gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003-2017 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 < capacity; 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
 252   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 253     {
 254       use_operand_p imm, old_imm;
 255       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 256       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
 257       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 258       relink_imm_use_stmt (imm, old_imm, new_phi);
 259     }
 260
 261   new_phi->capacity = len;
 262
 263   for (i = gimple_phi_num_args (new_phi); i < len; i++)
 264     {
 265       use_operand_p imm;
 266
 267       gimple_phi_arg_set_location (new_phi, i, UNKNOWN_LOCATION);
 268       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 269       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 270       imm->prev = NULL;
 271       imm->next = NULL;
 272       imm->loc.stmt = new_phi;
 273     }
 274
 275   return new_phi;
 276 }
 277
 278 /* Reserve PHI arguments for a new edge to basic block BB.  */
 279
 280 void
 281 reserve_phi_args_for_new_edge (basic_block bb)
 282 {
 283   size_t len = EDGE_COUNT (bb->preds);
 284   size_t cap = ideal_phi_node_len (len + 4);
 285   gphi_iterator gsi;
 286
 287   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 288     {
 289       gphi *stmt = gsi.phi ();
 290
 291       if (len > gimple_phi_capacity (stmt))
 292         {
 293           gphi *new_phi = resize_phi_node (stmt, cap);
 294
 295           /* The result of the PHI is defined by this PHI node.  */
 296           SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
 297           gsi_set_stmt (&gsi, new_phi);
 298
 299           release_phi_node (stmt);
 300           stmt = new_phi;
 301         }
 302
 303       /* We represent a "missing PHI argument" by placing NULL_TREE in
 304          the corresponding slot.  If PHI arguments were added
 305          immediately after an edge is created, this zeroing would not
 306          be necessary, but unfortunately this is not the case.  For
 307          example, the loop optimizer duplicates several basic blocks,
 308          redirects edges, and then fixes up PHI arguments later in
 309          batch.  */
 310       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
 311       gimple_phi_arg_set_location (stmt, len - 1, UNKNOWN_LOCATION);
 312
 313       stmt->nargs++;
 314     }
 315 }
 316
 317 /* Adds PHI to BB.  */
 318
 319 void
 320 add_phi_node_to_bb (gphi *phi, basic_block bb)
 321 {
 322   gimple_seq seq = phi_nodes (bb);
 323   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 324   if (seq == NULL)
 325     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
 326   else
 327     {
 328       gimple_seq_add_stmt (&seq, phi);
 329       gcc_assert (seq == phi_nodes (bb));
 330     }
 331
 332   /* Associate BB to the PHI node.  */
 333   gimple_set_bb (phi, bb);
 334
 335 }
 336
 337 /* Create a new PHI node for variable VAR at basic block BB.  */
 338
 339 gphi *
 340 create_phi_node (tree var, basic_block bb)
 341 {
 342   gphi *phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 343
 344   add_phi_node_to_bb (phi, bb);
 345   return phi;
 346 }
 347
 348
 349 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 350    definition and E is the edge through which DEF reaches PHI.  The new
 351    argument is added at the end of the argument list.
 352    If PHI has reached its maximum capacity, add a few slots.  In this case,
 353    PHI points to the reallocated phi node when we return.  */
 354
 355 void
 356 add_phi_arg (gphi *phi, tree def, edge e, source_location locus)
 357 {
 358   basic_block bb = e->dest;
 359
 360   gcc_assert (bb == gimple_bb (phi));
 361
 362   /* We resize PHI nodes upon edge creation.  We should always have
 363      enough room at this point.  */
 364   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 365
 366   /* We resize PHI nodes upon edge creation.  We should always have
 367      enough room at this point.  */
 368   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 369
 370   /* Copy propagation needs to know what object occur in abnormal
 371      PHI nodes.  This is a convenient place to record such information.  */
 372   if (e->flags & EDGE_ABNORMAL)
 373     {
 374       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 375       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 376     }
 377
 378   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 379   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 380 }
 381
 382
 383 /* Remove the Ith argument from PHI's argument list.  This routine
 384    implements removal by swapping the last alternative with the
 385    alternative we want to delete and then shrinking the vector, which
 386    is consistent with how we remove an edge from the edge vector.  */
 387
 388 static void
 389 remove_phi_arg_num (gphi *phi, int i)
 390 {
 391   int num_elem = gimple_phi_num_args (phi);
 392
 393   gcc_assert (i < num_elem);
 394
 395   /* Delink the item which is being removed.  */
 396   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 397
 398   /* If it is not the last element, move the last element
 399      to the element we want to delete, resetting all the links. */
 400   if (i != num_elem - 1)
 401     {
 402       use_operand_p old_p, new_p;
 403       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 404       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 405       /* Set use on new node, and link into last element's place.  */
 406       *(new_p->use) = *(old_p->use);
 407       relink_imm_use (new_p, old_p);
 408       /* Move the location as well.  */
 409       gimple_phi_arg_set_location (phi, i,
 410                                    gimple_phi_arg_location (phi, num_elem - 1));
 411     }
 412
 413   /* Shrink the vector and return.  Note that we do not have to clear
 414      PHI_ARG_DEF because the garbage collector will not look at those
 415      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 416   phi->nargs--;
 417 }
 418
 419
 420 /* Remove all PHI arguments associated with edge E.  */
 421
 422 void
 423 remove_phi_args (edge e)
 424 {
 425   gphi_iterator gsi;
 426
 427   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 428     remove_phi_arg_num (gsi.phi (),
 429                         e->dest_idx);
 430 }
 431
 432
 433 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 434    removal, iterator GSI is updated to point to the next PHI node in the
 435    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 436    into the free pool of SSA names.  */
 437
 438 void
 439 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 440 {
 441   gimple *phi = gsi_stmt (*gsi);
 442
 443   if (release_lhs_p)
 444     insert_debug_temps_for_defs (gsi);
 445
 446   gsi_remove (gsi, false);
 447
 448   /* If we are deleting the PHI node, then we should release the
 449      SSA_NAME node so that it can be reused.  */
 450   release_phi_node (phi);
 451   if (release_lhs_p)
 452     release_ssa_name (gimple_phi_result (phi));
 453 }
 454
 455 /* Remove all the phi nodes from BB.  */
 456
 457 void
 458 remove_phi_nodes (basic_block bb)
 459 {
 460   gphi_iterator gsi;
 461
 462   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 463     remove_phi_node (&gsi, true);
 464
 465   set_phi_nodes (bb, NULL);
 466 }
 467
 468 /* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
 469    NULL.  */
 470
 471 tree
 472 degenerate_phi_result (gphi *phi)
 473 {
 474   tree lhs = gimple_phi_result (phi);
 475   tree val = NULL;
 476   size_t i;
 477
 478   /* Ignoring arguments which are the same as LHS, if all the remaining
 479      arguments are the same, then the PHI is a degenerate and has the
 480      value of that common argument.  */
 481   for (i = 0; i < gimple_phi_num_args (phi); i++)
 482     {
 483       tree arg = gimple_phi_arg_def (phi, i);
 484
 485       if (arg == lhs)
 486         continue;
 487       else if (!arg)
 488         break;
 489       else if (!val)
 490         val = arg;
 491       else if (arg == val)
 492         continue;
 493       /* We bring in some of operand_equal_p not only to speed things
 494          up, but also to avoid crashing when dereferencing the type of
 495          a released SSA name.  */
 496       else if (TREE_CODE (val) != TREE_CODE (arg)
 497                || TREE_CODE (val) == SSA_NAME
 498                || !operand_equal_p (arg, val, 0))
 499         break;
 500     }
 501   return (i == gimple_phi_num_args (phi) ? val : NULL);
 502 }
 503
 504 /* Set PHI nodes of a basic block BB to SEQ.  */
 505
 506 void
 507 set_phi_nodes (basic_block bb, gimple_seq seq)
 508 {
 509   gimple_stmt_iterator i;
 510
 511   gcc_checking_assert (!(bb->flags & BB_RTL));
 512   bb->il.gimple.phi_nodes = seq;
 513   if (seq)
 514     for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (&i))
 515       gimple_set_bb (gsi_stmt (i), bb);
 516 }
 517
 518 #include "gt-tree-phinodes.h"