1/* Language independent return value optimizations
2 Copyright (C) 2004-2017 Free Software Foundation, Inc.
3
4This file is part of GCC.
5
6GCC is free software; you can redistribute it and/or modify
7it under the terms of the GNU General Public License as published by
8the Free Software Foundation; either version 3, or (at your option)
9any later version.
10
11GCC is distributed in the hope that it will be useful,
12but WITHOUT ANY WARRANTY; without even the implied warranty of
13MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14GNU General Public License for more details.
15
16You should have received a copy of the GNU General Public License
17along 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
47struct 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
59static 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
72static tree
73finalize_nrv_r (tree *tp, int *walk_subtrees, void *data)
74{
75 struct walk_stmt_info *wi = (struct walk_stmt_info *) data;
76 struct nrv_data_t *dp = (struct nrv_data_t *) wi->info;
77
78 /* No need to walk into types. */
79 if (TYPE_P (*tp))
80 *walk_subtrees = 0;
81
82 /* Otherwise replace all occurrences of VAR with RESULT. */
83 else if (*tp == dp->var)
84 {
85 *tp = dp->result;
86 dp->modified = 1;
87 }
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. */
104
105namespace {
106
107const 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
120class pass_nrv : public gimple_opt_pass
121{
122public:
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
134unsigned int
135pass_nrv::execute (function *fun)
136{
137 tree result = DECL_RESULT (current_function_decl);
138 tree result_type = TREE_TYPE (result);
139 tree found = NULL;
140 basic_block bb;
141 gimple_stmt_iterator gsi;
142 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 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 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 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 if (TREE_ADDRESSABLE (result))
162 return 0;
163
164 /* Look through each block for assignments to the RESULT_DECL. */
165 FOR_EACH_BB_FN (bb, fun)
166 {
167 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
168 {
169 gimple *stmt = gsi_stmt (gsi);
170 tree ret_val;
171
172 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 ret_val = gimple_return_retval (return_stmt);
178 if (ret_val)
179 gcc_assert (ret_val == result);
180 }
181 else if (gimple_has_lhs (stmt)
182 && gimple_get_lhs (stmt) == result)
183 {
184 tree rhs;
185
186 if (!gimple_assign_copy_p (stmt))
187 return 0;
188
189 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 if (found != NULL)
194 {
195 /* If we found a return statement using a different variable
196 than previous return statements, then we can not perform
197 NRV optimizations. */
198 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 if (!VAR_P (found)
207 || TREE_THIS_VOLATILE (found)
208 || !auto_var_in_fn_p (found, current_function_decl)
209 || TREE_ADDRESSABLE (found)
210 || DECL_ALIGN (found) > DECL_ALIGN (result)
211 || !useless_type_conversion_p (result_type,
212 TREE_TYPE (found)))
213 return 0;
214 }
215 else if (gimple_has_lhs (stmt))
216 {
217 tree addr = get_base_address (gimple_get_lhs (stmt));
218 /* If there's any MODIFY of component of RESULT,
219 then bail out. */
220 if (addr && addr == result)
221 return 0;
222 }
223 }
224 }
225
226 if (!found)
227 return 0;
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
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
252 TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found);
253
254 /* Now walk through the function changing all references to VAR to be
255 RESULT. */
256 data.var = found;
257 data.result = result;
258 FOR_EACH_BB_FN (bb, fun)
259 {
260 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); )
261 {
262 gimple *stmt = gsi_stmt (gsi);
263 /* If this is a copy from VAR to RESULT, remove it. */
264 if (gimple_assign_copy_p (stmt)
265 && gimple_assign_lhs (stmt) == result
266 && gimple_assign_rhs1 (stmt) == found)
267 {
268 unlink_stmt_vdef (stmt);
269 gsi_remove (&gsi, true);
270 release_defs (stmt);
271 }
272 else
273 {
274 struct walk_stmt_info wi;
275 memset (&wi, 0, sizeof (wi));
276 wi.info = &data;
277 data.modified = 0;
278 walk_gimple_op (stmt, finalize_nrv_r, &wi);
279 if (data.modified)
280 update_stmt (stmt);
281 gsi_next (&gsi);
282 }
283 }
284 }
285
286 SET_DECL_VALUE_EXPR (found, result);
287 DECL_HAS_VALUE_EXPR_P (found) = 1;
288
289 return 0;
290}
291
292} // anon namespace
293
294gimple_opt_pass *
295make_pass_nrv (gcc::context *ctxt)
296{
297 return new pass_nrv (ctxt);
298}
299
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
304 DEST is available if it is not clobbered or used by the call. */
305
306static bool
307dest_safe_for_nrv_p (gcall *call)
308{
309 tree dest = gimple_call_lhs (call);
310
311 dest = get_base_address (dest);
312 if (! dest)
313 return false;
314
315 if (TREE_CODE (dest) == SSA_NAME)
316 return true;
317
318 if (call_may_clobber_ref_p (call, dest)
319 || ref_maybe_used_by_stmt_p (call, dest))
320 return false;
321
322 return true;
323}
324
325/* Walk through the function looking for GIMPLE_ASSIGNs with calls that
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
337namespace {
338
339const pass_data pass_data_return_slot =
340{
341 GIMPLE_PASS, /* type */
342 "retslot", /* name */
343 OPTGROUP_NONE, /* optinfo_flags */
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 */
350};
351
352class pass_return_slot : public gimple_opt_pass
353{
354public:
355 pass_return_slot (gcc::context *ctxt)
356 : gimple_opt_pass (pass_data_return_slot, ctxt)
357 {}
358
359 /* opt_pass methods: */
360 virtual unsigned int execute (function *);
361
362}; // class pass_return_slot
363
364unsigned int
365pass_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 {
374 gcall *stmt;
375 bool slot_opt_p;
376
377 stmt = dyn_cast <gcall *> (gsi_stmt (gsi));
378 if (stmt
379 && gimple_call_lhs (stmt)
380 && !gimple_call_return_slot_opt_p (stmt)
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)))
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
400} // anon namespace
401
402gimple_opt_pass *
403make_pass_return_slot (gcc::context *ctxt)
404{
405 return new pass_return_slot (ctxt);
406}
407