1 /* Backward propagation of indirect loads through PHIs.
2 Copyright (C) 2007-2022 Free Software Foundation, Inc.
3 Contributed by Richard Guenther <rguenther@suse.de>
4
5 This file is part of GCC.
6
7 GCC is free software; you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation; either version 3, or (at your option)
10 any later version.
11
12 GCC is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
16
17 You should have received a copy of the GNU General Public License
18 along with GCC; see the file COPYING3. If not see
19 <http://www.gnu.org/licenses/>. */
20
21 #include "config.h"
22 #include "system.h"
23 #include "coretypes.h"
24 #include "backend.h"
25 #include "tree.h"
26 #include "gimple.h"
27 #include "tree-pass.h"
28 #include "ssa.h"
29 #include "gimple-pretty-print.h"
30 #include "fold-const.h"
31 #include "tree-eh.h"
32 #include "gimplify.h"
33 #include "gimple-iterator.h"
34 #include "stor-layout.h"
35 #include "tree-ssa-loop.h"
36
37 /* This pass propagates indirect loads through the PHI node for its
38 address to make the load source possibly non-addressable and to
39 allow for PHI optimization to trigger.
40
41 For example the pass changes
42
43 # addr_1 = PHI <&a, &b>
44 tmp_1 = *addr_1;
45
46 to
47
48 # tmp_1 = PHI <a, b>
49
50 but also handles more complex scenarios like
51
52 D.2077_2 = &this_1(D)->a1;
53 ...
54
55 # b_12 = PHI <&c(2), D.2077_2(3)>
56 D.2114_13 = *b_12;
57 ...
58
59 # b_15 = PHI <b_12(4), &b(5)>
60 D.2080_5 = &this_1(D)->a0;
61 ...
62
63 # b_18 = PHI <D.2080_5(6), &c(7)>
64 ...
65
66 # b_21 = PHI <b_15(8), b_18(9)>
67 D.2076_8 = *b_21;
68
69 where the addresses loaded are defined by PHIs itself.
70 The above happens for
71
72 std::max(std::min(a0, c), std::min(std::max(a1, c), b))
73
74 where this pass transforms it to a form later PHI optimization
75 recognizes and transforms it to the simple
76
77 D.2109_10 = this_1(D)->a1;
78 D.2110_11 = c;
79 D.2114_31 = MAX_EXPR <D.2109_10, D.2110_11>;
80 D.2115_14 = b;
81 D.2125_17 = MIN_EXPR <D.2115_14, D.2114_31>;
82 D.2119_16 = this_1(D)->a0;
83 D.2124_32 = MIN_EXPR <D.2110_11, D.2119_16>;
84 D.2076_33 = MAX_EXPR <D.2125_17, D.2124_32>;
85
86 The pass does a dominator walk processing loads using a basic-block
87 local analysis and stores the result for use by transformations on
88 dominated basic-blocks. */
89
90
91 /* Structure to keep track of the value of a dereferenced PHI result
92 and the virtual operand used for that dereference. */
93
94 struct phiprop_d
95 {
96 tree value;
97 tree vuse;
98 };
99
100 /* Verify if the value recorded for NAME in PHIVN is still valid at
101 the start of basic block BB. */
102
103 static bool
phivn_valid_p(struct phiprop_d * phivn,tree name,basic_block bb)104 phivn_valid_p (struct phiprop_d *phivn, tree name, basic_block bb)
105 {
106 tree vuse = phivn[SSA_NAME_VERSION (name)].vuse;
107 gimple *use_stmt;
108 imm_use_iterator ui2;
109 bool ok = true;
110
111 /* The def stmts of the virtual uses need to be dominated by bb. */
112 gcc_assert (vuse != NULL_TREE);
113
114 FOR_EACH_IMM_USE_STMT (use_stmt, ui2, vuse)
115 {
116 /* If BB does not dominate a VDEF, the value is invalid. */
117 if ((gimple_vdef (use_stmt) != NULL_TREE
118 || gimple_code (use_stmt) == GIMPLE_PHI)
119 && !dominated_by_p (CDI_DOMINATORS, gimple_bb (use_stmt), bb))
120 {
121 ok = false;
122 break;
123 }
124 }
125
126 return ok;
127 }
128
129 /* Insert a new phi node for the dereference of PHI at basic_block
130 BB with the virtual operands from USE_STMT. */
131
132 static tree
phiprop_insert_phi(basic_block bb,gphi * phi,gimple * use_stmt,struct phiprop_d * phivn,size_t n)133 phiprop_insert_phi (basic_block bb, gphi *phi, gimple *use_stmt,
134 struct phiprop_d *phivn, size_t n)
135 {
136 tree res;
137 gphi *new_phi = NULL;
138 edge_iterator ei;
139 edge e;
140
141 gcc_assert (is_gimple_assign (use_stmt)
142 && gimple_assign_rhs_code (use_stmt) == MEM_REF);
143
144 /* Build a new PHI node to replace the definition of
145 the indirect reference lhs. */
146 res = gimple_assign_lhs (use_stmt);
147 if (TREE_CODE (res) == SSA_NAME)
148 new_phi = create_phi_node (res, bb);
149
150 if (dump_file && (dump_flags & TDF_DETAILS))
151 {
152 fprintf (dump_file, "Inserting PHI for result of load ");
153 print_gimple_stmt (dump_file, use_stmt, 0);
154 }
155
156 /* Add PHI arguments for each edge inserting loads of the
157 addressable operands. */
158 FOR_EACH_EDGE (e, ei, bb->preds)
159 {
160 tree old_arg, new_var;
161 gassign *tmp;
162 location_t locus;
163
164 old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
165 locus = gimple_phi_arg_location_from_edge (phi, e);
166 while (TREE_CODE (old_arg) == SSA_NAME
167 && (SSA_NAME_VERSION (old_arg) >= n
168 || phivn[SSA_NAME_VERSION (old_arg)].value == NULL_TREE))
169 {
170 gimple *def_stmt = SSA_NAME_DEF_STMT (old_arg);
171 old_arg = gimple_assign_rhs1 (def_stmt);
172 locus = gimple_location (def_stmt);
173 }
174
175 if (TREE_CODE (old_arg) == SSA_NAME)
176 {
177 if (dump_file && (dump_flags & TDF_DETAILS))
178 {
179 fprintf (dump_file, " for edge defining ");
180 print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e));
181 fprintf (dump_file, " reusing PHI result ");
182 print_generic_expr (dump_file,
183 phivn[SSA_NAME_VERSION (old_arg)].value);
184 fprintf (dump_file, "\n");
185 }
186 /* Reuse a formerly created dereference. */
187 new_var = phivn[SSA_NAME_VERSION (old_arg)].value;
188 }
189 else
190 {
191 tree rhs = gimple_assign_rhs1 (use_stmt);
192 gcc_assert (TREE_CODE (old_arg) == ADDR_EXPR);
193 if (TREE_CODE (res) == SSA_NAME)
194 new_var = make_ssa_name (TREE_TYPE (rhs));
195 else
196 new_var = unshare_expr (res);
197 if (!is_gimple_min_invariant (old_arg))
198 old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
199 else
200 old_arg = unshare_expr (old_arg);
201 tmp = gimple_build_assign (new_var,
202 fold_build2 (MEM_REF, TREE_TYPE (rhs),
203 old_arg,
204 TREE_OPERAND (rhs, 1)));
205 gimple_set_location (tmp, locus);
206
207 gsi_insert_on_edge (e, tmp);
208 update_stmt (tmp);
209
210 if (dump_file && (dump_flags & TDF_DETAILS))
211 {
212 fprintf (dump_file, " for edge defining ");
213 print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e));
214 fprintf (dump_file, " inserting load ");
215 print_gimple_stmt (dump_file, tmp, 0);
216 }
217 }
218
219 if (new_phi)
220 add_phi_arg (new_phi, new_var, e, locus);
221 }
222
223 if (new_phi)
224 {
225 update_stmt (new_phi);
226
227 if (dump_file && (dump_flags & TDF_DETAILS))
228 print_gimple_stmt (dump_file, new_phi, 0);
229 }
230
231 return res;
232 }
233
234 /* Verify if *idx is available at *DATA. */
235
236 static bool
chk_uses(tree,tree * idx,void * data)237 chk_uses (tree, tree *idx, void *data)
238 {
239 basic_block dom = (basic_block) data;
240 if (TREE_CODE (*idx) == SSA_NAME)
241 return (SSA_NAME_IS_DEFAULT_DEF (*idx)
242 || ! dominated_by_p (CDI_DOMINATORS,
243 gimple_bb (SSA_NAME_DEF_STMT (*idx)), dom));
244 return true;
245 }
246
247 /* Propagate between the phi node arguments of PHI in BB and phi result
248 users. For now this matches
249 # p_2 = PHI <&x, &y>
250 <Lx>:;
251 p_3 = p_2;
252 z_2 = *p_3;
253 and converts it to
254 # z_2 = PHI <x, y>
255 <Lx>:;
256 Returns true if a transformation was done and edge insertions
257 need to be committed. Global data PHIVN and N is used to track
258 past transformation results. We need to be especially careful here
259 with aliasing issues as we are moving memory reads. */
260
261 static bool
propagate_with_phi(basic_block bb,gphi * phi,struct phiprop_d * phivn,size_t n)262 propagate_with_phi (basic_block bb, gphi *phi, struct phiprop_d *phivn,
263 size_t n)
264 {
265 tree ptr = PHI_RESULT (phi);
266 gimple *use_stmt;
267 tree res = NULL_TREE;
268 gimple_stmt_iterator gsi;
269 imm_use_iterator ui;
270 use_operand_p arg_p, use;
271 ssa_op_iter i;
272 bool phi_inserted;
273 bool changed;
274 tree type = NULL_TREE;
275
276 if (!POINTER_TYPE_P (TREE_TYPE (ptr))
277 || (!is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr)))
278 && TYPE_MODE (TREE_TYPE (TREE_TYPE (ptr))) == BLKmode))
279 return false;
280
281 /* Check if we can "cheaply" dereference all phi arguments. */
282 FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
283 {
284 tree arg = USE_FROM_PTR (arg_p);
285 /* Walk the ssa chain until we reach a ssa name we already
286 created a value for or we reach a definition of the form
287 ssa_name_n = &var; */
288 while (TREE_CODE (arg) == SSA_NAME
289 && !SSA_NAME_IS_DEFAULT_DEF (arg)
290 && (SSA_NAME_VERSION (arg) >= n
291 || phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
292 {
293 gimple *def_stmt = SSA_NAME_DEF_STMT (arg);
294 if (!gimple_assign_single_p (def_stmt))
295 return false;
296 arg = gimple_assign_rhs1 (def_stmt);
297 }
298 if (TREE_CODE (arg) != ADDR_EXPR
299 && !(TREE_CODE (arg) == SSA_NAME
300 && SSA_NAME_VERSION (arg) < n
301 && phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
302 && (!type
303 || types_compatible_p
304 (type, TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value)))
305 && phivn_valid_p (phivn, arg, bb)))
306 return false;
307 if (!type
308 && TREE_CODE (arg) == SSA_NAME)
309 type = TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value);
310 }
311
312 /* Find a dereferencing use. First follow (single use) ssa
313 copy chains for ptr. */
314 while (single_imm_use (ptr, &use, &use_stmt)
315 && gimple_assign_ssa_name_copy_p (use_stmt))
316 ptr = gimple_assign_lhs (use_stmt);
317
318 /* Replace the first dereference of *ptr if there is one and if we
319 can move the loads to the place of the ptr phi node. */
320 phi_inserted = false;
321 changed = false;
322 FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
323 {
324 gimple *def_stmt;
325 tree vuse;
326
327 /* Only replace loads in blocks that post-dominate the PHI node. That
328 makes sure we don't end up speculating loads. */
329 if (!dominated_by_p (CDI_POST_DOMINATORS,
330 bb, gimple_bb (use_stmt)))
331 continue;
332
333 /* Check whether this is a load of *ptr. */
334 if (!(is_gimple_assign (use_stmt)
335 && gimple_assign_rhs_code (use_stmt) == MEM_REF
336 && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
337 && integer_zerop (TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 1))
338 && (!type
339 || types_compatible_p
340 (TREE_TYPE (gimple_assign_lhs (use_stmt)), type))
341 /* We cannot replace a load that may throw or is volatile.
342 For volatiles the transform can change the number of
343 executions if the load is inside a loop but the address
344 computations outside (PR91812). We could relax this
345 if we guard against that appropriately. For loads that can
346 throw we could relax things if the moved loads all are
347 known to not throw. */
348 && !stmt_can_throw_internal (cfun, use_stmt)
349 && !gimple_has_volatile_ops (use_stmt)))
350 continue;
351
352 /* Check if we can move the loads. The def stmt of the virtual use
353 needs to be in a different basic block dominating bb. When the
354 def is an edge-inserted one we know it dominates us. */
355 vuse = gimple_vuse (use_stmt);
356 def_stmt = SSA_NAME_DEF_STMT (vuse);
357 if (!SSA_NAME_IS_DEFAULT_DEF (vuse)
358 && (gimple_bb (def_stmt) == bb
359 || (gimple_bb (def_stmt)
360 && !dominated_by_p (CDI_DOMINATORS,
361 bb, gimple_bb (def_stmt)))))
362 goto next;
363
364 /* Found a proper dereference with an aggregate copy. Just
365 insert aggregate copies on the edges instead. */
366 if (!is_gimple_reg_type (TREE_TYPE (gimple_assign_lhs (use_stmt))))
367 {
368 if (!gimple_vdef (use_stmt))
369 goto next;
370
371 /* As we replicate the lhs on each incoming edge all
372 used SSA names have to be available there. */
373 if (! for_each_index (gimple_assign_lhs_ptr (use_stmt),
374 chk_uses,
375 get_immediate_dominator (CDI_DOMINATORS,
376 gimple_bb (phi))))
377 goto next;
378
379 gimple *vuse_stmt;
380 imm_use_iterator vui;
381 use_operand_p vuse_p;
382 /* In order to move the aggregate copies earlier, make sure
383 there are no statements that could read from memory
384 aliasing the lhs in between the start of bb and use_stmt.
385 As we require use_stmt to have a VDEF above, loads after
386 use_stmt will use a different virtual SSA_NAME. */
387 FOR_EACH_IMM_USE_FAST (vuse_p, vui, vuse)
388 {
389 vuse_stmt = USE_STMT (vuse_p);
390 if (vuse_stmt == use_stmt)
391 continue;
392 if (!dominated_by_p (CDI_DOMINATORS,
393 gimple_bb (vuse_stmt), bb))
394 continue;
395 if (ref_maybe_used_by_stmt_p (vuse_stmt,
396 gimple_assign_lhs (use_stmt)))
397 goto next;
398 }
399
400 phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
401
402 /* Remove old stmt. The phi is taken care of by DCE. */
403 gsi = gsi_for_stmt (use_stmt);
404 /* Unlinking the VDEF here is fine as we are sure that we process
405 stmts in execution order due to aggregate copies having VDEFs
406 and we emit loads on the edges in the very same order.
407 We get multiple copies (or intermediate register loads) handled
408 only by walking PHIs or immediate uses in a lucky order though,
409 so we could signal the caller to re-start iterating over PHIs
410 when we come here which would make it quadratic in the number
411 of PHIs. */
412 unlink_stmt_vdef (use_stmt);
413 gsi_remove (&gsi, true);
414
415 changed = true;
416 }
417
418 /* Found a proper dereference. Insert a phi node if this
419 is the first load transformation. */
420 else if (!phi_inserted)
421 {
422 res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
423 type = TREE_TYPE (res);
424
425 /* Remember the value we created for *ptr. */
426 phivn[SSA_NAME_VERSION (ptr)].value = res;
427 phivn[SSA_NAME_VERSION (ptr)].vuse = vuse;
428
429 /* Remove old stmt. The phi is taken care of by DCE, if we
430 want to delete it here we also have to delete all intermediate
431 copies. */
432 gsi = gsi_for_stmt (use_stmt);
433 gsi_remove (&gsi, true);
434
435 phi_inserted = true;
436 changed = true;
437 }
438 else
439 {
440 /* Further replacements are easy, just make a copy out of the
441 load. */
442 gimple_assign_set_rhs1 (use_stmt, res);
443 update_stmt (use_stmt);
444 changed = true;
445 }
446
447 next:;
448 /* Continue searching for a proper dereference. */
449 }
450
451 return changed;
452 }
453
454 /* Main entry for phiprop pass. */
455
456 namespace {
457
458 const pass_data pass_data_phiprop =
459 {
460 GIMPLE_PASS, /* type */
461 "phiprop", /* name */
462 OPTGROUP_NONE, /* optinfo_flags */
463 TV_TREE_PHIPROP, /* tv_id */
464 ( PROP_cfg | PROP_ssa ), /* properties_required */
465 0, /* properties_provided */
466 0, /* properties_destroyed */
467 0, /* todo_flags_start */
468 TODO_update_ssa, /* todo_flags_finish */
469 };
470
471 class pass_phiprop : public gimple_opt_pass
472 {
473 public:
pass_phiprop(gcc::context * ctxt)474 pass_phiprop (gcc::context *ctxt)
475 : gimple_opt_pass (pass_data_phiprop, ctxt)
476 {}
477
478 /* opt_pass methods: */
gate(function *)479 virtual bool gate (function *) { return flag_tree_phiprop; }
480 virtual unsigned int execute (function *);
481
482 }; // class pass_phiprop
483
484 unsigned int
execute(function * fun)485 pass_phiprop::execute (function *fun)
486 {
487 struct phiprop_d *phivn;
488 bool did_something = false;
489 basic_block bb;
490 gphi_iterator gsi;
491 unsigned i;
492 size_t n;
493
494 calculate_dominance_info (CDI_DOMINATORS);
495 calculate_dominance_info (CDI_POST_DOMINATORS);
496
497 n = num_ssa_names;
498 phivn = XCNEWVEC (struct phiprop_d, n);
499
500 /* Walk the dominator tree in preorder. */
501 auto_vec<basic_block> bbs
502 = get_all_dominated_blocks (CDI_DOMINATORS,
503 single_succ (ENTRY_BLOCK_PTR_FOR_FN (fun)));
504 FOR_EACH_VEC_ELT (bbs, i, bb)
505 {
506 /* Since we're going to move dereferences across predecessor
507 edges avoid blocks with abnormal predecessors. */
508 if (bb_has_abnormal_pred (bb))
509 continue;
510 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
511 did_something |= propagate_with_phi (bb, gsi.phi (), phivn, n);
512 }
513
514 if (did_something)
515 gsi_commit_edge_inserts ();
516
517 free (phivn);
518
519 free_dominance_info (CDI_POST_DOMINATORS);
520
521 return 0;
522 }
523
524 } // anon namespace
525
526 gimple_opt_pass *
make_pass_phiprop(gcc::context * ctxt)527 make_pass_phiprop (gcc::context *ctxt)
528 {
529 return new pass_phiprop (ctxt);
530 }
531