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6de9cd9a | 1 | /* Language independent return value optimizations |
5624e564 | 2 | Copyright (C) 2004-2015 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" | |
c7131fb2 | 23 | #include "backend.h" |
40e23961 | 24 | #include "tree.h" |
c7131fb2 | 25 | #include "gimple.h" |
83685514 | 26 | #include "hard-reg-set.h" |
c7131fb2 AM |
27 | #include "ssa.h" |
28 | #include "alias.h" | |
29 | #include "fold-const.h" | |
cf835838 | 30 | #include "tree-pretty-print.h" |
2fb9a547 | 31 | #include "internal-fn.h" |
5be5c238 AM |
32 | #include "gimple-iterator.h" |
33 | #include "gimple-walk.h" | |
6de9cd9a DN |
34 | #include "tree-pass.h" |
35 | #include "langhooks.h" | |
40013784 SB |
36 | #include "flags.h" /* For "optimize" in gate_pass_return_slot. |
37 | FIXME: That should be up to the pass manager, | |
38 | but pass_nrv is not in pass_all_optimizations. */ | |
6de9cd9a DN |
39 | |
40 | /* This file implements return value optimizations for functions which | |
41 | return aggregate types. | |
42 | ||
43 | Basically this pass searches the function for return statements which | |
44 | return a local aggregate. When converted to RTL such statements will | |
45 | generate a copy from the local aggregate to final return value destination | |
46 | mandated by the target's ABI. | |
47 | ||
48 | That copy can often be avoided by directly constructing the return value | |
49 | into the final destination mandated by the target's ABI. | |
50 | ||
b8698a0f | 51 | This is basically a generic equivalent to the C++ front-end's |
6de9cd9a DN |
52 | Named Return Value optimization. */ |
53 | ||
11478306 | 54 | struct nrv_data_t |
6de9cd9a DN |
55 | { |
56 | /* This is the temporary (a VAR_DECL) which appears in all of | |
57 | this function's RETURN_EXPR statements. */ | |
58 | tree var; | |
59 | ||
1ea7e6ad | 60 | /* This is the function's RESULT_DECL. We will replace all occurrences |
6de9cd9a DN |
61 | of VAR with RESULT_DECL when we apply this optimization. */ |
62 | tree result; | |
4e3825db | 63 | int modified; |
6de9cd9a DN |
64 | }; |
65 | ||
66 | static tree finalize_nrv_r (tree *, int *, void *); | |
67 | ||
68 | /* Callback for the tree walker. | |
69 | ||
70 | If TP refers to a RETURN_EXPR, then set the expression being returned | |
71 | to nrv_data->result. | |
72 | ||
73 | If TP refers to nrv_data->var, then replace nrv_data->var with | |
74 | nrv_data->result. | |
75 | ||
76 | If we reach a node where we know all the subtrees are uninteresting, | |
77 | then set *WALK_SUBTREES to zero. */ | |
78 | ||
79 | static tree | |
80 | finalize_nrv_r (tree *tp, int *walk_subtrees, void *data) | |
81 | { | |
726a989a | 82 | struct walk_stmt_info *wi = (struct walk_stmt_info *) data; |
11478306 | 83 | struct nrv_data_t *dp = (struct nrv_data_t *) wi->info; |
6de9cd9a DN |
84 | |
85 | /* No need to walk into types. */ | |
86 | if (TYPE_P (*tp)) | |
87 | *walk_subtrees = 0; | |
500b9b49 | 88 | |
61ada8ae | 89 | /* Otherwise replace all occurrences of VAR with RESULT. */ |
6de9cd9a | 90 | else if (*tp == dp->var) |
4e3825db MM |
91 | { |
92 | *tp = dp->result; | |
93 | dp->modified = 1; | |
94 | } | |
6de9cd9a DN |
95 | |
96 | /* Keep iterating. */ | |
97 | return NULL_TREE; | |
98 | } | |
99 | ||
100 | /* Main entry point for return value optimizations. | |
101 | ||
102 | If this function always returns the same local variable, and that | |
103 | local variable is an aggregate type, then replace the variable with | |
104 | the function's DECL_RESULT. | |
105 | ||
106 | This is the equivalent of the C++ named return value optimization | |
107 | applied to optimized trees in a language independent form. If we | |
108 | ever encounter languages which prevent this kind of optimization, | |
109 | then we could either have the languages register the optimization or | |
110 | we could change the gating function to check the current language. */ | |
b8698a0f | 111 | |
be55bfe6 TS |
112 | namespace { |
113 | ||
114 | const pass_data pass_data_nrv = | |
115 | { | |
116 | GIMPLE_PASS, /* type */ | |
117 | "nrv", /* name */ | |
118 | OPTGROUP_NONE, /* optinfo_flags */ | |
be55bfe6 TS |
119 | TV_TREE_NRV, /* tv_id */ |
120 | ( PROP_ssa | PROP_cfg ), /* properties_required */ | |
121 | 0, /* properties_provided */ | |
122 | 0, /* properties_destroyed */ | |
123 | 0, /* todo_flags_start */ | |
124 | 0, /* todo_flags_finish */ | |
125 | }; | |
126 | ||
127 | class pass_nrv : public gimple_opt_pass | |
128 | { | |
129 | public: | |
130 | pass_nrv (gcc::context *ctxt) | |
131 | : gimple_opt_pass (pass_data_nrv, ctxt) | |
132 | {} | |
133 | ||
134 | /* opt_pass methods: */ | |
135 | virtual bool gate (function *) { return optimize > 0; } | |
136 | ||
137 | virtual unsigned int execute (function *); | |
138 | ||
139 | }; // class pass_nrv | |
140 | ||
141 | unsigned int | |
142 | pass_nrv::execute (function *fun) | |
6de9cd9a DN |
143 | { |
144 | tree result = DECL_RESULT (current_function_decl); | |
145 | tree result_type = TREE_TYPE (result); | |
146 | tree found = NULL; | |
147 | basic_block bb; | |
726a989a | 148 | gimple_stmt_iterator gsi; |
11478306 | 149 | struct nrv_data_t data; |
6de9cd9a DN |
150 | |
151 | /* If this function does not return an aggregate type in memory, then | |
152 | there is nothing to do. */ | |
153 | if (!aggregate_value_p (result, current_function_decl)) | |
c2924966 | 154 | return 0; |
6de9cd9a | 155 | |
12e19e05 JJ |
156 | /* If a GIMPLE type is returned in memory, finalize_nrv_r might create |
157 | non-GIMPLE. */ | |
158 | if (is_gimple_reg_type (result_type)) | |
159 | return 0; | |
160 | ||
7716876b JM |
161 | /* If the front end already did something like this, don't do it here. */ |
162 | if (DECL_NAME (result)) | |
163 | return 0; | |
164 | ||
f1c19648 RG |
165 | /* If the result has its address taken then it might be modified |
166 | by means not detected in the following loop. Bail out in this | |
167 | case. */ | |
168 | if (TREE_ADDRESSABLE (result)) | |
169 | return 0; | |
170 | ||
06208009 | 171 | /* Look through each block for assignments to the RESULT_DECL. */ |
be55bfe6 | 172 | FOR_EACH_BB_FN (bb, fun) |
6de9cd9a | 173 | { |
726a989a | 174 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) |
6de9cd9a | 175 | { |
726a989a RB |
176 | gimple stmt = gsi_stmt (gsi); |
177 | tree ret_val; | |
06208009 | 178 | |
538dd0b7 | 179 | if (greturn *return_stmt = dyn_cast <greturn *> (stmt)) |
6de9cd9a | 180 | { |
06208009 JM |
181 | /* In a function with an aggregate return value, the |
182 | gimplifier has changed all non-empty RETURN_EXPRs to | |
183 | return the RESULT_DECL. */ | |
538dd0b7 | 184 | ret_val = gimple_return_retval (return_stmt); |
726a989a RB |
185 | if (ret_val) |
186 | gcc_assert (ret_val == result); | |
06208009 | 187 | } |
7716876b JM |
188 | else if (gimple_has_lhs (stmt) |
189 | && gimple_get_lhs (stmt) == result) | |
06208009 | 190 | { |
726a989a RB |
191 | tree rhs; |
192 | ||
193 | if (!gimple_assign_copy_p (stmt)) | |
194 | return 0; | |
195 | ||
196 | rhs = gimple_assign_rhs1 (stmt); | |
06208009 JM |
197 | |
198 | /* Now verify that this return statement uses the same value | |
199 | as any previously encountered return statement. */ | |
200 | if (found != NULL) | |
201 | { | |
202 | /* If we found a return statement using a different variable | |
203 | than previous return statements, then we can not perform | |
204 | NRV optimizations. */ | |
726a989a | 205 | if (found != rhs) |
c2924966 | 206 | return 0; |
06208009 JM |
207 | } |
208 | else | |
726a989a | 209 | found = rhs; |
06208009 JM |
210 | |
211 | /* The returned value must be a local automatic variable of the | |
212 | same type and alignment as the function's result. */ | |
213 | if (TREE_CODE (found) != VAR_DECL | |
214 | || TREE_THIS_VOLATILE (found) | |
77ed3903 | 215 | || !auto_var_in_fn_p (found, current_function_decl) |
06208009 JM |
216 | || TREE_ADDRESSABLE (found) |
217 | || DECL_ALIGN (found) > DECL_ALIGN (result) | |
f4088621 | 218 | || !useless_type_conversion_p (result_type, |
f1c19648 | 219 | TREE_TYPE (found))) |
c2924966 | 220 | return 0; |
6de9cd9a | 221 | } |
7716876b | 222 | else if (gimple_has_lhs (stmt)) |
e31657e8 | 223 | { |
7716876b | 224 | tree addr = get_base_address (gimple_get_lhs (stmt)); |
b8698a0f | 225 | /* If there's any MODIFY of component of RESULT, |
e31657e8 SP |
226 | then bail out. */ |
227 | if (addr && addr == result) | |
228 | return 0; | |
229 | } | |
6de9cd9a DN |
230 | } |
231 | } | |
232 | ||
233 | if (!found) | |
c2924966 | 234 | return 0; |
6de9cd9a DN |
235 | |
236 | /* If dumping details, then note once and only the NRV replacement. */ | |
237 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
238 | { | |
239 | fprintf (dump_file, "NRV Replaced: "); | |
240 | print_generic_expr (dump_file, found, dump_flags); | |
241 | fprintf (dump_file, " with: "); | |
242 | print_generic_expr (dump_file, result, dump_flags); | |
243 | fprintf (dump_file, "\n"); | |
244 | } | |
245 | ||
246 | /* At this point we know that all the return statements return the | |
247 | same local which has suitable attributes for NRV. Copy debugging | |
7716876b JM |
248 | information from FOUND to RESULT if it will be useful. But don't set |
249 | DECL_ABSTRACT_ORIGIN to point at another function. */ | |
250 | if (!DECL_IGNORED_P (found) | |
251 | && !(DECL_ABSTRACT_ORIGIN (found) | |
252 | && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl)) | |
253 | { | |
254 | DECL_NAME (result) = DECL_NAME (found); | |
255 | DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found); | |
256 | DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found); | |
257 | } | |
258 | ||
f1c19648 | 259 | TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found); |
6de9cd9a DN |
260 | |
261 | /* Now walk through the function changing all references to VAR to be | |
262 | RESULT. */ | |
263 | data.var = found; | |
264 | data.result = result; | |
be55bfe6 | 265 | FOR_EACH_BB_FN (bb, fun) |
6de9cd9a | 266 | { |
726a989a | 267 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); ) |
06208009 | 268 | { |
726a989a | 269 | gimple stmt = gsi_stmt (gsi); |
06208009 | 270 | /* If this is a copy from VAR to RESULT, remove it. */ |
726a989a RB |
271 | if (gimple_assign_copy_p (stmt) |
272 | && gimple_assign_lhs (stmt) == result | |
273 | && gimple_assign_rhs1 (stmt) == found) | |
4e3825db MM |
274 | { |
275 | unlink_stmt_vdef (stmt); | |
276 | gsi_remove (&gsi, true); | |
3d3f2249 | 277 | release_defs (stmt); |
4e3825db | 278 | } |
06208009 JM |
279 | else |
280 | { | |
726a989a RB |
281 | struct walk_stmt_info wi; |
282 | memset (&wi, 0, sizeof (wi)); | |
283 | wi.info = &data; | |
4e3825db | 284 | data.modified = 0; |
726a989a | 285 | walk_gimple_op (stmt, finalize_nrv_r, &wi); |
4e3825db MM |
286 | if (data.modified) |
287 | update_stmt (stmt); | |
726a989a | 288 | gsi_next (&gsi); |
06208009 JM |
289 | } |
290 | } | |
6de9cd9a DN |
291 | } |
292 | ||
938650d8 JJ |
293 | SET_DECL_VALUE_EXPR (found, result); |
294 | DECL_HAS_VALUE_EXPR_P (found) = 1; | |
295 | ||
c2924966 | 296 | return 0; |
6de9cd9a DN |
297 | } |
298 | ||
27a4cd48 DM |
299 | } // anon namespace |
300 | ||
301 | gimple_opt_pass * | |
302 | make_pass_nrv (gcc::context *ctxt) | |
303 | { | |
304 | return new pass_nrv (ctxt); | |
305 | } | |
306 | ||
f0ce7858 JC |
307 | /* Determine (pessimistically) whether DEST is available for NRV |
308 | optimization, where DEST is expected to be the LHS of a modify | |
309 | expression where the RHS is a function returning an aggregate. | |
310 | ||
7f8ac3d7 | 311 | DEST is available if it is not clobbered or used by the call. */ |
f0ce7858 JC |
312 | |
313 | static bool | |
538dd0b7 | 314 | dest_safe_for_nrv_p (gcall *call) |
f0ce7858 | 315 | { |
12de6355 | 316 | tree dest = gimple_call_lhs (call); |
a2daf82c | 317 | |
12de6355 RG |
318 | dest = get_base_address (dest); |
319 | if (! dest) | |
a2daf82c JJ |
320 | return false; |
321 | ||
322 | if (TREE_CODE (dest) == SSA_NAME) | |
12de6355 | 323 | return true; |
a2daf82c | 324 | |
7f8ac3d7 RG |
325 | if (call_may_clobber_ref_p (call, dest) |
326 | || ref_maybe_used_by_stmt_p (call, dest)) | |
a2daf82c | 327 | return false; |
eee717aa | 328 | |
a2daf82c | 329 | return true; |
f0ce7858 JC |
330 | } |
331 | ||
726a989a | 332 | /* Walk through the function looking for GIMPLE_ASSIGNs with calls that |
fa47911c JM |
333 | return in memory on the RHS. For each of these, determine whether it is |
334 | safe to pass the address of the LHS as the return slot, and mark the | |
335 | call appropriately if so. | |
336 | ||
337 | The NRV shares the return slot with a local variable in the callee; this | |
338 | optimization shares the return slot with the target of the call within | |
339 | the caller. If the NRV is performed (which we can't know in general), | |
340 | this optimization is safe if the address of the target has not | |
341 | escaped prior to the call. If it has, modifications to the local | |
342 | variable will produce visible changes elsewhere, as in PR c++/19317. */ | |
343 | ||
27a4cd48 DM |
344 | namespace { |
345 | ||
346 | const pass_data pass_data_return_slot = | |
fa47911c | 347 | { |
27a4cd48 DM |
348 | GIMPLE_PASS, /* type */ |
349 | "retslot", /* name */ | |
350 | OPTGROUP_NONE, /* optinfo_flags */ | |
27a4cd48 DM |
351 | TV_NONE, /* tv_id */ |
352 | PROP_ssa, /* properties_required */ | |
353 | 0, /* properties_provided */ | |
354 | 0, /* properties_destroyed */ | |
355 | 0, /* todo_flags_start */ | |
356 | 0, /* todo_flags_finish */ | |
fa47911c | 357 | }; |
27a4cd48 DM |
358 | |
359 | class pass_return_slot : public gimple_opt_pass | |
360 | { | |
361 | public: | |
c3284718 RS |
362 | pass_return_slot (gcc::context *ctxt) |
363 | : gimple_opt_pass (pass_data_return_slot, ctxt) | |
27a4cd48 DM |
364 | {} |
365 | ||
366 | /* opt_pass methods: */ | |
be55bfe6 | 367 | virtual unsigned int execute (function *); |
27a4cd48 DM |
368 | |
369 | }; // class pass_return_slot | |
370 | ||
be55bfe6 TS |
371 | unsigned int |
372 | pass_return_slot::execute (function *fun) | |
373 | { | |
374 | basic_block bb; | |
375 | ||
376 | FOR_EACH_BB_FN (bb, fun) | |
377 | { | |
378 | gimple_stmt_iterator gsi; | |
379 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) | |
380 | { | |
538dd0b7 | 381 | gcall *stmt; |
be55bfe6 TS |
382 | bool slot_opt_p; |
383 | ||
538dd0b7 DM |
384 | stmt = dyn_cast <gcall *> (gsi_stmt (gsi)); |
385 | if (stmt | |
be55bfe6 TS |
386 | && gimple_call_lhs (stmt) |
387 | && !gimple_call_return_slot_opt_p (stmt) | |
388 | && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)), | |
389 | gimple_call_fndecl (stmt))) | |
390 | { | |
391 | /* Check if the location being assigned to is | |
392 | clobbered by the call. */ | |
393 | slot_opt_p = dest_safe_for_nrv_p (stmt); | |
394 | gimple_call_set_return_slot_opt (stmt, slot_opt_p); | |
395 | } | |
396 | } | |
397 | } | |
398 | return 0; | |
399 | } | |
400 | ||
27a4cd48 DM |
401 | } // anon namespace |
402 | ||
403 | gimple_opt_pass * | |
404 | make_pass_return_slot (gcc::context *ctxt) | |
405 | { | |
406 | return new pass_return_slot (ctxt); | |
407 | } |