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6de9cd9a | 1 | /* Language independent return value optimizations |
d1e082c2 | 2 | Copyright (C) 2004-2013 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" |
442b4905 | 28 | #include "gimple.h" |
5be5c238 AM |
29 | #include "gimple-iterator.h" |
30 | #include "gimple-walk.h" | |
442b4905 | 31 | #include "gimple-ssa.h" |
d8a2d370 | 32 | #include "stringpool.h" |
442b4905 | 33 | #include "tree-ssanames.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 | ||
54 | struct nrv_data | |
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 RB |
82 | struct walk_stmt_info *wi = (struct walk_stmt_info *) data; |
83 | struct nrv_data *dp = (struct nrv_data *) 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 | |
c2924966 | 112 | static unsigned int |
6de9cd9a DN |
113 | tree_nrv (void) |
114 | { | |
115 | tree result = DECL_RESULT (current_function_decl); | |
116 | tree result_type = TREE_TYPE (result); | |
117 | tree found = NULL; | |
118 | basic_block bb; | |
726a989a | 119 | gimple_stmt_iterator gsi; |
6de9cd9a DN |
120 | struct nrv_data data; |
121 | ||
122 | /* If this function does not return an aggregate type in memory, then | |
123 | there is nothing to do. */ | |
124 | if (!aggregate_value_p (result, current_function_decl)) | |
c2924966 | 125 | return 0; |
6de9cd9a | 126 | |
12e19e05 JJ |
127 | /* If a GIMPLE type is returned in memory, finalize_nrv_r might create |
128 | non-GIMPLE. */ | |
129 | if (is_gimple_reg_type (result_type)) | |
130 | return 0; | |
131 | ||
7716876b JM |
132 | /* If the front end already did something like this, don't do it here. */ |
133 | if (DECL_NAME (result)) | |
134 | return 0; | |
135 | ||
f1c19648 RG |
136 | /* If the result has its address taken then it might be modified |
137 | by means not detected in the following loop. Bail out in this | |
138 | case. */ | |
139 | if (TREE_ADDRESSABLE (result)) | |
140 | return 0; | |
141 | ||
06208009 | 142 | /* Look through each block for assignments to the RESULT_DECL. */ |
6de9cd9a DN |
143 | FOR_EACH_BB (bb) |
144 | { | |
726a989a | 145 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) |
6de9cd9a | 146 | { |
726a989a RB |
147 | gimple stmt = gsi_stmt (gsi); |
148 | tree ret_val; | |
06208009 | 149 | |
726a989a | 150 | if (gimple_code (stmt) == GIMPLE_RETURN) |
6de9cd9a | 151 | { |
06208009 JM |
152 | /* In a function with an aggregate return value, the |
153 | gimplifier has changed all non-empty RETURN_EXPRs to | |
154 | return the RESULT_DECL. */ | |
726a989a RB |
155 | ret_val = gimple_return_retval (stmt); |
156 | if (ret_val) | |
157 | gcc_assert (ret_val == result); | |
06208009 | 158 | } |
7716876b JM |
159 | else if (gimple_has_lhs (stmt) |
160 | && gimple_get_lhs (stmt) == result) | |
06208009 | 161 | { |
726a989a RB |
162 | tree rhs; |
163 | ||
164 | if (!gimple_assign_copy_p (stmt)) | |
165 | return 0; | |
166 | ||
167 | rhs = gimple_assign_rhs1 (stmt); | |
06208009 JM |
168 | |
169 | /* Now verify that this return statement uses the same value | |
170 | as any previously encountered return statement. */ | |
171 | if (found != NULL) | |
172 | { | |
173 | /* If we found a return statement using a different variable | |
174 | than previous return statements, then we can not perform | |
175 | NRV optimizations. */ | |
726a989a | 176 | if (found != rhs) |
c2924966 | 177 | return 0; |
06208009 JM |
178 | } |
179 | else | |
726a989a | 180 | found = rhs; |
06208009 JM |
181 | |
182 | /* The returned value must be a local automatic variable of the | |
183 | same type and alignment as the function's result. */ | |
184 | if (TREE_CODE (found) != VAR_DECL | |
185 | || TREE_THIS_VOLATILE (found) | |
186 | || DECL_CONTEXT (found) != current_function_decl | |
187 | || TREE_STATIC (found) | |
188 | || TREE_ADDRESSABLE (found) | |
189 | || DECL_ALIGN (found) > DECL_ALIGN (result) | |
f4088621 | 190 | || !useless_type_conversion_p (result_type, |
f1c19648 | 191 | TREE_TYPE (found))) |
c2924966 | 192 | return 0; |
6de9cd9a | 193 | } |
7716876b | 194 | else if (gimple_has_lhs (stmt)) |
e31657e8 | 195 | { |
7716876b | 196 | tree addr = get_base_address (gimple_get_lhs (stmt)); |
b8698a0f | 197 | /* If there's any MODIFY of component of RESULT, |
e31657e8 SP |
198 | then bail out. */ |
199 | if (addr && addr == result) | |
200 | return 0; | |
201 | } | |
6de9cd9a DN |
202 | } |
203 | } | |
204 | ||
205 | if (!found) | |
c2924966 | 206 | return 0; |
6de9cd9a DN |
207 | |
208 | /* If dumping details, then note once and only the NRV replacement. */ | |
209 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
210 | { | |
211 | fprintf (dump_file, "NRV Replaced: "); | |
212 | print_generic_expr (dump_file, found, dump_flags); | |
213 | fprintf (dump_file, " with: "); | |
214 | print_generic_expr (dump_file, result, dump_flags); | |
215 | fprintf (dump_file, "\n"); | |
216 | } | |
217 | ||
218 | /* At this point we know that all the return statements return the | |
219 | same local which has suitable attributes for NRV. Copy debugging | |
7716876b JM |
220 | information from FOUND to RESULT if it will be useful. But don't set |
221 | DECL_ABSTRACT_ORIGIN to point at another function. */ | |
222 | if (!DECL_IGNORED_P (found) | |
223 | && !(DECL_ABSTRACT_ORIGIN (found) | |
224 | && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl)) | |
225 | { | |
226 | DECL_NAME (result) = DECL_NAME (found); | |
227 | DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found); | |
228 | DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found); | |
229 | } | |
230 | ||
f1c19648 | 231 | TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found); |
6de9cd9a DN |
232 | |
233 | /* Now walk through the function changing all references to VAR to be | |
234 | RESULT. */ | |
235 | data.var = found; | |
236 | data.result = result; | |
237 | FOR_EACH_BB (bb) | |
238 | { | |
726a989a | 239 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); ) |
06208009 | 240 | { |
726a989a | 241 | gimple stmt = gsi_stmt (gsi); |
06208009 | 242 | /* If this is a copy from VAR to RESULT, remove it. */ |
726a989a RB |
243 | if (gimple_assign_copy_p (stmt) |
244 | && gimple_assign_lhs (stmt) == result | |
245 | && gimple_assign_rhs1 (stmt) == found) | |
4e3825db MM |
246 | { |
247 | unlink_stmt_vdef (stmt); | |
248 | gsi_remove (&gsi, true); | |
3d3f2249 | 249 | release_defs (stmt); |
4e3825db | 250 | } |
06208009 JM |
251 | else |
252 | { | |
726a989a RB |
253 | struct walk_stmt_info wi; |
254 | memset (&wi, 0, sizeof (wi)); | |
255 | wi.info = &data; | |
4e3825db | 256 | data.modified = 0; |
726a989a | 257 | walk_gimple_op (stmt, finalize_nrv_r, &wi); |
4e3825db MM |
258 | if (data.modified) |
259 | update_stmt (stmt); | |
726a989a | 260 | gsi_next (&gsi); |
06208009 JM |
261 | } |
262 | } | |
6de9cd9a DN |
263 | } |
264 | ||
938650d8 JJ |
265 | SET_DECL_VALUE_EXPR (found, result); |
266 | DECL_HAS_VALUE_EXPR_P (found) = 1; | |
267 | ||
c2924966 | 268 | return 0; |
6de9cd9a DN |
269 | } |
270 | ||
c72321c9 JH |
271 | static bool |
272 | gate_pass_return_slot (void) | |
273 | { | |
274 | return optimize > 0; | |
275 | } | |
276 | ||
27a4cd48 DM |
277 | namespace { |
278 | ||
279 | const pass_data pass_data_nrv = | |
6de9cd9a | 280 | { |
27a4cd48 DM |
281 | GIMPLE_PASS, /* type */ |
282 | "nrv", /* name */ | |
283 | OPTGROUP_NONE, /* optinfo_flags */ | |
284 | true, /* has_gate */ | |
285 | true, /* has_execute */ | |
286 | TV_TREE_NRV, /* tv_id */ | |
287 | ( PROP_ssa | PROP_cfg ), /* properties_required */ | |
288 | 0, /* properties_provided */ | |
289 | 0, /* properties_destroyed */ | |
290 | 0, /* todo_flags_start */ | |
291 | 0, /* todo_flags_finish */ | |
6de9cd9a | 292 | }; |
fa47911c | 293 | |
27a4cd48 DM |
294 | class pass_nrv : public gimple_opt_pass |
295 | { | |
296 | public: | |
c3284718 RS |
297 | pass_nrv (gcc::context *ctxt) |
298 | : gimple_opt_pass (pass_data_nrv, ctxt) | |
27a4cd48 DM |
299 | {} |
300 | ||
301 | /* opt_pass methods: */ | |
302 | bool gate () { return gate_pass_return_slot (); } | |
303 | unsigned int execute () { return tree_nrv (); } | |
304 | ||
305 | }; // class pass_nrv | |
306 | ||
307 | } // anon namespace | |
308 | ||
309 | gimple_opt_pass * | |
310 | make_pass_nrv (gcc::context *ctxt) | |
311 | { | |
312 | return new pass_nrv (ctxt); | |
313 | } | |
314 | ||
f0ce7858 JC |
315 | /* Determine (pessimistically) whether DEST is available for NRV |
316 | optimization, where DEST is expected to be the LHS of a modify | |
317 | expression where the RHS is a function returning an aggregate. | |
318 | ||
7f8ac3d7 | 319 | DEST is available if it is not clobbered or used by the call. */ |
f0ce7858 JC |
320 | |
321 | static bool | |
12de6355 | 322 | dest_safe_for_nrv_p (gimple call) |
f0ce7858 | 323 | { |
12de6355 | 324 | tree dest = gimple_call_lhs (call); |
a2daf82c | 325 | |
12de6355 RG |
326 | dest = get_base_address (dest); |
327 | if (! dest) | |
a2daf82c JJ |
328 | return false; |
329 | ||
330 | if (TREE_CODE (dest) == SSA_NAME) | |
12de6355 | 331 | return true; |
a2daf82c | 332 | |
7f8ac3d7 RG |
333 | if (call_may_clobber_ref_p (call, dest) |
334 | || ref_maybe_used_by_stmt_p (call, dest)) | |
a2daf82c | 335 | return false; |
eee717aa | 336 | |
a2daf82c | 337 | return true; |
f0ce7858 JC |
338 | } |
339 | ||
726a989a | 340 | /* Walk through the function looking for GIMPLE_ASSIGNs with calls that |
fa47911c JM |
341 | return in memory on the RHS. For each of these, determine whether it is |
342 | safe to pass the address of the LHS as the return slot, and mark the | |
343 | call appropriately if so. | |
344 | ||
345 | The NRV shares the return slot with a local variable in the callee; this | |
346 | optimization shares the return slot with the target of the call within | |
347 | the caller. If the NRV is performed (which we can't know in general), | |
348 | this optimization is safe if the address of the target has not | |
349 | escaped prior to the call. If it has, modifications to the local | |
350 | variable will produce visible changes elsewhere, as in PR c++/19317. */ | |
351 | ||
c2924966 | 352 | static unsigned int |
fa47911c JM |
353 | execute_return_slot_opt (void) |
354 | { | |
355 | basic_block bb; | |
356 | ||
357 | FOR_EACH_BB (bb) | |
358 | { | |
726a989a RB |
359 | gimple_stmt_iterator gsi; |
360 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) | |
fa47911c | 361 | { |
726a989a RB |
362 | gimple stmt = gsi_stmt (gsi); |
363 | bool slot_opt_p; | |
364 | ||
365 | if (is_gimple_call (stmt) | |
366 | && gimple_call_lhs (stmt) | |
367 | && !gimple_call_return_slot_opt_p (stmt) | |
368 | && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)), | |
7f8ac3d7 | 369 | gimple_call_fndecl (stmt))) |
726a989a RB |
370 | { |
371 | /* Check if the location being assigned to is | |
12de6355 RG |
372 | clobbered by the call. */ |
373 | slot_opt_p = dest_safe_for_nrv_p (stmt); | |
726a989a RB |
374 | gimple_call_set_return_slot_opt (stmt, slot_opt_p); |
375 | } | |
fa47911c JM |
376 | } |
377 | } | |
c2924966 | 378 | return 0; |
fa47911c JM |
379 | } |
380 | ||
27a4cd48 DM |
381 | namespace { |
382 | ||
383 | const pass_data pass_data_return_slot = | |
fa47911c | 384 | { |
27a4cd48 DM |
385 | GIMPLE_PASS, /* type */ |
386 | "retslot", /* name */ | |
387 | OPTGROUP_NONE, /* optinfo_flags */ | |
388 | false, /* has_gate */ | |
389 | true, /* has_execute */ | |
390 | TV_NONE, /* tv_id */ | |
391 | PROP_ssa, /* properties_required */ | |
392 | 0, /* properties_provided */ | |
393 | 0, /* properties_destroyed */ | |
394 | 0, /* todo_flags_start */ | |
395 | 0, /* todo_flags_finish */ | |
fa47911c | 396 | }; |
27a4cd48 DM |
397 | |
398 | class pass_return_slot : public gimple_opt_pass | |
399 | { | |
400 | public: | |
c3284718 RS |
401 | pass_return_slot (gcc::context *ctxt) |
402 | : gimple_opt_pass (pass_data_return_slot, ctxt) | |
27a4cd48 DM |
403 | {} |
404 | ||
405 | /* opt_pass methods: */ | |
406 | unsigned int execute () { return execute_return_slot_opt (); } | |
407 | ||
408 | }; // class pass_return_slot | |
409 | ||
410 | } // anon namespace | |
411 | ||
412 | gimple_opt_pass * | |
413 | make_pass_return_slot (gcc::context *ctxt) | |
414 | { | |
415 | return new pass_return_slot (ctxt); | |
416 | } |