1/* Generic routines for manipulating PHIs
2 Copyright (C) 2003-2023 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 "ssa.h"
27#include "fold-const.h"
28#include "gimple-iterator.h"
29#include "tree-ssa.h"
30
31/* Rewriting a function into SSA form can create a huge number of PHIs
32 many of which may be thrown away shortly after their creation if jumps
33 were threaded through PHI nodes.
34
35 While our garbage collection mechanisms will handle this situation, it
36 is extremely wasteful to create nodes and throw them away, especially
37 when the nodes can be reused.
38
39 For PR 8361, we can significantly reduce the number of nodes allocated
40 and thus the total amount of memory allocated by managing PHIs a
41 little. This additionally helps reduce the amount of work done by the
42 garbage collector. Similar results have been seen on a wider variety
43 of tests (such as the compiler itself).
44
45 PHI nodes have different sizes, so we can't have a single list of all
46 the PHI nodes as it would be too expensive to walk down that list to
47 find a PHI of a suitable size.
48
49 Instead we have an array of lists of free PHI nodes. The array is
50 indexed by the number of PHI alternatives that PHI node can hold.
51 Except for the last array member, which holds all remaining PHI
52 nodes.
53
54 So to find a free PHI node, we compute its index into the free PHI
55 node array and see if there are any elements with an exact match.
56 If so, then we are done. Otherwise, we test the next larger size
57 up and continue until we are in the last array element.
58
59 We do not actually walk members of the last array element. While it
60 might allow us to pick up a few reusable PHI nodes, it could potentially
61 be very expensive if the program has released a bunch of large PHI nodes,
62 but keeps asking for even larger PHI nodes. Experiments have shown that
63 walking the elements of the last array entry would result in finding less
64 than .1% additional reusable PHI nodes.
65
66 Note that we can never have less than two PHI argument slots. Thus,
67 the -2 on all the calculations below. */
68
69#define NUM_BUCKETS 10
70static GTY ((deletable (""))) vec<gimple *, va_gc> *free_phinodes[NUM_BUCKETS - 2];
71static unsigned long free_phinode_count;
72
73static int ideal_phi_node_len (int);
74
75unsigned int phi_nodes_reused;
76unsigned int phi_nodes_created;
77
78/* Dump some simple statistics regarding the re-use of PHI nodes. */
79
80void
81phinodes_print_statistics (void)
82{
83 fprintf (stderr, format: "%-32s" PRsa (11) "\n", "PHI nodes allocated:",
84 SIZE_AMOUNT (phi_nodes_created));
85 fprintf (stderr, format: "%-32s" PRsa (11) "\n", "PHI nodes reused:",
86 SIZE_AMOUNT (phi_nodes_reused));
87}
88
89/* Allocate a PHI node with at least LEN arguments. If the free list
90 happens to contain a PHI node with LEN arguments or more, return
91 that one. */
92
93static inline gphi *
94allocate_phi_node (size_t len)
95{
96 gphi *phi;
97 size_t bucket = NUM_BUCKETS - 2;
98 size_t size = sizeof (struct gphi)
99 + (len - 1) * sizeof (struct phi_arg_d);
100
101 if (free_phinode_count)
102 for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
103 if (free_phinodes[bucket])
104 break;
105
106 /* If our free list has an element, then use it. */
107 if (bucket < NUM_BUCKETS - 2
108 && gimple_phi_capacity (gs: (*free_phinodes[bucket])[0]) >= len)
109 {
110 free_phinode_count--;
111 phi = as_a <gphi *> (p: free_phinodes[bucket]->pop ());
112 if (free_phinodes[bucket]->is_empty ())
113 vec_free (v&: free_phinodes[bucket]);
114 if (GATHER_STATISTICS)
115 phi_nodes_reused++;
116 }
117 else
118 {
119 phi = static_cast <gphi *> (ggc_internal_alloc (s: size));
120 if (GATHER_STATISTICS)
121 {
122 enum gimple_alloc_kind kind = gimple_alloc_kind (code: GIMPLE_PHI);
123 phi_nodes_created++;
124 gimple_alloc_counts[(int) kind]++;
125 gimple_alloc_sizes[(int) kind] += size;
126 }
127 }
128
129 return phi;
130}
131
132/* Given LEN, the original number of requested PHI arguments, return
133 a new, "ideal" length for the PHI node. The "ideal" length rounds
134 the total size of the PHI node up to the next power of two bytes.
135
136 Rounding up will not result in wasting any memory since the size request
137 will be rounded up by the GC system anyway. [ Note this is not entirely
138 true since the original length might have fit on one of the special
139 GC pages. ] By rounding up, we may avoid the need to reallocate the
140 PHI node later if we increase the number of arguments for the PHI. */
141
142static int
143ideal_phi_node_len (int len)
144{
145 size_t size, new_size;
146 int log2, new_len;
147
148 /* We do not support allocations of less than two PHI argument slots. */
149 if (len < 2)
150 len = 2;
151
152 /* Compute the number of bytes of the original request. */
153 size = sizeof (struct gphi)
154 + (len - 1) * sizeof (struct phi_arg_d);
155
156 /* Round it up to the next power of two. */
157 log2 = ceil_log2 (x: size);
158 new_size = 1 << log2;
159
160 /* Now compute and return the number of PHI argument slots given an
161 ideal size allocation. */
162 new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
163 return new_len;
164}
165
166/* Return a PHI node with LEN argument slots for variable VAR. */
167
168static gphi *
169make_phi_node (tree var, int len)
170{
171 gphi *phi;
172 int capacity, i;
173
174 capacity = ideal_phi_node_len (len);
175
176 phi = allocate_phi_node (len: capacity);
177
178 /* We need to clear the entire PHI node, including the argument
179 portion, because we represent a "missing PHI argument" by placing
180 NULL_TREE in PHI_ARG_DEF. */
181 memset (s: phi, c: 0, n: (sizeof (struct gphi)
182 - sizeof (struct phi_arg_d)
183 + sizeof (struct phi_arg_d) * len));
184 phi->code = GIMPLE_PHI;
185 gimple_init_singleton (g: phi);
186 phi->nargs = len;
187 phi->capacity = capacity;
188 if (!var)
189 ;
190 else if (TREE_CODE (var) == SSA_NAME)
191 gimple_phi_set_result (phi, result: var);
192 else
193 gimple_phi_set_result (phi, result: make_ssa_name (var, stmt: phi));
194
195 for (i = 0; i < len; i++)
196 {
197 use_operand_p imm;
198
199 gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
200 imm = gimple_phi_arg_imm_use_ptr (gs: phi, i);
201 imm->use = gimple_phi_arg_def_ptr (phi, index: i);
202 imm->prev = NULL;
203 imm->next = NULL;
204 imm->loc.stmt = phi;
205 }
206
207 return phi;
208}
209
210/* We no longer need PHI, release it so that it may be reused. */
211
212static void
213release_phi_node (gimple *phi)
214{
215 size_t bucket;
216 size_t len = gimple_phi_capacity (gs: phi);
217 size_t x;
218
219 for (x = 0; x < gimple_phi_num_args (gs: phi); x++)
220 {
221 use_operand_p imm;
222 imm = gimple_phi_arg_imm_use_ptr (gs: phi, i: x);
223 delink_imm_use (linknode: imm);
224 }
225
226 bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
227 bucket -= 2;
228 vec_safe_push (v&: free_phinodes[bucket], obj: phi);
229 free_phinode_count++;
230}
231
232
233/* Resize an existing PHI node. The only way is up. Return the
234 possibly relocated phi. */
235
236static gphi *
237resize_phi_node (gphi *phi, size_t len)
238{
239 size_t old_size, i;
240 gphi *new_phi;
241
242 gcc_assert (len > gimple_phi_capacity (phi));
243
244 /* The garbage collector will not look at the PHI node beyond the
245 first PHI_NUM_ARGS elements. Therefore, all we have to copy is a
246 portion of the PHI node currently in use. */
247 old_size = sizeof (struct gphi)
248 + (gimple_phi_num_args (gs: phi) - 1) * sizeof (struct phi_arg_d);
249
250 new_phi = allocate_phi_node (len);
251
252 memcpy (dest: new_phi, src: phi, n: old_size);
253 memset (s: (char *)new_phi + old_size, c: 0,
254 n: (sizeof (struct gphi)
255 - sizeof (struct phi_arg_d)
256 + sizeof (struct phi_arg_d) * len) - old_size);
257
258 for (i = 0; i < gimple_phi_num_args (gs: new_phi); i++)
259 {
260 use_operand_p imm, old_imm;
261 imm = gimple_phi_arg_imm_use_ptr (gs: new_phi, i);
262 old_imm = gimple_phi_arg_imm_use_ptr (gs: phi, i);
263 imm->use = gimple_phi_arg_def_ptr (phi: new_phi, index: i);
264 relink_imm_use_stmt (linknode: imm, old: old_imm, stmt: new_phi);
265 }
266
267 new_phi->capacity = len;
268
269 return new_phi;
270}
271
272/* Reserve PHI arguments for a new edge to basic block BB. */
273
274void
275reserve_phi_args_for_new_edge (basic_block bb)
276{
277 size_t len = EDGE_COUNT (bb->preds);
278 size_t cap = ideal_phi_node_len (len: len + 4);
279 gphi_iterator gsi;
280
281 for (gsi = gsi_start_phis (bb); !gsi_end_p (i: gsi); gsi_next (i: &gsi))
282 {
283 gphi *stmt = gsi.phi ();
284
285 if (len > gimple_phi_capacity (gs: stmt))
286 {
287 gphi *new_phi = resize_phi_node (phi: stmt, len: cap);
288
289 /* The result of the PHI is defined by this PHI node. */
290 SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
291 gsi_set_stmt (&gsi, new_phi);
292
293 release_phi_node (phi: stmt);
294 stmt = new_phi;
295 }
296
297 stmt->nargs++;
298
299 /* We represent a "missing PHI argument" by placing NULL_TREE in
300 the corresponding slot. If PHI arguments were added
301 immediately after an edge is created, this zeroing would not
302 be necessary, but unfortunately this is not the case. For
303 example, the loop optimizer duplicates several basic blocks,
304 redirects edges, and then fixes up PHI arguments later in
305 batch. */
306 use_operand_p imm = gimple_phi_arg_imm_use_ptr (gs: stmt, i: len - 1);
307 imm->use = gimple_phi_arg_def_ptr (phi: stmt, index: len - 1);
308 imm->prev = NULL;
309 imm->next = NULL;
310 imm->loc.stmt = stmt;
311 SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
312 gimple_phi_arg_set_location (phi: stmt, i: len - 1, UNKNOWN_LOCATION);
313 }
314}
315
316/* Adds PHI to BB. */
317
318static void
319add_phi_node_to_bb (gphi *phi, basic_block bb)
320{
321 gimple_seq seq = phi_nodes (bb);
322 /* Add the new PHI node to the list of PHI nodes for block BB. */
323 if (seq == NULL)
324 set_phi_nodes (bb, gimple_seq_alloc_with_stmt (stmt: phi));
325 else
326 {
327 gimple_seq_add_stmt (&seq, phi);
328 gcc_assert (seq == phi_nodes (bb));
329 }
330
331 /* Associate BB to the PHI node. */
332 gimple_set_bb (phi, bb);
333}
334
335/* Create a new PHI node for variable VAR at basic block BB. */
336
337gphi *
338create_phi_node (tree var, basic_block bb)
339{
340 gphi *phi = make_phi_node (var, EDGE_COUNT (bb->preds));
341
342 add_phi_node_to_bb (phi, bb);
343 return phi;
344}
345
346
347/* Add a new argument to PHI node PHI. DEF is the incoming reaching
348 definition and E is the edge through which DEF reaches PHI. The new
349 argument is added at the end of the argument list.
350 If PHI has reached its maximum capacity, add a few slots. In this case,
351 PHI points to the reallocated phi node when we return. */
352
353void
354add_phi_arg (gphi *phi, tree def, edge e, location_t locus)
355{
356 basic_block bb = e->dest;
357
358 gcc_assert (bb == gimple_bb (phi));
359
360 /* We resize PHI nodes upon edge creation. We should always have
361 enough room at this point. */
362 gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
363
364 /* We resize PHI nodes upon edge creation. We should always have
365 enough room at this point. */
366 gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
367
368 /* Copy propagation needs to know what object occur in abnormal
369 PHI nodes. This is a convenient place to record such information. */
370 if (e->flags & EDGE_ABNORMAL)
371 {
372 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
373 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
374 }
375
376 SET_PHI_ARG_DEF (phi, e->dest_idx, def);
377 gimple_phi_arg_set_location (phi, i: e->dest_idx, loc: locus);
378}
379
380
381/* Remove the Ith argument from PHI's argument list. This routine
382 implements removal by swapping the last alternative with the
383 alternative we want to delete and then shrinking the vector, which
384 is consistent with how we remove an edge from the edge vector. */
385
386static void
387remove_phi_arg_num (gphi *phi, int i)
388{
389 int num_elem = gimple_phi_num_args (gs: phi);
390
391 gcc_assert (i < num_elem);
392
393 /* Delink the item which is being removed. */
394 delink_imm_use (linknode: gimple_phi_arg_imm_use_ptr (gs: phi, i));
395
396 /* If it is not the last element, move the last element
397 to the element we want to delete, resetting all the links. */
398 if (i != num_elem - 1)
399 {
400 use_operand_p old_p, new_p;
401 old_p = gimple_phi_arg_imm_use_ptr (gs: phi, i: num_elem - 1);
402 new_p = gimple_phi_arg_imm_use_ptr (gs: phi, i);
403 /* Set use on new node, and link into last element's place. */
404 *(new_p->use) = *(old_p->use);
405 relink_imm_use (node: new_p, old: old_p);
406 /* Move the location as well. */
407 gimple_phi_arg_set_location (phi, i,
408 loc: gimple_phi_arg_location (phi, i: num_elem - 1));
409 }
410
411 /* Shrink the vector and return. Note that we do not have to clear
412 PHI_ARG_DEF because the garbage collector will not look at those
413 elements beyond the first PHI_NUM_ARGS elements of the array. */
414 phi->nargs--;
415}
416
417
418/* Remove all PHI arguments associated with edge E. */
419
420void
421remove_phi_args (edge e)
422{
423 gphi_iterator gsi;
424
425 for (gsi = gsi_start_phis (e->dest); !gsi_end_p (i: gsi); gsi_next (i: &gsi))
426 remove_phi_arg_num (phi: gsi.phi (),
427 i: e->dest_idx);
428}
429
430
431/* Remove the PHI node pointed-to by iterator GSI from basic block BB. After
432 removal, iterator GSI is updated to point to the next PHI node in the
433 sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
434 into the free pool of SSA names. */
435
436void
437remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
438{
439 gimple *phi = gsi_stmt (i: *gsi);
440
441 if (release_lhs_p)
442 insert_debug_temps_for_defs (gsi);
443
444 gsi_remove (gsi, false);
445
446 /* If we are deleting the PHI node, then we should release the
447 SSA_NAME node so that it can be reused. */
448 release_phi_node (phi);
449 if (release_lhs_p)
450 release_ssa_name (name: gimple_phi_result (gs: phi));
451}
452
453/* Remove all the phi nodes from BB. */
454
455void
456remove_phi_nodes (basic_block bb)
457{
458 gphi_iterator gsi;
459
460 for (gsi = gsi_start_phis (bb); !gsi_end_p (i: gsi); )
461 remove_phi_node (gsi: &gsi, release_lhs_p: true);
462
463 set_phi_nodes (bb, NULL);
464}
465
466/* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
467 NULL. */
468
469tree
470degenerate_phi_result (gphi *phi)
471{
472 tree lhs = gimple_phi_result (gs: phi);
473 tree val = NULL;
474 size_t i;
475
476 /* Ignoring arguments which are the same as LHS, if all the remaining
477 arguments are the same, then the PHI is a degenerate and has the
478 value of that common argument. */
479 for (i = 0; i < gimple_phi_num_args (gs: phi); i++)
480 {
481 tree arg = gimple_phi_arg_def (gs: phi, index: i);
482
483 if (arg == lhs)
484 continue;
485 else if (!arg)
486 break;
487 else if (!val)
488 val = arg;
489 else if (arg == val)
490 continue;
491 /* We bring in some of operand_equal_p not only to speed things
492 up, but also to avoid crashing when dereferencing the type of
493 a released SSA name. */
494 else if (TREE_CODE (val) != TREE_CODE (arg)
495 || TREE_CODE (val) == SSA_NAME
496 || !operand_equal_p (arg, val, flags: 0))
497 break;
498 }
499 return (i == gimple_phi_num_args (gs: phi) ? val : NULL);
500}
501
502/* Set PHI nodes of a basic block BB to SEQ. */
503
504void
505set_phi_nodes (basic_block bb, gimple_seq seq)
506{
507 gimple_stmt_iterator i;
508
509 gcc_checking_assert (!(bb->flags & BB_RTL));
510 bb->il.gimple.phi_nodes = seq;
511 if (seq)
512 for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (i: &i))
513 gimple_set_bb (gsi_stmt (i), bb);
514}
515
516#include "gt-tree-phinodes.h"
517

source code of gcc/tree-phinodes.cc