1/* Generic routines for manipulating PHIs
2 Copyright (C) 2003-2024 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 /* Immediately return the memory to the allocator when we would
227 only ever re-use it for a smaller size allocation. */
228 if (len - 2 >= NUM_BUCKETS - 2)
229 {
230 ggc_free (phi);
231 return;
232 }
233
234 bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
235 bucket -= 2;
236 vec_safe_push (v&: free_phinodes[bucket], obj: phi);
237 free_phinode_count++;
238}
239
240
241/* Resize an existing PHI node. The only way is up. Return the
242 possibly relocated phi. */
243
244static gphi *
245resize_phi_node (gphi *phi, size_t len)
246{
247 size_t old_size, i;
248 gphi *new_phi;
249
250 gcc_assert (len > gimple_phi_capacity (phi));
251
252 /* The garbage collector will not look at the PHI node beyond the
253 first PHI_NUM_ARGS elements. Therefore, all we have to copy is a
254 portion of the PHI node currently in use. */
255 old_size = sizeof (struct gphi)
256 + (gimple_phi_num_args (gs: phi) - 1) * sizeof (struct phi_arg_d);
257
258 new_phi = allocate_phi_node (len);
259
260 memcpy (dest: new_phi, src: phi, n: old_size);
261 memset (s: (char *)new_phi + old_size, c: 0,
262 n: (sizeof (struct gphi)
263 - sizeof (struct phi_arg_d)
264 + sizeof (struct phi_arg_d) * len) - old_size);
265
266 for (i = 0; i < gimple_phi_num_args (gs: new_phi); i++)
267 {
268 use_operand_p imm, old_imm;
269 imm = gimple_phi_arg_imm_use_ptr (gs: new_phi, i);
270 old_imm = gimple_phi_arg_imm_use_ptr (gs: phi, i);
271 imm->use = gimple_phi_arg_def_ptr (phi: new_phi, index: i);
272 relink_imm_use_stmt (linknode: imm, old: old_imm, stmt: new_phi);
273 }
274
275 new_phi->capacity = len;
276
277 return new_phi;
278}
279
280/* Reserve PHI arguments for a new edge to basic block BB. */
281
282void
283reserve_phi_args_for_new_edge (basic_block bb)
284{
285 size_t len = EDGE_COUNT (bb->preds);
286 size_t cap = ideal_phi_node_len (len: len + 4);
287 gphi_iterator gsi;
288
289 for (gsi = gsi_start_phis (bb); !gsi_end_p (i: gsi); gsi_next (i: &gsi))
290 {
291 gphi *stmt = gsi.phi ();
292
293 if (len > gimple_phi_capacity (gs: stmt))
294 {
295 gphi *new_phi = resize_phi_node (phi: stmt, len: cap);
296
297 /* The result of the PHI is defined by this PHI node. */
298 SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
299 gsi_set_stmt (&gsi, new_phi);
300
301 release_phi_node (phi: stmt);
302 stmt = new_phi;
303 }
304
305 stmt->nargs++;
306
307 /* We represent a "missing PHI argument" by placing NULL_TREE in
308 the corresponding slot. If PHI arguments were added
309 immediately after an edge is created, this zeroing would not
310 be necessary, but unfortunately this is not the case. For
311 example, the loop optimizer duplicates several basic blocks,
312 redirects edges, and then fixes up PHI arguments later in
313 batch. */
314 use_operand_p imm = gimple_phi_arg_imm_use_ptr (gs: stmt, i: len - 1);
315 imm->use = gimple_phi_arg_def_ptr (phi: stmt, index: len - 1);
316 imm->prev = NULL;
317 imm->next = NULL;
318 imm->loc.stmt = stmt;
319 SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
320 gimple_phi_arg_set_location (phi: stmt, i: len - 1, UNKNOWN_LOCATION);
321 }
322}
323
324/* Adds PHI to BB. */
325
326static void
327add_phi_node_to_bb (gphi *phi, basic_block bb)
328{
329 gimple_seq seq = phi_nodes (bb);
330 /* Add the new PHI node to the list of PHI nodes for block BB. */
331 if (seq == NULL)
332 set_phi_nodes (bb, gimple_seq_alloc_with_stmt (stmt: phi));
333 else
334 {
335 gimple_seq_add_stmt (&seq, phi);
336 gcc_assert (seq == phi_nodes (bb));
337 }
338
339 /* Associate BB to the PHI node. */
340 gimple_set_bb (phi, bb);
341}
342
343/* Create a new PHI node for variable VAR at basic block BB. */
344
345gphi *
346create_phi_node (tree var, basic_block bb)
347{
348 gphi *phi = make_phi_node (var, EDGE_COUNT (bb->preds));
349
350 add_phi_node_to_bb (phi, bb);
351 return phi;
352}
353
354
355/* Add a new argument to PHI node PHI. DEF is the incoming reaching
356 definition and E is the edge through which DEF reaches PHI. The new
357 argument is added at the end of the argument list.
358 If PHI has reached its maximum capacity, add a few slots. In this case,
359 PHI points to the reallocated phi node when we return. */
360
361void
362add_phi_arg (gphi *phi, tree def, edge e, location_t locus)
363{
364 basic_block bb = e->dest;
365
366 gcc_assert (bb == gimple_bb (phi));
367
368 /* We resize PHI nodes upon edge creation. We should always have
369 enough room at this point. */
370 gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
371
372 /* We resize PHI nodes upon edge creation. We should always have
373 enough room at this point. */
374 gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
375
376 /* Copy propagation needs to know what object occur in abnormal
377 PHI nodes. This is a convenient place to record such information. */
378 if (e->flags & EDGE_ABNORMAL)
379 {
380 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
381 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
382 }
383
384 SET_PHI_ARG_DEF (phi, e->dest_idx, def);
385 gimple_phi_arg_set_location (phi, i: e->dest_idx, loc: locus);
386}
387
388
389/* Remove the Ith argument from PHI's argument list. This routine
390 implements removal by swapping the last alternative with the
391 alternative we want to delete and then shrinking the vector, which
392 is consistent with how we remove an edge from the edge vector. */
393
394static void
395remove_phi_arg_num (gphi *phi, int i)
396{
397 int num_elem = gimple_phi_num_args (gs: phi);
398
399 gcc_assert (i < num_elem);
400
401 /* Delink the item which is being removed. */
402 delink_imm_use (linknode: gimple_phi_arg_imm_use_ptr (gs: phi, i));
403
404 /* If it is not the last element, move the last element
405 to the element we want to delete, resetting all the links. */
406 if (i != num_elem - 1)
407 {
408 use_operand_p old_p, new_p;
409 old_p = gimple_phi_arg_imm_use_ptr (gs: phi, i: num_elem - 1);
410 new_p = gimple_phi_arg_imm_use_ptr (gs: phi, i);
411 /* Set use on new node, and link into last element's place. */
412 *(new_p->use) = *(old_p->use);
413 relink_imm_use (node: new_p, old: old_p);
414 /* Move the location as well. */
415 gimple_phi_arg_set_location (phi, i,
416 loc: gimple_phi_arg_location (phi, i: num_elem - 1));
417 }
418
419 /* Shrink the vector and return. Note that we do not have to clear
420 PHI_ARG_DEF because the garbage collector will not look at those
421 elements beyond the first PHI_NUM_ARGS elements of the array. */
422 phi->nargs--;
423}
424
425
426/* Remove all PHI arguments associated with edge E. */
427
428void
429remove_phi_args (edge e)
430{
431 gphi_iterator gsi;
432
433 for (gsi = gsi_start_phis (e->dest); !gsi_end_p (i: gsi); gsi_next (i: &gsi))
434 remove_phi_arg_num (phi: gsi.phi (),
435 i: e->dest_idx);
436}
437
438
439/* Remove the PHI node pointed-to by iterator GSI from basic block BB. After
440 removal, iterator GSI is updated to point to the next PHI node in the
441 sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
442 into the free pool of SSA names. */
443
444void
445remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
446{
447 gimple *phi = gsi_stmt (i: *gsi);
448
449 if (release_lhs_p)
450 insert_debug_temps_for_defs (gsi);
451
452 gsi_remove (gsi, false);
453
454 /* If we are deleting the PHI node, then we should release the
455 SSA_NAME node so that it can be reused. */
456 if (release_lhs_p)
457 release_ssa_name (name: gimple_phi_result (gs: phi));
458 release_phi_node (phi);
459}
460
461/* Remove all the phi nodes from BB. */
462
463void
464remove_phi_nodes (basic_block bb)
465{
466 gphi_iterator gsi;
467
468 for (gsi = gsi_start_phis (bb); !gsi_end_p (i: gsi); )
469 remove_phi_node (gsi: &gsi, release_lhs_p: true);
470
471 set_phi_nodes (bb, NULL);
472}
473
474/* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
475 NULL. */
476
477tree
478degenerate_phi_result (gphi *phi)
479{
480 tree lhs = gimple_phi_result (gs: phi);
481 tree val = NULL;
482 size_t i;
483
484 /* Ignoring arguments which are the same as LHS, if all the remaining
485 arguments are the same, then the PHI is a degenerate and has the
486 value of that common argument. */
487 for (i = 0; i < gimple_phi_num_args (gs: phi); i++)
488 {
489 tree arg = gimple_phi_arg_def (gs: phi, index: i);
490
491 if (arg == lhs)
492 continue;
493 else if (!arg)
494 break;
495 else if (!val)
496 val = arg;
497 else if (arg == val)
498 continue;
499 /* We bring in some of operand_equal_p not only to speed things
500 up, but also to avoid crashing when dereferencing the type of
501 a released SSA name. */
502 else if (TREE_CODE (val) != TREE_CODE (arg)
503 || TREE_CODE (val) == SSA_NAME
504 || !operand_equal_p (arg, val, flags: 0))
505 break;
506 }
507 return (i == gimple_phi_num_args (gs: phi) ? val : NULL);
508}
509
510/* Set PHI nodes of a basic block BB to SEQ. */
511
512void
513set_phi_nodes (basic_block bb, gimple_seq seq)
514{
515 gimple_stmt_iterator i;
516
517 gcc_checking_assert (!(bb->flags & BB_RTL));
518 bb->il.gimple.phi_nodes = seq;
519 if (seq)
520 for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (i: &i))
521 gimple_set_bb (gsi_stmt (i), bb);
522}
523
524#include "gt-tree-phinodes.h"
525

source code of gcc/tree-phinodes.cc