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4ee9c684 | 1 | /* Language independent return value optimizations |
3aea1f79 | 2 | Copyright (C) 2004-2014 Free Software Foundation, Inc. |
4ee9c684 | 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 | |
8c4c00c1 | 8 | the Free Software Foundation; either version 3, or (at your option) |
4ee9c684 | 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 | |
8c4c00c1 | 17 | along with GCC; see the file COPYING3. If not see |
18 | <http://www.gnu.org/licenses/>. */ | |
4ee9c684 | 19 | |
20 | #include "config.h" | |
21 | #include "system.h" | |
22 | #include "coretypes.h" | |
23 | #include "tm.h" | |
24 | #include "tree.h" | |
a3020f2f | 25 | #include "hashtab.h" |
26 | #include "hash-set.h" | |
27 | #include "vec.h" | |
28 | #include "machmode.h" | |
29 | #include "hard-reg-set.h" | |
30 | #include "input.h" | |
4ee9c684 | 31 | #include "function.h" |
94ea8568 | 32 | #include "predict.h" |
33 | #include "dominance.h" | |
34 | #include "cfg.h" | |
4ee9c684 | 35 | #include "basic-block.h" |
ce084dfc | 36 | #include "tree-pretty-print.h" |
bc61cadb | 37 | #include "tree-ssa-alias.h" |
38 | #include "internal-fn.h" | |
39 | #include "gimple-expr.h" | |
40 | #include "is-a.h" | |
073c1fd5 | 41 | #include "gimple.h" |
dcf1a1ec | 42 | #include "gimple-iterator.h" |
43 | #include "gimple-walk.h" | |
073c1fd5 | 44 | #include "gimple-ssa.h" |
9ed99284 | 45 | #include "stringpool.h" |
073c1fd5 | 46 | #include "tree-ssanames.h" |
4ee9c684 | 47 | #include "tree-pass.h" |
48 | #include "langhooks.h" | |
a7a46268 | 49 | #include "flags.h" /* For "optimize" in gate_pass_return_slot. |
50 | FIXME: That should be up to the pass manager, | |
51 | but pass_nrv is not in pass_all_optimizations. */ | |
4ee9c684 | 52 | |
53 | /* This file implements return value optimizations for functions which | |
54 | return aggregate types. | |
55 | ||
56 | Basically this pass searches the function for return statements which | |
57 | return a local aggregate. When converted to RTL such statements will | |
58 | generate a copy from the local aggregate to final return value destination | |
59 | mandated by the target's ABI. | |
60 | ||
61 | That copy can often be avoided by directly constructing the return value | |
62 | into the final destination mandated by the target's ABI. | |
63 | ||
48e1416a | 64 | This is basically a generic equivalent to the C++ front-end's |
4ee9c684 | 65 | Named Return Value optimization. */ |
66 | ||
9908fe4d | 67 | struct nrv_data_t |
4ee9c684 | 68 | { |
69 | /* This is the temporary (a VAR_DECL) which appears in all of | |
70 | this function's RETURN_EXPR statements. */ | |
71 | tree var; | |
72 | ||
365db11e | 73 | /* This is the function's RESULT_DECL. We will replace all occurrences |
4ee9c684 | 74 | of VAR with RESULT_DECL when we apply this optimization. */ |
75 | tree result; | |
a8dd994c | 76 | int modified; |
4ee9c684 | 77 | }; |
78 | ||
79 | static tree finalize_nrv_r (tree *, int *, void *); | |
80 | ||
81 | /* Callback for the tree walker. | |
82 | ||
83 | If TP refers to a RETURN_EXPR, then set the expression being returned | |
84 | to nrv_data->result. | |
85 | ||
86 | If TP refers to nrv_data->var, then replace nrv_data->var with | |
87 | nrv_data->result. | |
88 | ||
89 | If we reach a node where we know all the subtrees are uninteresting, | |
90 | then set *WALK_SUBTREES to zero. */ | |
91 | ||
92 | static tree | |
93 | finalize_nrv_r (tree *tp, int *walk_subtrees, void *data) | |
94 | { | |
75a70cf9 | 95 | struct walk_stmt_info *wi = (struct walk_stmt_info *) data; |
9908fe4d | 96 | struct nrv_data_t *dp = (struct nrv_data_t *) wi->info; |
4ee9c684 | 97 | |
98 | /* No need to walk into types. */ | |
99 | if (TYPE_P (*tp)) | |
100 | *walk_subtrees = 0; | |
15a9f039 | 101 | |
2c763ed4 | 102 | /* Otherwise replace all occurrences of VAR with RESULT. */ |
4ee9c684 | 103 | else if (*tp == dp->var) |
a8dd994c | 104 | { |
105 | *tp = dp->result; | |
106 | dp->modified = 1; | |
107 | } | |
4ee9c684 | 108 | |
109 | /* Keep iterating. */ | |
110 | return NULL_TREE; | |
111 | } | |
112 | ||
113 | /* Main entry point for return value optimizations. | |
114 | ||
115 | If this function always returns the same local variable, and that | |
116 | local variable is an aggregate type, then replace the variable with | |
117 | the function's DECL_RESULT. | |
118 | ||
119 | This is the equivalent of the C++ named return value optimization | |
120 | applied to optimized trees in a language independent form. If we | |
121 | ever encounter languages which prevent this kind of optimization, | |
122 | then we could either have the languages register the optimization or | |
123 | we could change the gating function to check the current language. */ | |
48e1416a | 124 | |
65b0537f | 125 | namespace { |
126 | ||
127 | const pass_data pass_data_nrv = | |
128 | { | |
129 | GIMPLE_PASS, /* type */ | |
130 | "nrv", /* name */ | |
131 | OPTGROUP_NONE, /* optinfo_flags */ | |
65b0537f | 132 | TV_TREE_NRV, /* tv_id */ |
133 | ( PROP_ssa | PROP_cfg ), /* properties_required */ | |
134 | 0, /* properties_provided */ | |
135 | 0, /* properties_destroyed */ | |
136 | 0, /* todo_flags_start */ | |
137 | 0, /* todo_flags_finish */ | |
138 | }; | |
139 | ||
140 | class pass_nrv : public gimple_opt_pass | |
141 | { | |
142 | public: | |
143 | pass_nrv (gcc::context *ctxt) | |
144 | : gimple_opt_pass (pass_data_nrv, ctxt) | |
145 | {} | |
146 | ||
147 | /* opt_pass methods: */ | |
148 | virtual bool gate (function *) { return optimize > 0; } | |
149 | ||
150 | virtual unsigned int execute (function *); | |
151 | ||
152 | }; // class pass_nrv | |
153 | ||
154 | unsigned int | |
155 | pass_nrv::execute (function *fun) | |
4ee9c684 | 156 | { |
157 | tree result = DECL_RESULT (current_function_decl); | |
158 | tree result_type = TREE_TYPE (result); | |
159 | tree found = NULL; | |
160 | basic_block bb; | |
75a70cf9 | 161 | gimple_stmt_iterator gsi; |
9908fe4d | 162 | struct nrv_data_t data; |
4ee9c684 | 163 | |
164 | /* If this function does not return an aggregate type in memory, then | |
165 | there is nothing to do. */ | |
166 | if (!aggregate_value_p (result, current_function_decl)) | |
2a1990e9 | 167 | return 0; |
4ee9c684 | 168 | |
e264d515 | 169 | /* If a GIMPLE type is returned in memory, finalize_nrv_r might create |
170 | non-GIMPLE. */ | |
171 | if (is_gimple_reg_type (result_type)) | |
172 | return 0; | |
173 | ||
93ef9154 | 174 | /* If the front end already did something like this, don't do it here. */ |
175 | if (DECL_NAME (result)) | |
176 | return 0; | |
177 | ||
a97b4ced | 178 | /* If the result has its address taken then it might be modified |
179 | by means not detected in the following loop. Bail out in this | |
180 | case. */ | |
181 | if (TREE_ADDRESSABLE (result)) | |
182 | return 0; | |
183 | ||
84ef41be | 184 | /* Look through each block for assignments to the RESULT_DECL. */ |
65b0537f | 185 | FOR_EACH_BB_FN (bb, fun) |
4ee9c684 | 186 | { |
75a70cf9 | 187 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) |
4ee9c684 | 188 | { |
75a70cf9 | 189 | gimple stmt = gsi_stmt (gsi); |
190 | tree ret_val; | |
84ef41be | 191 | |
1a91d914 | 192 | if (greturn *return_stmt = dyn_cast <greturn *> (stmt)) |
4ee9c684 | 193 | { |
84ef41be | 194 | /* In a function with an aggregate return value, the |
195 | gimplifier has changed all non-empty RETURN_EXPRs to | |
196 | return the RESULT_DECL. */ | |
1a91d914 | 197 | ret_val = gimple_return_retval (return_stmt); |
75a70cf9 | 198 | if (ret_val) |
199 | gcc_assert (ret_val == result); | |
84ef41be | 200 | } |
93ef9154 | 201 | else if (gimple_has_lhs (stmt) |
202 | && gimple_get_lhs (stmt) == result) | |
84ef41be | 203 | { |
75a70cf9 | 204 | tree rhs; |
205 | ||
206 | if (!gimple_assign_copy_p (stmt)) | |
207 | return 0; | |
208 | ||
209 | rhs = gimple_assign_rhs1 (stmt); | |
84ef41be | 210 | |
211 | /* Now verify that this return statement uses the same value | |
212 | as any previously encountered return statement. */ | |
213 | if (found != NULL) | |
214 | { | |
215 | /* If we found a return statement using a different variable | |
216 | than previous return statements, then we can not perform | |
217 | NRV optimizations. */ | |
75a70cf9 | 218 | if (found != rhs) |
2a1990e9 | 219 | return 0; |
84ef41be | 220 | } |
221 | else | |
75a70cf9 | 222 | found = rhs; |
84ef41be | 223 | |
224 | /* The returned value must be a local automatic variable of the | |
225 | same type and alignment as the function's result. */ | |
226 | if (TREE_CODE (found) != VAR_DECL | |
227 | || TREE_THIS_VOLATILE (found) | |
6e986d60 | 228 | || !auto_var_in_fn_p (found, current_function_decl) |
84ef41be | 229 | || TREE_ADDRESSABLE (found) |
230 | || DECL_ALIGN (found) > DECL_ALIGN (result) | |
c8ca3ee7 | 231 | || !useless_type_conversion_p (result_type, |
a97b4ced | 232 | TREE_TYPE (found))) |
2a1990e9 | 233 | return 0; |
4ee9c684 | 234 | } |
93ef9154 | 235 | else if (gimple_has_lhs (stmt)) |
fdf6266d | 236 | { |
93ef9154 | 237 | tree addr = get_base_address (gimple_get_lhs (stmt)); |
48e1416a | 238 | /* If there's any MODIFY of component of RESULT, |
fdf6266d | 239 | then bail out. */ |
240 | if (addr && addr == result) | |
241 | return 0; | |
242 | } | |
4ee9c684 | 243 | } |
244 | } | |
245 | ||
246 | if (!found) | |
2a1990e9 | 247 | return 0; |
4ee9c684 | 248 | |
249 | /* If dumping details, then note once and only the NRV replacement. */ | |
250 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
251 | { | |
252 | fprintf (dump_file, "NRV Replaced: "); | |
253 | print_generic_expr (dump_file, found, dump_flags); | |
254 | fprintf (dump_file, " with: "); | |
255 | print_generic_expr (dump_file, result, dump_flags); | |
256 | fprintf (dump_file, "\n"); | |
257 | } | |
258 | ||
259 | /* At this point we know that all the return statements return the | |
260 | same local which has suitable attributes for NRV. Copy debugging | |
93ef9154 | 261 | information from FOUND to RESULT if it will be useful. But don't set |
262 | DECL_ABSTRACT_ORIGIN to point at another function. */ | |
263 | if (!DECL_IGNORED_P (found) | |
264 | && !(DECL_ABSTRACT_ORIGIN (found) | |
265 | && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl)) | |
266 | { | |
267 | DECL_NAME (result) = DECL_NAME (found); | |
268 | DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found); | |
269 | DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found); | |
270 | } | |
271 | ||
a97b4ced | 272 | TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found); |
4ee9c684 | 273 | |
274 | /* Now walk through the function changing all references to VAR to be | |
275 | RESULT. */ | |
276 | data.var = found; | |
277 | data.result = result; | |
65b0537f | 278 | FOR_EACH_BB_FN (bb, fun) |
4ee9c684 | 279 | { |
75a70cf9 | 280 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); ) |
84ef41be | 281 | { |
75a70cf9 | 282 | gimple stmt = gsi_stmt (gsi); |
84ef41be | 283 | /* If this is a copy from VAR to RESULT, remove it. */ |
75a70cf9 | 284 | if (gimple_assign_copy_p (stmt) |
285 | && gimple_assign_lhs (stmt) == result | |
286 | && gimple_assign_rhs1 (stmt) == found) | |
a8dd994c | 287 | { |
288 | unlink_stmt_vdef (stmt); | |
289 | gsi_remove (&gsi, true); | |
bc8a8451 | 290 | release_defs (stmt); |
a8dd994c | 291 | } |
84ef41be | 292 | else |
293 | { | |
75a70cf9 | 294 | struct walk_stmt_info wi; |
295 | memset (&wi, 0, sizeof (wi)); | |
296 | wi.info = &data; | |
a8dd994c | 297 | data.modified = 0; |
75a70cf9 | 298 | walk_gimple_op (stmt, finalize_nrv_r, &wi); |
a8dd994c | 299 | if (data.modified) |
300 | update_stmt (stmt); | |
75a70cf9 | 301 | gsi_next (&gsi); |
84ef41be | 302 | } |
303 | } | |
4ee9c684 | 304 | } |
305 | ||
e641a23b | 306 | SET_DECL_VALUE_EXPR (found, result); |
307 | DECL_HAS_VALUE_EXPR_P (found) = 1; | |
308 | ||
2a1990e9 | 309 | return 0; |
4ee9c684 | 310 | } |
311 | ||
cbe8bda8 | 312 | } // anon namespace |
313 | ||
314 | gimple_opt_pass * | |
315 | make_pass_nrv (gcc::context *ctxt) | |
316 | { | |
317 | return new pass_nrv (ctxt); | |
318 | } | |
319 | ||
166cf564 | 320 | /* Determine (pessimistically) whether DEST is available for NRV |
321 | optimization, where DEST is expected to be the LHS of a modify | |
322 | expression where the RHS is a function returning an aggregate. | |
323 | ||
095a3ce9 | 324 | DEST is available if it is not clobbered or used by the call. */ |
166cf564 | 325 | |
326 | static bool | |
1a91d914 | 327 | dest_safe_for_nrv_p (gcall *call) |
166cf564 | 328 | { |
4debfcfc | 329 | tree dest = gimple_call_lhs (call); |
fbb5d9ac | 330 | |
4debfcfc | 331 | dest = get_base_address (dest); |
332 | if (! dest) | |
fbb5d9ac | 333 | return false; |
334 | ||
335 | if (TREE_CODE (dest) == SSA_NAME) | |
4debfcfc | 336 | return true; |
fbb5d9ac | 337 | |
095a3ce9 | 338 | if (call_may_clobber_ref_p (call, dest) |
339 | || ref_maybe_used_by_stmt_p (call, dest)) | |
fbb5d9ac | 340 | return false; |
fb0ac376 | 341 | |
fbb5d9ac | 342 | return true; |
166cf564 | 343 | } |
344 | ||
75a70cf9 | 345 | /* Walk through the function looking for GIMPLE_ASSIGNs with calls that |
ea523851 | 346 | return in memory on the RHS. For each of these, determine whether it is |
347 | safe to pass the address of the LHS as the return slot, and mark the | |
348 | call appropriately if so. | |
349 | ||
350 | The NRV shares the return slot with a local variable in the callee; this | |
351 | optimization shares the return slot with the target of the call within | |
352 | the caller. If the NRV is performed (which we can't know in general), | |
353 | this optimization is safe if the address of the target has not | |
354 | escaped prior to the call. If it has, modifications to the local | |
355 | variable will produce visible changes elsewhere, as in PR c++/19317. */ | |
356 | ||
cbe8bda8 | 357 | namespace { |
358 | ||
359 | const pass_data pass_data_return_slot = | |
ea523851 | 360 | { |
cbe8bda8 | 361 | GIMPLE_PASS, /* type */ |
362 | "retslot", /* name */ | |
363 | OPTGROUP_NONE, /* optinfo_flags */ | |
cbe8bda8 | 364 | TV_NONE, /* tv_id */ |
365 | PROP_ssa, /* properties_required */ | |
366 | 0, /* properties_provided */ | |
367 | 0, /* properties_destroyed */ | |
368 | 0, /* todo_flags_start */ | |
369 | 0, /* todo_flags_finish */ | |
ea523851 | 370 | }; |
cbe8bda8 | 371 | |
372 | class pass_return_slot : public gimple_opt_pass | |
373 | { | |
374 | public: | |
9af5ce0c | 375 | pass_return_slot (gcc::context *ctxt) |
376 | : gimple_opt_pass (pass_data_return_slot, ctxt) | |
cbe8bda8 | 377 | {} |
378 | ||
379 | /* opt_pass methods: */ | |
65b0537f | 380 | virtual unsigned int execute (function *); |
cbe8bda8 | 381 | |
382 | }; // class pass_return_slot | |
383 | ||
65b0537f | 384 | unsigned int |
385 | pass_return_slot::execute (function *fun) | |
386 | { | |
387 | basic_block bb; | |
388 | ||
389 | FOR_EACH_BB_FN (bb, fun) | |
390 | { | |
391 | gimple_stmt_iterator gsi; | |
392 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) | |
393 | { | |
1a91d914 | 394 | gcall *stmt; |
65b0537f | 395 | bool slot_opt_p; |
396 | ||
1a91d914 | 397 | stmt = dyn_cast <gcall *> (gsi_stmt (gsi)); |
398 | if (stmt | |
65b0537f | 399 | && gimple_call_lhs (stmt) |
400 | && !gimple_call_return_slot_opt_p (stmt) | |
401 | && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)), | |
402 | gimple_call_fndecl (stmt))) | |
403 | { | |
404 | /* Check if the location being assigned to is | |
405 | clobbered by the call. */ | |
406 | slot_opt_p = dest_safe_for_nrv_p (stmt); | |
407 | gimple_call_set_return_slot_opt (stmt, slot_opt_p); | |
408 | } | |
409 | } | |
410 | } | |
411 | return 0; | |
412 | } | |
413 | ||
cbe8bda8 | 414 | } // anon namespace |
415 | ||
416 | gimple_opt_pass * | |
417 | make_pass_return_slot (gcc::context *ctxt) | |
418 | { | |
419 | return new pass_return_slot (ctxt); | |
420 | } |