72321c9 JH |
259 | static bool |
260 | gate_pass_return_slot (void) | |
261 | { | |
262 | return optimize > 0; | |
263 | } | |
264 | ||
b8698a0f | 265 | struct gimple_opt_pass pass_nrv = |
6de9cd9a | 266 | { |
8ddbbcae JH |
267 | { |
268 | GIMPLE_PASS, | |
6de9cd9a | 269 | "nrv", /* name */ |
c72321c9 | 270 | gate_pass_return_slot, /* gate */ |
6de9cd9a DN |
271 | tree_nrv, /* execute */ |
272 | NULL, /* sub */ | |
273 | NULL, /* next */ | |
274 | 0, /* static_pass_number */ | |
275 | TV_TREE_NRV, /* tv_id */ | |
4e3825db | 276 | PROP_ssa | PROP_cfg, /* properties_required */ |
6de9cd9a DN |
277 | 0, /* properties_provided */ |
278 | 0, /* properties_destroyed */ | |
279 | 0, /* todo_flags_start */ | |
8ddbbcae JH |
280 | TODO_dump_func | TODO_ggc_collect /* todo_flags_finish */ |
281 | } | |
6de9cd9a | 282 | }; |
fa47911c | 283 | |
f0ce7858 JC |
284 | /* Determine (pessimistically) whether DEST is available for NRV |
285 | optimization, where DEST is expected to be the LHS of a modify | |
286 | expression where the RHS is a function returning an aggregate. | |
287 | ||
5611cf0b RG |
288 | We search for a base VAR_DECL and look to see if it is call clobbered. |
289 | Note that we could do better, for example, by | |
f0ce7858 JC |
290 | attempting to doing points-to analysis on INDIRECT_REFs. */ |
291 | ||
292 | static bool | |
293 | dest_safe_for_nrv_p (tree dest) | |
294 | { | |
a2daf82c JJ |
295 | while (handled_component_p (dest)) |
296 | dest = TREE_OPERAND (dest, 0); | |
297 | ||
298 | if (! SSA_VAR_P (dest)) | |
299 | return false; | |
300 | ||
301 | if (TREE_CODE (dest) == SSA_NAME) | |
302 | dest = SSA_NAME_VAR (dest); | |
303 | ||
115340c7 | 304 | if (is_call_used (dest)) |
a2daf82c | 305 | return false; |
eee717aa | 306 | |
a2daf82c | 307 | return true; |
f0ce7858 JC |
308 | } |
309 | ||
726a989a | 310 | /* Walk through the function looking for GIMPLE_ASSIGNs with calls that |
fa47911c JM |
311 | return in memory on the RHS. For each of these, determine whether it is |
312 | safe to pass the address of the LHS as the return slot, and mark the | |
313 | call appropriately if so. | |
314 | ||
315 | The NRV shares the return slot with a local variable in the callee; this | |
316 | optimization shares the return slot with the target of the call within | |
317 | the caller. If the NRV is performed (which we can't know in general), | |
318 | this optimization is safe if the address of the target has not | |
319 | escaped prior to the call. If it has, modifications to the local | |
320 | variable will produce visible changes elsewhere, as in PR c++/19317. */ | |
321 | ||
c2924966 | 322 | static unsigned int |
fa47911c JM |
323 | execute_return_slot_opt (void) |
324 | { | |
325 | basic_block bb; | |
326 | ||
327 | FOR_EACH_BB (bb) | |
328 | { | |
726a989a RB |
329 | gimple_stmt_iterator gsi; |
330 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) | |
fa47911c | 331 | { |
726a989a RB |
332 | gimple stmt = gsi_stmt (gsi); |
333 | bool slot_opt_p; | |
334 | ||
335 | if (is_gimple_call (stmt) | |
336 | && gimple_call_lhs (stmt) | |
337 | && !gimple_call_return_slot_opt_p (stmt) | |
338 | && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)), | |
339 | gimple_call_fndecl (stmt)) | |
340 | ) | |
341 | { | |
342 | /* Check if the location being assigned to is | |
343 | call-clobbered. */ | |
344 | slot_opt_p = dest_safe_for_nrv_p (gimple_call_lhs (stmt)); | |
345 | gimple_call_set_return_slot_opt (stmt, slot_opt_p); | |
346 | } | |
fa47911c JM |
347 | } |
348 | } | |
c2924966 | 349 | return 0; |
fa47911c JM |
350 | } |
351 | ||
b8698a0f | 352 | struct gimple_opt_pass pass_return_slot = |
fa47911c | 353 | { |
8ddbbcae JH |
354 | { |
355 | GIMPLE_PASS, | |
fa47911c JM |
356 | "retslot", /* name */ |
357 | NULL, /* gate */ | |
358 | execute_return_slot_opt, /* execute */ | |
359 | NULL, /* sub */ | |
360 | NULL, /* next */ | |
361 | 0, /* static_pass_number */ | |
7072a650 | 362 | TV_NONE, /* tv_id */ |
4effdf02 | 363 | PROP_ssa, /* properties_required */ |
fa47911c JM |
364 | 0, /* properties_provided */ |
365 | 0, /* properties_destroyed */ | |
366 | 0, /* todo_flags_start */ | |
8ddbbcae JH |
367 | 0 /* todo_flags_finish */ |
368 | } | |
fa47911c | 369 | }; |