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4ee9c684 | 1 | /* Language independent return value optimizations |
fbd26352 | 2 | Copyright (C) 2004-2019 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" | |
9ef16211 | 23 | #include "backend.h" |
b20a8bb4 | 24 | #include "tree.h" |
9ef16211 | 25 | #include "gimple.h" |
7c29e30e | 26 | #include "tree-pass.h" |
9ef16211 | 27 | #include "ssa.h" |
7c29e30e | 28 | #include "tree-pretty-print.h" |
dcf1a1ec | 29 | #include "gimple-iterator.h" |
30 | #include "gimple-walk.h" | |
d992f757 | 31 | #include "internal-fn.h" |
4ee9c684 | 32 | |
33 | /* This file implements return value optimizations for functions which | |
34 | return aggregate types. | |
35 | ||
36 | Basically this pass searches the function for return statements which | |
37 | return a local aggregate. When converted to RTL such statements will | |
38 | generate a copy from the local aggregate to final return value destination | |
39 | mandated by the target's ABI. | |
40 | ||
41 | That copy can often be avoided by directly constructing the return value | |
42 | into the final destination mandated by the target's ABI. | |
43 | ||
48e1416a | 44 | This is basically a generic equivalent to the C++ front-end's |
4ee9c684 | 45 | Named Return Value optimization. */ |
46 | ||
9908fe4d | 47 | struct nrv_data_t |
4ee9c684 | 48 | { |
49 | /* This is the temporary (a VAR_DECL) which appears in all of | |
50 | this function's RETURN_EXPR statements. */ | |
51 | tree var; | |
52 | ||
365db11e | 53 | /* This is the function's RESULT_DECL. We will replace all occurrences |
4ee9c684 | 54 | of VAR with RESULT_DECL when we apply this optimization. */ |
55 | tree result; | |
a8dd994c | 56 | int modified; |
4ee9c684 | 57 | }; |
58 | ||
59 | static tree finalize_nrv_r (tree *, int *, void *); | |
60 | ||
61 | /* Callback for the tree walker. | |
62 | ||
63 | If TP refers to a RETURN_EXPR, then set the expression being returned | |
64 | to nrv_data->result. | |
65 | ||
66 | If TP refers to nrv_data->var, then replace nrv_data->var with | |
67 | nrv_data->result. | |
68 | ||
69 | If we reach a node where we know all the subtrees are uninteresting, | |
70 | then set *WALK_SUBTREES to zero. */ | |
71 | ||
72 | static tree | |
73 | finalize_nrv_r (tree *tp, int *walk_subtrees, void *data) | |
74 | { | |
75a70cf9 | 75 | struct walk_stmt_info *wi = (struct walk_stmt_info *) data; |
9908fe4d | 76 | struct nrv_data_t *dp = (struct nrv_data_t *) wi->info; |
4ee9c684 | 77 | |
78 | /* No need to walk into types. */ | |
79 | if (TYPE_P (*tp)) | |
80 | *walk_subtrees = 0; | |
15a9f039 | 81 | |
2c763ed4 | 82 | /* Otherwise replace all occurrences of VAR with RESULT. */ |
4ee9c684 | 83 | else if (*tp == dp->var) |
a8dd994c | 84 | { |
85 | *tp = dp->result; | |
86 | dp->modified = 1; | |
87 | } | |
4ee9c684 | 88 | |
89 | /* Keep iterating. */ | |
90 | return NULL_TREE; | |
91 | } | |
92 | ||
93 | /* Main entry point for return value optimizations. | |
94 | ||
95 | If this function always returns the same local variable, and that | |
96 | local variable is an aggregate type, then replace the variable with | |
97 | the function's DECL_RESULT. | |
98 | ||
99 | This is the equivalent of the C++ named return value optimization | |
100 | applied to optimized trees in a language independent form. If we | |
101 | ever encounter languages which prevent this kind of optimization, | |
102 | then we could either have the languages register the optimization or | |
103 | we could change the gating function to check the current language. */ | |
48e1416a | 104 | |
65b0537f | 105 | namespace { |
106 | ||
107 | const pass_data pass_data_nrv = | |
108 | { | |
109 | GIMPLE_PASS, /* type */ | |
110 | "nrv", /* name */ | |
111 | OPTGROUP_NONE, /* optinfo_flags */ | |
65b0537f | 112 | TV_TREE_NRV, /* tv_id */ |
113 | ( PROP_ssa | PROP_cfg ), /* properties_required */ | |
114 | 0, /* properties_provided */ | |
115 | 0, /* properties_destroyed */ | |
116 | 0, /* todo_flags_start */ | |
117 | 0, /* todo_flags_finish */ | |
118 | }; | |
119 | ||
120 | class pass_nrv : public gimple_opt_pass | |
121 | { | |
122 | public: | |
123 | pass_nrv (gcc::context *ctxt) | |
124 | : gimple_opt_pass (pass_data_nrv, ctxt) | |
125 | {} | |
126 | ||
127 | /* opt_pass methods: */ | |
128 | virtual bool gate (function *) { return optimize > 0; } | |
129 | ||
130 | virtual unsigned int execute (function *); | |
131 | ||
132 | }; // class pass_nrv | |
133 | ||
134 | unsigned int | |
135 | pass_nrv::execute (function *fun) | |
4ee9c684 | 136 | { |
137 | tree result = DECL_RESULT (current_function_decl); | |
138 | tree result_type = TREE_TYPE (result); | |
139 | tree found = NULL; | |
140 | basic_block bb; | |
75a70cf9 | 141 | gimple_stmt_iterator gsi; |
9908fe4d | 142 | struct nrv_data_t data; |
4ee9c684 | 143 | |
144 | /* If this function does not return an aggregate type in memory, then | |
145 | there is nothing to do. */ | |
146 | if (!aggregate_value_p (result, current_function_decl)) | |
2a1990e9 | 147 | return 0; |
4ee9c684 | 148 | |
e264d515 | 149 | /* If a GIMPLE type is returned in memory, finalize_nrv_r might create |
150 | non-GIMPLE. */ | |
151 | if (is_gimple_reg_type (result_type)) | |
152 | return 0; | |
153 | ||
93ef9154 | 154 | /* If the front end already did something like this, don't do it here. */ |
155 | if (DECL_NAME (result)) | |
156 | return 0; | |
157 | ||
a97b4ced | 158 | /* If the result has its address taken then it might be modified |
159 | by means not detected in the following loop. Bail out in this | |
160 | case. */ | |
161 | if (TREE_ADDRESSABLE (result)) | |
162 | return 0; | |
163 | ||
84ef41be | 164 | /* Look through each block for assignments to the RESULT_DECL. */ |
65b0537f | 165 | FOR_EACH_BB_FN (bb, fun) |
4ee9c684 | 166 | { |
75a70cf9 | 167 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) |
4ee9c684 | 168 | { |
42acab1c | 169 | gimple *stmt = gsi_stmt (gsi); |
75a70cf9 | 170 | tree ret_val; |
84ef41be | 171 | |
1a91d914 | 172 | if (greturn *return_stmt = dyn_cast <greturn *> (stmt)) |
4ee9c684 | 173 | { |
84ef41be | 174 | /* In a function with an aggregate return value, the |
175 | gimplifier has changed all non-empty RETURN_EXPRs to | |
176 | return the RESULT_DECL. */ | |
1a91d914 | 177 | ret_val = gimple_return_retval (return_stmt); |
75a70cf9 | 178 | if (ret_val) |
179 | gcc_assert (ret_val == result); | |
84ef41be | 180 | } |
93ef9154 | 181 | else if (gimple_has_lhs (stmt) |
182 | && gimple_get_lhs (stmt) == result) | |
84ef41be | 183 | { |
75a70cf9 | 184 | tree rhs; |
185 | ||
186 | if (!gimple_assign_copy_p (stmt)) | |
187 | return 0; | |
188 | ||
189 | rhs = gimple_assign_rhs1 (stmt); | |
84ef41be | 190 | |
191 | /* Now verify that this return statement uses the same value | |
192 | as any previously encountered return statement. */ | |
193 | if (found != NULL) | |
194 | { | |
195 | /* If we found a return statement using a different variable | |
f4d3c071 | 196 | than previous return statements, then we cannot perform |
84ef41be | 197 | NRV optimizations. */ |
75a70cf9 | 198 | if (found != rhs) |
2a1990e9 | 199 | return 0; |
84ef41be | 200 | } |
201 | else | |
75a70cf9 | 202 | found = rhs; |
84ef41be | 203 | |
204 | /* The returned value must be a local automatic variable of the | |
205 | same type and alignment as the function's result. */ | |
53e9c5c4 | 206 | if (!VAR_P (found) |
84ef41be | 207 | || TREE_THIS_VOLATILE (found) |
6e986d60 | 208 | || !auto_var_in_fn_p (found, current_function_decl) |
84ef41be | 209 | || TREE_ADDRESSABLE (found) |
210 | || DECL_ALIGN (found) > DECL_ALIGN (result) | |
c8ca3ee7 | 211 | || !useless_type_conversion_p (result_type, |
a97b4ced | 212 | TREE_TYPE (found))) |
2a1990e9 | 213 | return 0; |
4ee9c684 | 214 | } |
93ef9154 | 215 | else if (gimple_has_lhs (stmt)) |
fdf6266d | 216 | { |
93ef9154 | 217 | tree addr = get_base_address (gimple_get_lhs (stmt)); |
48e1416a | 218 | /* If there's any MODIFY of component of RESULT, |
fdf6266d | 219 | then bail out. */ |
220 | if (addr && addr == result) | |
221 | return 0; | |
222 | } | |
4ee9c684 | 223 | } |
224 | } | |
225 | ||
226 | if (!found) | |
2a1990e9 | 227 | return 0; |
4ee9c684 | 228 | |
229 | /* If dumping details, then note once and only the NRV replacement. */ | |
230 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
231 | { | |
232 | fprintf (dump_file, "NRV Replaced: "); | |
233 | print_generic_expr (dump_file, found, dump_flags); | |
234 | fprintf (dump_file, " with: "); | |
235 | print_generic_expr (dump_file, result, dump_flags); | |
236 | fprintf (dump_file, "\n"); | |
237 | } | |
238 | ||
239 | /* At this point we know that all the return statements return the | |
240 | same local which has suitable attributes for NRV. Copy debugging | |
93ef9154 | 241 | information from FOUND to RESULT if it will be useful. But don't set |
242 | DECL_ABSTRACT_ORIGIN to point at another function. */ | |
243 | if (!DECL_IGNORED_P (found) | |
244 | && !(DECL_ABSTRACT_ORIGIN (found) | |
245 | && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl)) | |
246 | { | |
247 | DECL_NAME (result) = DECL_NAME (found); | |
248 | DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found); | |
249 | DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found); | |
250 | } | |
251 | ||
a97b4ced | 252 | TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found); |
4ee9c684 | 253 | |
254 | /* Now walk through the function changing all references to VAR to be | |
255 | RESULT. */ | |
256 | data.var = found; | |
257 | data.result = result; | |
65b0537f | 258 | FOR_EACH_BB_FN (bb, fun) |
4ee9c684 | 259 | { |
75a70cf9 | 260 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); ) |
84ef41be | 261 | { |
42acab1c | 262 | gimple *stmt = gsi_stmt (gsi); |
84ef41be | 263 | /* If this is a copy from VAR to RESULT, remove it. */ |
75a70cf9 | 264 | if (gimple_assign_copy_p (stmt) |
265 | && gimple_assign_lhs (stmt) == result | |
266 | && gimple_assign_rhs1 (stmt) == found) | |
a8dd994c | 267 | { |
268 | unlink_stmt_vdef (stmt); | |
269 | gsi_remove (&gsi, true); | |
bc8a8451 | 270 | release_defs (stmt); |
a8dd994c | 271 | } |
84ef41be | 272 | else |
273 | { | |
75a70cf9 | 274 | struct walk_stmt_info wi; |
275 | memset (&wi, 0, sizeof (wi)); | |
276 | wi.info = &data; | |
a8dd994c | 277 | data.modified = 0; |
75a70cf9 | 278 | walk_gimple_op (stmt, finalize_nrv_r, &wi); |
a8dd994c | 279 | if (data.modified) |
280 | update_stmt (stmt); | |
75a70cf9 | 281 | gsi_next (&gsi); |
84ef41be | 282 | } |
283 | } | |
4ee9c684 | 284 | } |
285 | ||
e641a23b | 286 | SET_DECL_VALUE_EXPR (found, result); |
287 | DECL_HAS_VALUE_EXPR_P (found) = 1; | |
288 | ||
2a1990e9 | 289 | return 0; |
4ee9c684 | 290 | } |
291 | ||
cbe8bda8 | 292 | } // anon namespace |
293 | ||
294 | gimple_opt_pass * | |
295 | make_pass_nrv (gcc::context *ctxt) | |
296 | { | |
297 | return new pass_nrv (ctxt); | |
298 | } | |
299 | ||
166cf564 | 300 | /* Determine (pessimistically) whether DEST is available for NRV |
301 | optimization, where DEST is expected to be the LHS of a modify | |
302 | expression where the RHS is a function returning an aggregate. | |
303 | ||
095a3ce9 | 304 | DEST is available if it is not clobbered or used by the call. */ |
166cf564 | 305 | |
306 | static bool | |
1a91d914 | 307 | dest_safe_for_nrv_p (gcall *call) |
166cf564 | 308 | { |
4debfcfc | 309 | tree dest = gimple_call_lhs (call); |
fbb5d9ac | 310 | |
4debfcfc | 311 | dest = get_base_address (dest); |
312 | if (! dest) | |
fbb5d9ac | 313 | return false; |
314 | ||
315 | if (TREE_CODE (dest) == SSA_NAME) | |
4debfcfc | 316 | return true; |
fbb5d9ac | 317 | |
095a3ce9 | 318 | if (call_may_clobber_ref_p (call, dest) |
319 | || ref_maybe_used_by_stmt_p (call, dest)) | |
fbb5d9ac | 320 | return false; |
fb0ac376 | 321 | |
fbb5d9ac | 322 | return true; |
166cf564 | 323 | } |
324 | ||
75a70cf9 | 325 | /* Walk through the function looking for GIMPLE_ASSIGNs with calls that |
ea523851 | 326 | return in memory on the RHS. For each of these, determine whether it is |
327 | safe to pass the address of the LHS as the return slot, and mark the | |
328 | call appropriately if so. | |
329 | ||
330 | The NRV shares the return slot with a local variable in the callee; this | |
331 | optimization shares the return slot with the target of the call within | |
332 | the caller. If the NRV is performed (which we can't know in general), | |
333 | this optimization is safe if the address of the target has not | |
334 | escaped prior to the call. If it has, modifications to the local | |
335 | variable will produce visible changes elsewhere, as in PR c++/19317. */ | |
336 | ||
cbe8bda8 | 337 | namespace { |
338 | ||
339 | const pass_data pass_data_return_slot = | |
ea523851 | 340 | { |
cbe8bda8 | 341 | GIMPLE_PASS, /* type */ |
342 | "retslot", /* name */ | |
343 | OPTGROUP_NONE, /* optinfo_flags */ | |
cbe8bda8 | 344 | TV_NONE, /* tv_id */ |
345 | PROP_ssa, /* properties_required */ | |
346 | 0, /* properties_provided */ | |
347 | 0, /* properties_destroyed */ | |
348 | 0, /* todo_flags_start */ | |
349 | 0, /* todo_flags_finish */ | |
ea523851 | 350 | }; |
cbe8bda8 | 351 | |
352 | class pass_return_slot : public gimple_opt_pass | |
353 | { | |
354 | public: | |
9af5ce0c | 355 | pass_return_slot (gcc::context *ctxt) |
356 | : gimple_opt_pass (pass_data_return_slot, ctxt) | |
cbe8bda8 | 357 | {} |
358 | ||
359 | /* opt_pass methods: */ | |
65b0537f | 360 | virtual unsigned int execute (function *); |
cbe8bda8 | 361 | |
362 | }; // class pass_return_slot | |
363 | ||
65b0537f | 364 | unsigned int |
365 | pass_return_slot::execute (function *fun) | |
366 | { | |
367 | basic_block bb; | |
368 | ||
369 | FOR_EACH_BB_FN (bb, fun) | |
370 | { | |
371 | gimple_stmt_iterator gsi; | |
372 | for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) | |
373 | { | |
1a91d914 | 374 | gcall *stmt; |
65b0537f | 375 | bool slot_opt_p; |
376 | ||
1a91d914 | 377 | stmt = dyn_cast <gcall *> (gsi_stmt (gsi)); |
378 | if (stmt | |
65b0537f | 379 | && gimple_call_lhs (stmt) |
380 | && !gimple_call_return_slot_opt_p (stmt) | |
d992f757 | 381 | /* Ignore internal functions without direct optabs, |
382 | those are expanded specially and aggregate_value_p | |
383 | on their result might result in undesirable warnings | |
384 | with some backends. */ | |
385 | && (!gimple_call_internal_p (stmt) | |
386 | || direct_internal_fn_p (gimple_call_internal_fn (stmt))) | |
65b0537f | 387 | && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)), |
388 | gimple_call_fndecl (stmt))) | |
389 | { | |
390 | /* Check if the location being assigned to is | |
391 | clobbered by the call. */ | |
392 | slot_opt_p = dest_safe_for_nrv_p (stmt); | |
393 | gimple_call_set_return_slot_opt (stmt, slot_opt_p); | |
394 | } | |
395 | } | |
396 | } | |
397 | return 0; | |
398 | } | |
399 | ||
cbe8bda8 | 400 | } // anon namespace |
401 | ||
402 | gimple_opt_pass * | |
403 | make_pass_return_slot (gcc::context *ctxt) | |
404 | { | |
405 | return new pass_return_slot (ctxt); | |
406 | } |