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6de9cd9a | 1 | /* Language independent return value optimizations |
23a5b65a | 2 | Copyright (C) 2004-2014 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" | |
6de9cd9a DN |
25 | #include "function.h" |
26 | #include "basic-block.h" | |
cf835838 | 27 | #include "tree-pretty-print.h" |
2fb9a547 AM |
28 | #include "tree-ssa-alias.h" |
29 | #include "internal-fn.h" | |
30 | #include "gimple-expr.h" | |
31 | #include "is-a.h" | |
442b4905 | 32 | #include "gimple.h" |
5be5c238 AM |
33 | #include "gimple-iterator.h" |
34 | #include "gimple-walk.h" | |
442b4905 | 35 | #include "gimple-ssa.h" |
d8a2d370 | 36 | #include "stringpool.h" |
442b4905 | 37 | #include "tree-ssanames.h" |
6de9cd9a DN |
38 | #include "tree-pass.h" |
39 | #include "langhooks.h" | |
40013784 SB |
40 | #include "flags.h" /* For "optimize" in gate_pass_return_slot. |
41 | FIXME: That should be up to the pass manager, | |
42 | but pass_nrv is not in pass_all_optimizations. */ | |
6de9cd9a DN |
43 | |
44 | /* This file implements return value optimizations for functions which | |
45 | return aggregate types. | |
46 | ||
47 | Basically this pass searches the function for return statements which | |
48 | return a local aggregate. When converted to RTL such statements will | |
49 | generate a copy from the local aggregate to final return value destination | |
50 | mandated by the target's ABI. | |
51 | ||
52 | That copy can often be avoided by directly constructing the return value | |
53 | into the final destination mandated by the target's ABI. | |
54 | ||
b8698a0f | 55 | This is basically a generic equivalent to the C++ front-end's |
6de9cd9a DN |
56 | Named Return Value optimization. */ |
57 | ||
58 | struct nrv_data | |
59 | { | |
60 | /* This is the temporary (a VAR_DECL) which appears in all of | |
61 | this function's RETURN_EXPR statements. */ | |
62 | tree var; | |
63 | ||
1ea7e6ad | 64 | /* This is the function's RESULT_DECL. We will replace all occurrences |
6de9cd9a DN |
65 | of VAR with RESULT_DECL when we apply this optimization. */ |
66 | tree result; | |
4e3825db | 67 | int modified; |
6de9cd9a DN |
68 | }; |
69 | ||
70 | static tree finalize_nrv_r (tree *, int *, void *); | |
71 | ||
72 | /* Callback for the tree walker. | |
73 | ||
74 | If TP refers to a RETURN_EXPR, then set the expression being returned | |
75 | to nrv_data->result. | |
76 | ||
77 | If TP refers to nrv_data->var, then replace nrv_data->var with | |
78 | nrv_data->result. | |
79 | ||
80 | If we reach a node where we know all the subtrees are uninteresting, | |
81 | then set *WALK_SUBTREES to zero. */ | |
82 | ||
83 | static tree | |
84 | finalize_nrv_r (tree *tp, int *walk_subtrees, void *data) | |
85 | { | |
726a989a RB |
86 | struct walk_stmt_info *wi = (struct walk_stmt_info *) data; |
87 | struct nrv_data *dp = (struct nrv_data *) wi->info; | |
6de9cd9a DN |
88 | |
89 | /* No need to walk into types. */ | |
90 | if (TYPE_P (*tp)) | |
91 | *walk_subtrees = 0; | |
500b9b49 | 92 | |
61ada8ae | 93 | /* Otherwise replace all occurrences of VAR with RESULT. */ |
6de9cd9a | 94 | else if (*tp == dp->var) |
4e3825db MM |
95 | { |
96 | *tp = dp->result; | |
97 | dp->modified = 1; | |
98 | } | |
6de9cd9a DN |
99 | |
100 | /* Keep iterating. */ | |
101 | return NULL_TREE; | |
102 | } | |
103 | ||
104 | /* Main entry point for return value optimizations. | |
105 | ||
106 | If this function always returns the same local variable, and that | |
107 | local variable is an aggregate type, then replace the variable with | |
108 | the function's DECL_RESULT. | |
109 | ||
110 | This is the equivalent of the C++ named return value optimization | |
111 | applied to optimized trees in a language independent form. If we | |
112 | ever encounter languages which prevent this kind of optimization, | |
113 | then we could either have the languages register the optimization or | |
114 | we could change the gating function to check the current language. */ | |
b8698a0f | 115 | |
be55bfe6 TS |
116 | namespace { |
117 | ||
118 | const pass_data pass_data_nrv = | |
119 | { | |
120 | GIMPLE_PASS, /* type */ | |
121 | "nrv", /* name */ | |
122 | OPTGROUP_NONE, /* optinfo_flags */ | |
be55bfe6 TS |
123 | TV_TREE_NRV, /* tv_id */ |
124 | ( PROP_ssa | PROP_cfg ), /* properties_required */ | |
125 | 0, /* properties_provided */ | |
126 | 0, /* properties_destroyed */ | |
127 | 0, /* todo_flags_start */ | |
128 | 0, /* todo_flags_finish */ | |
129 | }; | |
130 | ||
131 | class pass_nrv : public gimple_opt_pass | |
132 | { | |
133 | public: | |
134 | pass_nrv (gcc::context *ctxt) | |
135 | : gimple_opt_pass (pass_data_nrv, ctxt) | |
136 | {} | |
137 | ||
138 | /* opt_pass methods: */ | |
139 | virtual bool gate (function *) { return optimize > 0; } | |
140 | ||
141 | virtual unsigned int execute (function *); | |
142 | ||
143 | }; // class pass_nrv | |
144 | ||
145 | unsigned int | |
146 | pass_nrv::execute (function *fun) | |
6de9cd9a DN |
147 | { |
148 | tree result = DECL_RESULT (current_function_decl); | |
149 | tree result_type = TREE_TYPE (result); | |
150 | tree found = NULL; | |
151 | basic_block bb; | |
726a989a | 152 | gimple_stmt_iterator gsi; |
6de9cd9a DN |
153 | struct nrv_data data; |
154 | ||
155 | /* If this function does not return an aggregate type in memory, then | |
156 | there is nothing to do. */ | |
157 | if (!aggregate_value_p (result, current_function_decl)) | |
c2924966 | 158 | return 0; |
6de9cd9a | 159 | |
12e19e05 JJ |
160 | /* If a GIMPLE type is returned in memory, finalize_nrv_r might create |
161 | non-GIMPLE. */ | |
162 | if (is_gimple_reg_type (result_type)) | |
163 | return 0; | |
164 | ||
7716876b JM |
165 | /* If the front end already did something like this, don't do it here. */ |
166 | if (DECL_NAME (result)) | |
167 | return 0; | |
168 | ||
f1c19648 RG |
169 | /* If the result has its address taken then it might be modified |
170 | by means not detected in the following loop. Bail out in this | |
171 | case. */ | |
172 | if (TREE_ADDRESSABLE (result)) | |
173 | return 0; | |
174 | ||
06208009 | 175 | /* Look through each block for assignments to the RESULT_DECL. */ |
be55bfe6 | 176 | FOR_EACH_BB_FN (bb, fun) |
6de9cd9a | 177 | { |
726a989a | 178 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) |
6de9cd9a | 179 | { |
726a989a RB |
180 | gimple stmt = gsi_stmt (gsi); |
181 | tree ret_val; | |
06208009 | 182 | |
726a989a | 183 | if (gimple_code (stmt) == GIMPLE_RETURN) |
6de9cd9a | 184 | { |
06208009 JM |
185 | /* In a function with an aggregate return value, the |
186 | gimplifier has changed all non-empty RETURN_EXPRs to | |
187 | return the RESULT_DECL. */ | |
726a989a RB |
188 | ret_val = gimple_return_retval (stmt); |
189 | if (ret_val) | |
190 | gcc_assert (ret_val == result); | |
06208009 | 191 | } |
7716876b JM |
192 | else if (gimple_has_lhs (stmt) |
193 | && gimple_get_lhs (stmt) == result) | |
06208009 | 194 | { |
726a989a RB |
195 | tree rhs; |
196 | ||
197 | if (!gimple_assign_copy_p (stmt)) | |
198 | return 0; | |
199 | ||
200 | rhs = gimple_assign_rhs1 (stmt); | |
06208009 JM |
201 | |
202 | /* Now verify that this return statement uses the same value | |
203 | as any previously encountered return statement. */ | |
204 | if (found != NULL) | |
205 | { | |
206 | /* If we found a return statement using a different variable | |
207 | than previous return statements, then we can not perform | |
208 | NRV optimizations. */ | |
726a989a | 209 | if (found != rhs) |
c2924966 | 210 | return 0; |
06208009 JM |
211 | } |
212 | else | |
726a989a | 213 | found = rhs; |
06208009 JM |
214 | |
215 | /* The returned value must be a local automatic variable of the | |
216 | same type and alignment as the function's result. */ | |
217 | if (TREE_CODE (found) != VAR_DECL | |
218 | || TREE_THIS_VOLATILE (found) | |
219 | || DECL_CONTEXT (found) != current_function_decl | |
220 | || TREE_STATIC (found) | |
221 | || TREE_ADDRESSABLE (found) | |
222 | || DECL_ALIGN (found) > DECL_ALIGN (result) | |
f4088621 | 223 | || !useless_type_conversion_p (result_type, |
f1c19648 | 224 | TREE_TYPE (found))) |
c2924966 | 225 | return 0; |
6de9cd9a | 226 | } |
7716876b | 227 | else if (gimple_has_lhs (stmt)) |
e31657e8 | 228 | { |
7716876b | 229 | tree addr = get_base_address (gimple_get_lhs (stmt)); |
b8698a0f | 230 | /* If there's any MODIFY of component of RESULT, |
e31657e8 SP |
231 | then bail out. */ |
232 | if (addr && addr == result) | |
233 | return 0; | |
234 | } | |
6de9cd9a DN |
235 | } |
236 | } | |
237 | ||
238 | if (!found) | |
c2924966 | 239 | return 0; |
6de9cd9a DN |
240 | |
241 | /* If dumping details, then note once and only the NRV replacement. */ | |
242 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
243 | { | |
244 | fprintf (dump_file, "NRV Replaced: "); | |
245 | print_generic_expr (dump_file, found, dump_flags); | |
246 | fprintf (dump_file, " with: "); | |
247 | print_generic_expr (dump_file, result, dump_flags); | |
248 | fprintf (dump_file, "\n"); | |
249 | } | |
250 | ||
251 | /* At this point we know that all the return statements return the | |
252 | same local which has suitable attributes for NRV. Copy debugging | |
7716876b JM |
253 | information from FOUND to RESULT if it will be useful. But don't set |
254 | DECL_ABSTRACT_ORIGIN to point at another function. */ | |
255 | if (!DECL_IGNORED_P (found) | |
256 | && !(DECL_ABSTRACT_ORIGIN (found) | |
257 | && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl)) | |
258 | { | |
259 | DECL_NAME (result) = DECL_NAME (found); | |
260 | DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found); | |
261 | DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found); | |
262 | } | |
263 | ||
f1c19648 | 264 | TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found); |
6de9cd9a DN |
265 | |
266 | /* Now walk through the function changing all references to VAR to be | |
267 | RESULT. */ | |
268 | data.var = found; | |
269 | data.result = result; | |
be55bfe6 | 270 | FOR_EACH_BB_FN (bb, fun) |
6de9cd9a | 271 | { |
726a989a | 272 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); ) |
06208009 | 273 | { |
726a989a | 274 | gimple stmt = gsi_stmt (gsi); |
06208009 | 275 | /* If this is a copy from VAR to RESULT, remove it. */ |
726a989a RB |
276 | if (gimple_assign_copy_p (stmt) |
277 | && gimple_assign_lhs (stmt) == result | |
278 | && gimple_assign_rhs1 (stmt) == found) | |
4e3825db MM |
279 | { |
280 | unlink_stmt_vdef (stmt); | |
281 | gsi_remove (&gsi, true); | |
3d3f2249 | 282 | release_defs (stmt); |
4e3825db | 283 | } |
06208009 JM |
284 | else |
285 | { | |
726a989a RB |
286 | struct walk_stmt_info wi; |
287 | memset (&wi, 0, sizeof (wi)); | |
288 | wi.info = &data; | |
4e3825db | 289 | data.modified = 0; |
726a989a | 290 | walk_gimple_op (stmt, finalize_nrv_r, &wi); |
4e3825db MM |
291 | if (data.modified) |
292 | update_stmt (stmt); | |
726a989a | 293 | gsi_next (&gsi); |
06208009 JM |
294 | } |
295 | } | |
6de9cd9a DN |
296 | } |
297 | ||
938650d8 JJ |
298 | SET_DECL_VALUE_EXPR (found, result); |
299 | DECL_HAS_VALUE_EXPR_P (found) = 1; | |
300 | ||
c2924966 | 301 | return 0; |
6de9cd9a DN |
302 | } |
303 | ||
27a4cd48 DM |
304 | } // anon namespace |
305 | ||
306 | gimple_opt_pass * | |
307 | make_pass_nrv (gcc::context *ctxt) | |
308 | { | |
309 | return new pass_nrv (ctxt); | |
310 | } | |
311 | ||
f0ce7858 JC |
312 | /* Determine (pessimistically) whether DEST is available for NRV |
313 | optimization, where DEST is expected to be the LHS of a modify | |
314 | expression where the RHS is a function returning an aggregate. | |
315 | ||
7f8ac3d7 | 316 | DEST is available if it is not clobbered or used by the call. */ |
f0ce7858 JC |
317 | |
318 | static bool | |
12de6355 | 319 | dest_safe_for_nrv_p (gimple call) |
f0ce7858 | 320 | { |
12de6355 | 321 | tree dest = gimple_call_lhs (call); |
a2daf82c | 322 | |
12de6355 RG |
323 | dest = get_base_address (dest); |
324 | if (! dest) | |
a2daf82c JJ |
325 | return false; |
326 | ||
327 | if (TREE_CODE (dest) == SSA_NAME) | |
12de6355 | 328 | return true; |
a2daf82c | 329 | |
7f8ac3d7 RG |
330 | if (call_may_clobber_ref_p (call, dest) |
331 | || ref_maybe_used_by_stmt_p (call, dest)) | |
a2daf82c | 332 | return false; |
eee717aa | 333 | |
a2daf82c | 334 | return true; |
f0ce7858 JC |
335 | } |
336 | ||
726a989a | 337 | /* Walk through the function looking for GIMPLE_ASSIGNs with calls that |
fa47911c JM |
338 | return in memory on the RHS. For each of these, determine whether it is |
339 | safe to pass the address of the LHS as the return slot, and mark the | |
340 | call appropriately if so. | |
341 | ||
342 | The NRV shares the return slot with a local variable in the callee; this | |
343 | optimization shares the return slot with the target of the call within | |
344 | the caller. If the NRV is performed (which we can't know in general), | |
345 | this optimization is safe if the address of the target has not | |
346 | escaped prior to the call. If it has, modifications to the local | |
347 | variable will produce visible changes elsewhere, as in PR c++/19317. */ | |
348 | ||
27a4cd48 DM |
349 | namespace { |
350 | ||
351 | const pass_data pass_data_return_slot = | |
fa47911c | 352 | { |
27a4cd48 DM |
353 | GIMPLE_PASS, /* type */ |
354 | "retslot", /* name */ | |
355 | OPTGROUP_NONE, /* optinfo_flags */ | |
27a4cd48 DM |
356 | TV_NONE, /* tv_id */ |
357 | PROP_ssa, /* properties_required */ | |
358 | 0, /* properties_provided */ | |
359 | 0, /* properties_destroyed */ | |
360 | 0, /* todo_flags_start */ | |
361 | 0, /* todo_flags_finish */ | |
fa47911c | 362 | }; |
27a4cd48 DM |
363 | |
364 | class pass_return_slot : public gimple_opt_pass | |
365 | { | |
366 | public: | |
c3284718 RS |
367 | pass_return_slot (gcc::context *ctxt) |
368 | : gimple_opt_pass (pass_data_return_slot, ctxt) | |
27a4cd48 DM |
369 | {} |
370 | ||
371 | /* opt_pass methods: */ | |
be55bfe6 | 372 | virtual unsigned int execute (function *); |
27a4cd48 DM |
373 | |
374 | }; // class pass_return_slot | |
375 | ||
be55bfe6 TS |
376 | unsigned int |
377 | pass_return_slot::execute (function *fun) | |
378 | { | |
379 | basic_block bb; | |
380 | ||
381 | FOR_EACH_BB_FN (bb, fun) | |
382 | { | |
383 | gimple_stmt_iterator gsi; | |
384 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) | |
385 | { | |
386 | gimple stmt = gsi_stmt (gsi); | |
387 | bool slot_opt_p; | |
388 | ||
389 | if (is_gimple_call (stmt) | |
390 | && gimple_call_lhs (stmt) | |
391 | && !gimple_call_return_slot_opt_p (stmt) | |
392 | && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)), | |
393 | gimple_call_fndecl (stmt))) | |
394 | { | |
395 | /* Check if the location being assigned to is | |
396 | clobbered by the call. */ | |
397 | slot_opt_p = dest_safe_for_nrv_p (stmt); | |
398 | gimple_call_set_return_slot_opt (stmt, slot_opt_p); | |
399 | } | |
400 | } | |
401 | } | |
402 | return 0; | |
403 | } | |
404 | ||
27a4cd48 DM |
405 | } // anon namespace |
406 | ||
407 | gimple_opt_pass * | |
408 | make_pass_return_slot (gcc::context *ctxt) | |
409 | { | |
410 | return new pass_return_slot (ctxt); | |
411 | } |