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1 : : /* Language independent return value optimizations
2 : : Copyright (C) 2004-2024 Free Software Foundation, Inc.
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
8 : : the Free Software Foundation; either version 3, or (at your option)
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
17 : : along with GCC; see the file COPYING3. If not see
18 : : <http://www.gnu.org/licenses/>. */
19 : :
20 : : #include "config.h"
21 : : #include "system.h"
22 : : #include "coretypes.h"
23 : : #include "backend.h"
24 : : #include "tree.h"
25 : : #include "gimple.h"
26 : : #include "tree-pass.h"
27 : : #include "ssa.h"
28 : : #include "tree-pretty-print.h"
29 : : #include "gimple-iterator.h"
30 : : #include "gimple-walk.h"
31 : : #include "internal-fn.h"
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 : :
44 : : This is basically a generic equivalent to the C++ front-end's
45 : : Named Return Value optimization. */
46 : :
47 : : struct nrv_data_t
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 : :
53 : : /* This is the function's RESULT_DECL. We will replace all occurrences
54 : : of VAR with RESULT_DECL when we apply this optimization. */
55 : : tree result;
56 : : int modified;
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 : 144083 : finalize_nrv_r (tree *tp, int *walk_subtrees, void *data)
74 : : {
75 : 144083 : struct walk_stmt_info *wi = (struct walk_stmt_info *) data;
76 : 144083 : struct nrv_data_t *dp = (struct nrv_data_t *) wi->info;
77 : :
78 : : /* No need to walk into types. */
79 : 144083 : if (TYPE_P (*tp))
80 : 0 : *walk_subtrees = 0;
81 : :
82 : : /* Otherwise replace all occurrences of VAR with RESULT. */
83 : 144083 : else if (*tp == dp->var)
84 : : {
85 : 1744 : *tp = dp->result;
86 : 1744 : dp->modified = 1;
87 : : }
88 : :
89 : : /* Keep iterating. */
90 : 144083 : 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. */
104 : :
105 : : namespace {
106 : :
107 : : const pass_data pass_data_nrv =
108 : : {
109 : : GIMPLE_PASS, /* type */
110 : : "nrv", /* name */
111 : : OPTGROUP_NONE, /* optinfo_flags */
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 : 285189 : pass_nrv (gcc::context *ctxt)
124 : 570378 : : gimple_opt_pass (pass_data_nrv, ctxt)
125 : : {}
126 : :
127 : : /* opt_pass methods: */
128 : 1419128 : bool gate (function *) final override { return optimize > 0; }
129 : :
130 : : unsigned int execute (function *) final override;
131 : :
132 : : }; // class pass_nrv
133 : :
134 : : unsigned int
135 : 979498 : pass_nrv::execute (function *fun)
136 : : {
137 : 979498 : tree result = DECL_RESULT (current_function_decl);
138 : 979498 : tree result_type = TREE_TYPE (result);
139 : 979498 : tree found = NULL;
140 : 979498 : basic_block bb;
141 : 979498 : gimple_stmt_iterator gsi;
142 : 979498 : struct nrv_data_t data;
143 : :
144 : : /* If this function does not return an aggregate type in memory, then
145 : : there is nothing to do. */
146 : 979498 : if (!aggregate_value_p (result, current_function_decl))
147 : : return 0;
148 : :
149 : : /* If a GIMPLE type is returned in memory, finalize_nrv_r might create
150 : : non-GIMPLE. */
151 : 52492 : if (is_gimple_reg_type (result_type))
152 : : return 0;
153 : :
154 : : /* If the front end already did something like this, don't do it here. */
155 : 43224 : if (DECL_NAME (result))
156 : : return 0;
157 : :
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 : 42519 : if (TREE_ADDRESSABLE (result))
162 : : return 0;
163 : :
164 : : /* Look through each block for assignments to the RESULT_DECL. */
165 : 598219 : FOR_EACH_BB_FN (bb, fun)
166 : : {
167 : 3978146 : for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
168 : : {
169 : 2824070 : gimple *stmt = gsi_stmt (gsi);
170 : 2824070 : tree ret_val;
171 : :
172 : 2824070 : if (greturn *return_stmt = dyn_cast <greturn *> (stmt))
173 : : {
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. */
177 : 901 : ret_val = gimple_return_retval (return_stmt);
178 : 901 : if (ret_val)
179 : 901 : gcc_assert (ret_val == result);
180 : : }
181 : 2823169 : else if (gimple_has_lhs (stmt)
182 : 1337626 : && gimple_get_lhs (stmt) == result)
183 : : {
184 : 3152 : tree rhs;
185 : :
186 : 3152 : if (!gimple_assign_copy_p (stmt))
187 : : return 0;
188 : :
189 : 2854 : rhs = gimple_assign_rhs1 (stmt);
190 : :
191 : : /* Now verify that this return statement uses the same value
192 : : as any previously encountered return statement. */
193 : 2854 : if (found != NULL)
194 : : {
195 : : /* If we found a return statement using a different variable
196 : : than previous return statements, then we cannot perform
197 : : NRV optimizations. */
198 : 38 : if (found != rhs)
199 : : return 0;
200 : : }
201 : : else
202 : : found = rhs;
203 : :
204 : : /* The returned value must be a local automatic variable of the
205 : : same type and alignment as the function's result. */
206 : 2828 : if (!VAR_P (found)
207 : 2828 : || TREE_THIS_VOLATILE (found)
208 : 2547 : || !auto_var_in_fn_p (found, current_function_decl)
209 : 1659 : || TREE_ADDRESSABLE (found)
210 : 1513 : || DECL_ALIGN (found) > DECL_ALIGN (result)
211 : 3556 : || !useless_type_conversion_p (result_type,
212 : 728 : TREE_TYPE (found)))
213 : 2100 : return 0;
214 : : }
215 : 5841883 : else if (gimple_has_lhs (stmt))
216 : : {
217 : 1235624 : tree addr = get_base_address (gimple_get_lhs (stmt));
218 : : /* If there's any MODIFY of component of RESULT,
219 : : then bail out. */
220 : 1235624 : if (addr && addr == result)
221 : : return 0;
222 : : }
223 : : }
224 : : }
225 : :
226 : 800 : if (!found)
227 : : return 0;
228 : :
229 : : /* If dumping details, then note once and only the NRV replacement. */
230 : 690 : if (dump_file && (dump_flags & TDF_DETAILS))
231 : : {
232 : 0 : fprintf (dump_file, "NRV Replaced: ");
233 : 0 : print_generic_expr (dump_file, found, dump_flags);
234 : 0 : fprintf (dump_file, " with: ");
235 : 0 : print_generic_expr (dump_file, result, dump_flags);
236 : 0 : fprintf (dump_file, "\n");
237 : : }
238 : :
239 : 690 : TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found);
240 : :
241 : : /* Now walk through the function changing all references to VAR to be
242 : : RESULT. */
243 : 690 : data.var = found;
244 : 690 : data.result = result;
245 : 9294 : FOR_EACH_BB_FN (bb, fun)
246 : : {
247 : 57640 : for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); )
248 : : {
249 : 40432 : gimple *stmt = gsi_stmt (gsi);
250 : : /* If this is a copy from VAR to RESULT, remove it. */
251 : 40432 : if (gimple_assign_copy_p (stmt)
252 : 8746 : && gimple_assign_lhs (stmt) == result
253 : 41134 : && gimple_assign_rhs1 (stmt) == found)
254 : : {
255 : 702 : unlink_stmt_vdef (stmt);
256 : 702 : gsi_remove (&gsi, true);
257 : 702 : release_defs (stmt);
258 : : }
259 : : else
260 : : {
261 : 39730 : struct walk_stmt_info wi;
262 : 39730 : memset (&wi, 0, sizeof (wi));
263 : 39730 : wi.info = &data;
264 : 39730 : data.modified = 0;
265 : 39730 : walk_gimple_op (stmt, finalize_nrv_r, &wi);
266 : 39730 : if (data.modified)
267 : : {
268 : : /* If this is a CLOBBER of VAR, remove it. */
269 : 1744 : if (gimple_clobber_p (stmt))
270 : : {
271 : 687 : unlink_stmt_vdef (stmt);
272 : 687 : gsi_remove (&gsi, true);
273 : 687 : release_defs (stmt);
274 : 687 : continue;
275 : : }
276 : 1057 : update_stmt (stmt);
277 : : }
278 : 39043 : gsi_next (&gsi);
279 : : }
280 : : }
281 : : }
282 : :
283 : 690 : SET_DECL_VALUE_EXPR (found, result);
284 : 690 : DECL_HAS_VALUE_EXPR_P (found) = 1;
285 : :
286 : 690 : return 0;
287 : : }
288 : :
289 : : } // anon namespace
290 : :
291 : : gimple_opt_pass *
292 : 285189 : make_pass_nrv (gcc::context *ctxt)
293 : : {
294 : 285189 : return new pass_nrv (ctxt);
295 : : }
296 : :
297 : : /* Determine (pessimistically) whether DEST is available for NRV
298 : : optimization, where DEST is expected to be the LHS of a modify
299 : : expression where the RHS is a function returning an aggregate.
300 : :
301 : : DEST is available if it is not clobbered or used by the call. */
302 : :
303 : : static bool
304 : 7244 : dest_safe_for_nrv_p (gcall *call)
305 : : {
306 : 7244 : tree dest = gimple_call_lhs (call);
307 : :
308 : 7244 : dest = get_base_address (dest);
309 : 7244 : if (! dest)
310 : : return false;
311 : :
312 : 7244 : if (TREE_CODE (dest) == SSA_NAME)
313 : : return true;
314 : :
315 : 3471 : if (call_may_clobber_ref_p (call, dest, false)
316 : 3471 : || ref_maybe_used_by_stmt_p (call, dest, false))
317 : 715 : return false;
318 : :
319 : : return true;
320 : : }
321 : :
322 : : /* Walk through the function looking for GIMPLE_ASSIGNs with calls that
323 : : return in memory on the RHS. For each of these, determine whether it is
324 : : safe to pass the address of the LHS as the return slot, and mark the
325 : : call appropriately if so.
326 : :
327 : : The NRV shares the return slot with a local variable in the callee; this
328 : : optimization shares the return slot with the target of the call within
329 : : the caller. If the NRV is performed (which we can't know in general),
330 : : this optimization is safe if the address of the target has not
331 : : escaped prior to the call. If it has, modifications to the local
332 : : variable will produce visible changes elsewhere, as in PR c++/19317. */
333 : :
334 : : namespace {
335 : :
336 : : const pass_data pass_data_return_slot =
337 : : {
338 : : GIMPLE_PASS, /* type */
339 : : "retslot", /* name */
340 : : OPTGROUP_NONE, /* optinfo_flags */
341 : : TV_NONE, /* tv_id */
342 : : PROP_ssa, /* properties_required */
343 : : 0, /* properties_provided */
344 : : 0, /* properties_destroyed */
345 : : 0, /* todo_flags_start */
346 : : 0, /* todo_flags_finish */
347 : : };
348 : :
349 : : class pass_return_slot : public gimple_opt_pass
350 : : {
351 : : public:
352 : 285189 : pass_return_slot (gcc::context *ctxt)
353 : 570378 : : gimple_opt_pass (pass_data_return_slot, ctxt)
354 : : {}
355 : :
356 : : /* opt_pass methods: */
357 : : unsigned int execute (function *) final override;
358 : :
359 : : }; // class pass_return_slot
360 : :
361 : : unsigned int
362 : 976884 : pass_return_slot::execute (function *fun)
363 : : {
364 : 976884 : basic_block bb;
365 : :
366 : 10820317 : FOR_EACH_BB_FN (bb, fun)
367 : : {
368 : 9843433 : gimple_stmt_iterator gsi;
369 : 88271508 : for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
370 : : {
371 : 68584642 : gcall *stmt;
372 : 68584642 : bool slot_opt_p;
373 : :
374 : 68584642 : stmt = dyn_cast <gcall *> (gsi_stmt (gsi));
375 : 5130875 : if (stmt
376 : 5130875 : && gimple_call_lhs (stmt)
377 : 1923534 : && !gimple_call_return_slot_opt_p (stmt)
378 : : /* Ignore internal functions, those are expanded specially
379 : : and aggregate_value_p on their result might result in
380 : : undesirable warnings with some backends. */
381 : 1733798 : && !gimple_call_internal_p (stmt)
382 : 1663573 : && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)),
383 : 1663573 : gimple_call_fndecl (stmt)))
384 : : {
385 : : /* Check if the location being assigned to is
386 : : clobbered by the call. */
387 : 7244 : slot_opt_p = dest_safe_for_nrv_p (stmt);
388 : 68591886 : gimple_call_set_return_slot_opt (stmt, slot_opt_p);
389 : : }
390 : : }
391 : : }
392 : 976884 : return 0;
393 : : }
394 : :
395 : : } // anon namespace
396 : :
397 : : gimple_opt_pass *
398 : 285189 : make_pass_return_slot (gcc::context *ctxt)
399 : : {
400 : 285189 : return new pass_return_slot (ctxt);
401 : : }
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