1 /* Generic routines for manipulating PHIs
2    Copyright (C) 2003-2018 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 "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
70 static GTY ((deletable (""))) vec<gimple *, va_gc> *free_phinodes[NUM_BUCKETS - 2];
71 static unsigned long free_phinode_count;
72 
73 static int ideal_phi_node_len (int);
74 
75 unsigned int phi_nodes_reused;
76 unsigned int phi_nodes_created;
77 
78 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
79 
80 void
81 phinodes_print_statistics (void)
82 {
83   fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
84   fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
85 }
86 
87 /* Allocate a PHI node with at least LEN arguments.  If the free list
88    happens to contain a PHI node with LEN arguments or more, return
89    that one.  */
90 
91 static inline gphi *
92 allocate_phi_node (size_t len)
93 {
94   gphi *phi;
95   size_t bucket = NUM_BUCKETS - 2;
96   size_t size = sizeof (struct gphi)
97 	        + (len - 1) * sizeof (struct phi_arg_d);
98 
99   if (free_phinode_count)
100     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
101       if (free_phinodes[bucket])
102 	break;
103 
104   /* If our free list has an element, then use it.  */
105   if (bucket < NUM_BUCKETS - 2
106       && gimple_phi_capacity ((*free_phinodes[bucket])[0]) >= len)
107     {
108       free_phinode_count--;
109       phi = as_a <gphi *> (free_phinodes[bucket]->pop ());
110       if (free_phinodes[bucket]->is_empty ())
111 	vec_free (free_phinodes[bucket]);
112       if (GATHER_STATISTICS)
113 	phi_nodes_reused++;
114     }
115   else
116     {
117       phi = static_cast <gphi *> (ggc_internal_alloc (size));
118       if (GATHER_STATISTICS)
119 	{
120 	  enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
121 	  phi_nodes_created++;
122 	  gimple_alloc_counts[(int) kind]++;
123 	  gimple_alloc_sizes[(int) kind] += size;
124 	}
125     }
126 
127   return phi;
128 }
129 
130 /* Given LEN, the original number of requested PHI arguments, return
131    a new, "ideal" length for the PHI node.  The "ideal" length rounds
132    the total size of the PHI node up to the next power of two bytes.
133 
134    Rounding up will not result in wasting any memory since the size request
135    will be rounded up by the GC system anyway.  [ Note this is not entirely
136    true since the original length might have fit on one of the special
137    GC pages. ]  By rounding up, we may avoid the need to reallocate the
138    PHI node later if we increase the number of arguments for the PHI.  */
139 
140 static int
141 ideal_phi_node_len (int len)
142 {
143   size_t size, new_size;
144   int log2, new_len;
145 
146   /* We do not support allocations of less than two PHI argument slots.  */
147   if (len < 2)
148     len = 2;
149 
150   /* Compute the number of bytes of the original request.  */
151   size = sizeof (struct gphi)
152 	 + (len - 1) * sizeof (struct phi_arg_d);
153 
154   /* Round it up to the next power of two.  */
155   log2 = ceil_log2 (size);
156   new_size = 1 << log2;
157 
158   /* Now compute and return the number of PHI argument slots given an
159      ideal size allocation.  */
160   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
161   return new_len;
162 }
163 
164 /* Return a PHI node with LEN argument slots for variable VAR.  */
165 
166 static gphi *
167 make_phi_node (tree var, int len)
168 {
169   gphi *phi;
170   int capacity, i;
171 
172   capacity = ideal_phi_node_len (len);
173 
174   phi = allocate_phi_node (capacity);
175 
176   /* We need to clear the entire PHI node, including the argument
177      portion, because we represent a "missing PHI argument" by placing
178      NULL_TREE in PHI_ARG_DEF.  */
179   memset (phi, 0, (sizeof (struct gphi)
180 		   - sizeof (struct phi_arg_d)
181 		   + sizeof (struct phi_arg_d) * len));
182   phi->code = GIMPLE_PHI;
183   gimple_init_singleton (phi);
184   phi->nargs = len;
185   phi->capacity = capacity;
186   if (!var)
187     ;
188   else if (TREE_CODE (var) == SSA_NAME)
189     gimple_phi_set_result (phi, var);
190   else
191     gimple_phi_set_result (phi, make_ssa_name (var, phi));
192 
193   for (i = 0; i < len; i++)
194     {
195       use_operand_p  imm;
196 
197       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
198       imm = gimple_phi_arg_imm_use_ptr (phi, i);
199       imm->use = gimple_phi_arg_def_ptr (phi, i);
200       imm->prev = NULL;
201       imm->next = NULL;
202       imm->loc.stmt = phi;
203     }
204 
205   return phi;
206 }
207 
208 /* We no longer need PHI, release it so that it may be reused.  */
209 
210 static void
211 release_phi_node (gimple *phi)
212 {
213   size_t bucket;
214   size_t len = gimple_phi_capacity (phi);
215   size_t x;
216 
217   for (x = 0; x < gimple_phi_num_args (phi); x++)
218     {
219       use_operand_p  imm;
220       imm = gimple_phi_arg_imm_use_ptr (phi, x);
221       delink_imm_use (imm);
222     }
223 
224   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
225   bucket -= 2;
226   vec_safe_push (free_phinodes[bucket], phi);
227   free_phinode_count++;
228 }
229 
230 
231 /* Resize an existing PHI node.  The only way is up.  Return the
232    possibly relocated phi.  */
233 
234 static gphi *
235 resize_phi_node (gphi *phi, size_t len)
236 {
237   size_t old_size, i;
238   gphi *new_phi;
239 
240   gcc_assert (len > gimple_phi_capacity (phi));
241 
242   /* The garbage collector will not look at the PHI node beyond the
243      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
244      portion of the PHI node currently in use.  */
245   old_size = sizeof (struct gphi)
246 	     + (gimple_phi_num_args (phi) - 1) * sizeof (struct phi_arg_d);
247 
248   new_phi = allocate_phi_node (len);
249 
250   memcpy (new_phi, phi, old_size);
251   memset ((char *)new_phi + old_size, 0,
252 	  (sizeof (struct gphi)
253 	   - sizeof (struct phi_arg_d)
254 	   + sizeof (struct phi_arg_d) * len) - old_size);
255 
256   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
257     {
258       use_operand_p imm, old_imm;
259       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
260       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
261       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
262       relink_imm_use_stmt (imm, old_imm, new_phi);
263     }
264 
265   new_phi->capacity = len;
266 
267   return new_phi;
268 }
269 
270 /* Reserve PHI arguments for a new edge to basic block BB.  */
271 
272 void
273 reserve_phi_args_for_new_edge (basic_block bb)
274 {
275   size_t len = EDGE_COUNT (bb->preds);
276   size_t cap = ideal_phi_node_len (len + 4);
277   gphi_iterator gsi;
278 
279   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
280     {
281       gphi *stmt = gsi.phi ();
282 
283       if (len > gimple_phi_capacity (stmt))
284 	{
285 	  gphi *new_phi = resize_phi_node (stmt, cap);
286 
287 	  /* The result of the PHI is defined by this PHI node.  */
288 	  SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
289 	  gsi_set_stmt (&gsi, new_phi);
290 
291 	  release_phi_node (stmt);
292 	  stmt = new_phi;
293 	}
294 
295       stmt->nargs++;
296 
297       /* We represent a "missing PHI argument" by placing NULL_TREE in
298 	 the corresponding slot.  If PHI arguments were added
299 	 immediately after an edge is created, this zeroing would not
300 	 be necessary, but unfortunately this is not the case.  For
301 	 example, the loop optimizer duplicates several basic blocks,
302 	 redirects edges, and then fixes up PHI arguments later in
303 	 batch.  */
304       use_operand_p imm = gimple_phi_arg_imm_use_ptr (stmt, len - 1);
305       imm->use = gimple_phi_arg_def_ptr (stmt, len - 1);
306       imm->prev = NULL;
307       imm->next = NULL;
308       imm->loc.stmt = stmt;
309       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
310       gimple_phi_arg_set_location (stmt, len - 1, UNKNOWN_LOCATION);
311     }
312 }
313 
314 /* Adds PHI to BB.  */
315 
316 void
317 add_phi_node_to_bb (gphi *phi, basic_block bb)
318 {
319   gimple_seq seq = phi_nodes (bb);
320   /* Add the new PHI node to the list of PHI nodes for block BB.  */
321   if (seq == NULL)
322     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
323   else
324     {
325       gimple_seq_add_stmt (&seq, phi);
326       gcc_assert (seq == phi_nodes (bb));
327     }
328 
329   /* Associate BB to the PHI node.  */
330   gimple_set_bb (phi, bb);
331 
332 }
333 
334 /* Create a new PHI node for variable VAR at basic block BB.  */
335 
336 gphi *
337 create_phi_node (tree var, basic_block bb)
338 {
339   gphi *phi = make_phi_node (var, EDGE_COUNT (bb->preds));
340 
341   add_phi_node_to_bb (phi, bb);
342   return phi;
343 }
344 
345 
346 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
347    definition and E is the edge through which DEF reaches PHI.  The new
348    argument is added at the end of the argument list.
349    If PHI has reached its maximum capacity, add a few slots.  In this case,
350    PHI points to the reallocated phi node when we return.  */
351 
352 void
353 add_phi_arg (gphi *phi, tree def, edge e, source_location locus)
354 {
355   basic_block bb = e->dest;
356 
357   gcc_assert (bb == gimple_bb (phi));
358 
359   /* We resize PHI nodes upon edge creation.  We should always have
360      enough room at this point.  */
361   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
362 
363   /* We resize PHI nodes upon edge creation.  We should always have
364      enough room at this point.  */
365   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
366 
367   /* Copy propagation needs to know what object occur in abnormal
368      PHI nodes.  This is a convenient place to record such information.  */
369   if (e->flags & EDGE_ABNORMAL)
370     {
371       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
372       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
373     }
374 
375   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
376   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
377 }
378 
379 
380 /* Remove the Ith argument from PHI's argument list.  This routine
381    implements removal by swapping the last alternative with the
382    alternative we want to delete and then shrinking the vector, which
383    is consistent with how we remove an edge from the edge vector.  */
384 
385 static void
386 remove_phi_arg_num (gphi *phi, int i)
387 {
388   int num_elem = gimple_phi_num_args (phi);
389 
390   gcc_assert (i < num_elem);
391 
392   /* Delink the item which is being removed.  */
393   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
394 
395   /* If it is not the last element, move the last element
396      to the element we want to delete, resetting all the links. */
397   if (i != num_elem - 1)
398     {
399       use_operand_p old_p, new_p;
400       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
401       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
402       /* Set use on new node, and link into last element's place.  */
403       *(new_p->use) = *(old_p->use);
404       relink_imm_use (new_p, old_p);
405       /* Move the location as well.  */
406       gimple_phi_arg_set_location (phi, i,
407 				   gimple_phi_arg_location (phi, num_elem - 1));
408     }
409 
410   /* Shrink the vector and return.  Note that we do not have to clear
411      PHI_ARG_DEF because the garbage collector will not look at those
412      elements beyond the first PHI_NUM_ARGS elements of the array.  */
413   phi->nargs--;
414 }
415 
416 
417 /* Remove all PHI arguments associated with edge E.  */
418 
419 void
420 remove_phi_args (edge e)
421 {
422   gphi_iterator gsi;
423 
424   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
425     remove_phi_arg_num (gsi.phi (),
426 			e->dest_idx);
427 }
428 
429 
430 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
431    removal, iterator GSI is updated to point to the next PHI node in the
432    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
433    into the free pool of SSA names.  */
434 
435 void
436 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
437 {
438   gimple *phi = gsi_stmt (*gsi);
439 
440   if (release_lhs_p)
441     insert_debug_temps_for_defs (gsi);
442 
443   gsi_remove (gsi, false);
444 
445   /* If we are deleting the PHI node, then we should release the
446      SSA_NAME node so that it can be reused.  */
447   release_phi_node (phi);
448   if (release_lhs_p)
449     release_ssa_name (gimple_phi_result (phi));
450 }
451 
452 /* Remove all the phi nodes from BB.  */
453 
454 void
455 remove_phi_nodes (basic_block bb)
456 {
457   gphi_iterator gsi;
458 
459   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
460     remove_phi_node (&gsi, true);
461 
462   set_phi_nodes (bb, NULL);
463 }
464 
465 /* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
466    NULL.  */
467 
468 tree
469 degenerate_phi_result (gphi *phi)
470 {
471   tree lhs = gimple_phi_result (phi);
472   tree val = NULL;
473   size_t i;
474 
475   /* Ignoring arguments which are the same as LHS, if all the remaining
476      arguments are the same, then the PHI is a degenerate and has the
477      value of that common argument.  */
478   for (i = 0; i < gimple_phi_num_args (phi); i++)
479     {
480       tree arg = gimple_phi_arg_def (phi, i);
481 
482       if (arg == lhs)
483 	continue;
484       else if (!arg)
485 	break;
486       else if (!val)
487 	val = arg;
488       else if (arg == val)
489 	continue;
490       /* We bring in some of operand_equal_p not only to speed things
491 	 up, but also to avoid crashing when dereferencing the type of
492 	 a released SSA name.  */
493       else if (TREE_CODE (val) != TREE_CODE (arg)
494 	       || TREE_CODE (val) == SSA_NAME
495 	       || !operand_equal_p (arg, val, 0))
496 	break;
497     }
498   return (i == gimple_phi_num_args (phi) ? val : NULL);
499 }
500 
501 /* Set PHI nodes of a basic block BB to SEQ.  */
502 
503 void
504 set_phi_nodes (basic_block bb, gimple_seq seq)
505 {
506   gimple_stmt_iterator i;
507 
508   gcc_checking_assert (!(bb->flags & BB_RTL));
509   bb->il.gimple.phi_nodes = seq;
510   if (seq)
511     for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (&i))
512       gimple_set_bb (gsi_stmt (i), bb);
513 }
514 
515 #include "gt-tree-phinodes.h"
516