gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003-2014 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 "tm.h"
  24 #include "tree.h"
  25 #include "basic-block.h"
  26 #include "tree-ssa-alias.h"
  27 #include "internal-fn.h"
  28 #include "gimple-expr.h"
  29 #include "is-a.h"
  30 #include "gimple.h"
  31 #include "gimple-iterator.h"
  32 #include "gimple-ssa.h"
  33 #include "tree-phinodes.h"
  34 #include "ssa-iterators.h"
  35 #include "stringpool.h"
  36 #include "tree-ssanames.h"
  37 #include "tree-ssa.h"
  38 #include "diagnostic-core.h"
  39
  40 /* Rewriting a function into SSA form can create a huge number of PHIs
  41    many of which may be thrown away shortly after their creation if jumps
  42    were threaded through PHI nodes.
  43
  44    While our garbage collection mechanisms will handle this situation, it
  45    is extremely wasteful to create nodes and throw them away, especially
  46    when the nodes can be reused.
  47
  48    For PR 8361, we can significantly reduce the number of nodes allocated
  49    and thus the total amount of memory allocated by managing PHIs a
  50    little.  This additionally helps reduce the amount of work done by the
  51    garbage collector.  Similar results have been seen on a wider variety
  52    of tests (such as the compiler itself).
  53
  54    PHI nodes have different sizes, so we can't have a single list of all
  55    the PHI nodes as it would be too expensive to walk down that list to
  56    find a PHI of a suitable size.
  57
  58    Instead we have an array of lists of free PHI nodes.  The array is
  59    indexed by the number of PHI alternatives that PHI node can hold.
  60    Except for the last array member, which holds all remaining PHI
  61    nodes.
  62
  63    So to find a free PHI node, we compute its index into the free PHI
  64    node array and see if there are any elements with an exact match.
  65    If so, then we are done.  Otherwise, we test the next larger size
  66    up and continue until we are in the last array element.
  67
  68    We do not actually walk members of the last array element.  While it
  69    might allow us to pick up a few reusable PHI nodes, it could potentially
  70    be very expensive if the program has released a bunch of large PHI nodes,
  71    but keeps asking for even larger PHI nodes.  Experiments have shown that
  72    walking the elements of the last array entry would result in finding less
  73    than .1% additional reusable PHI nodes.
  74
  75    Note that we can never have less than two PHI argument slots.  Thus,
  76    the -2 on all the calculations below.  */
  77
  78 #define NUM_BUCKETS 10
  79 static GTY ((deletable (""))) vec<gimple, va_gc> *free_phinodes[NUM_BUCKETS - 2];
  80 static unsigned long free_phinode_count;
  81
  82 static int ideal_phi_node_len (int);
  83
  84 unsigned int phi_nodes_reused;
  85 unsigned int phi_nodes_created;
  86
  87 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
  88
  89 void
  90 phinodes_print_statistics (void)
  91 {
  92   fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
  93   fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
  94 }
  95
  96 /* Allocate a PHI node with at least LEN arguments.  If the free list
  97    happens to contain a PHI node with LEN arguments or more, return
  98    that one.  */
  99
 100 static inline gimple_phi
 101 allocate_phi_node (size_t len)
 102 {
 103   gimple_phi phi;
 104   size_t bucket = NUM_BUCKETS - 2;
 105   size_t size = sizeof (struct gimple_statement_phi)
 106                 + (len - 1) * sizeof (struct phi_arg_d);
 107
 108   if (free_phinode_count)
 109     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 110       if (free_phinodes[bucket])
 111         break;
 112
 113   /* If our free list has an element, then use it.  */
 114   if (bucket < NUM_BUCKETS - 2
 115       && gimple_phi_capacity ((*free_phinodes[bucket])[0]) >= len)
 116     {
 117       free_phinode_count--;
 118       phi = as_a <gimple_statement_phi *> (free_phinodes[bucket]->pop ());
 119       if (free_phinodes[bucket]->is_empty ())
 120         vec_free (free_phinodes[bucket]);
 121       if (GATHER_STATISTICS)
 122         phi_nodes_reused++;
 123     }
 124   else
 125     {
 126       phi = static_cast <gimple_phi> (ggc_internal_alloc (size));
 127       if (GATHER_STATISTICS)
 128         {
 129           enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
 130           phi_nodes_created++;
 131           gimple_alloc_counts[(int) kind]++;
 132           gimple_alloc_sizes[(int) kind] += size;
 133         }
 134     }
 135
 136   return phi;
 137 }
 138
 139 /* Given LEN, the original number of requested PHI arguments, return
 140    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 141    the total size of the PHI node up to the next power of two bytes.
 142
 143    Rounding up will not result in wasting any memory since the size request
 144    will be rounded up by the GC system anyway.  [ Note this is not entirely
 145    true since the original length might have fit on one of the special
 146    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 147    PHI node later if we increase the number of arguments for the PHI.  */
 148
 149 static int
 150 ideal_phi_node_len (int len)
 151 {
 152   size_t size, new_size;
 153   int log2, new_len;
 154
 155   /* We do not support allocations of less than two PHI argument slots.  */
 156   if (len < 2)
 157     len = 2;
 158
 159   /* Compute the number of bytes of the original request.  */
 160   size = sizeof (struct gimple_statement_phi)
 161          + (len - 1) * sizeof (struct phi_arg_d);
 162
 163   /* Round it up to the next power of two.  */
 164   log2 = ceil_log2 (size);
 165   new_size = 1 << log2;
 166
 167   /* Now compute and return the number of PHI argument slots given an
 168      ideal size allocation.  */
 169   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 170   return new_len;
 171 }
 172
 173 /* Return a PHI node with LEN argument slots for variable VAR.  */
 174
 175 static gimple_phi
 176 make_phi_node (tree var, int len)
 177 {
 178   gimple_phi phi;
 179   int capacity, i;
 180
 181   capacity = ideal_phi_node_len (len);
 182
 183   phi = allocate_phi_node (capacity);
 184
 185   /* We need to clear the entire PHI node, including the argument
 186      portion, because we represent a "missing PHI argument" by placing
 187      NULL_TREE in PHI_ARG_DEF.  */
 188   memset (phi, 0, (sizeof (struct gimple_statement_phi)
 189                    - sizeof (struct phi_arg_d)
 190                    + sizeof (struct phi_arg_d) * len));
 191   phi->code = GIMPLE_PHI;
 192   gimple_init_singleton (phi);
 193   phi->nargs = len;
 194   phi->capacity = capacity;
 195   if (!var)
 196     ;
 197   else if (TREE_CODE (var) == SSA_NAME)
 198     gimple_phi_set_result (phi, var);
 199   else
 200     gimple_phi_set_result (phi, make_ssa_name (var, phi));
 201
 202   for (i = 0; i < capacity; i++)
 203     {
 204       use_operand_p  imm;
 205
 206       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
 207       imm = gimple_phi_arg_imm_use_ptr (phi, i);
 208       imm->use = gimple_phi_arg_def_ptr (phi, i);
 209       imm->prev = NULL;
 210       imm->next = NULL;
 211       imm->loc.stmt = phi;
 212     }
 213
 214   return phi;
 215 }
 216
 217 /* We no longer need PHI, release it so that it may be reused.  */
 218
 219 void
 220 release_phi_node (gimple phi)
 221 {
 222   size_t bucket;
 223   size_t len = gimple_phi_capacity (phi);
 224   size_t x;
 225
 226   for (x = 0; x < gimple_phi_num_args (phi); x++)
 227     {
 228       use_operand_p  imm;
 229       imm = gimple_phi_arg_imm_use_ptr (phi, x);
 230       delink_imm_use (imm);
 231     }
 232
 233   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 234   bucket -= 2;
 235   vec_safe_push (free_phinodes[bucket], phi);
 236   free_phinode_count++;
 237 }
 238
 239
 240 /* Resize an existing PHI node.  The only way is up.  Return the
 241    possibly relocated phi.  */
 242
 243 static gimple_phi
 244 resize_phi_node (gimple_phi phi, size_t len)
 245 {
 246   size_t old_size, i;
 247   gimple_phi new_phi;
 248
 249   gcc_assert (len > gimple_phi_capacity (phi));
 250
 251   /* The garbage collector will not look at the PHI node beyond the
 252      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 253      portion of the PHI node currently in use.  */
 254   old_size = sizeof (struct gimple_statement_phi)
 255              + (gimple_phi_num_args (phi) - 1) * sizeof (struct phi_arg_d);
 256
 257   new_phi = allocate_phi_node (len);
 258
 259   memcpy (new_phi, phi, old_size);
 260
 261   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 262     {
 263       use_operand_p imm, old_imm;
 264       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 265       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
 266       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 267       relink_imm_use_stmt (imm, old_imm, new_phi);
 268     }
 269
 270   new_phi->capacity = len;
 271
 272   for (i = gimple_phi_num_args (new_phi); i < len; i++)
 273     {
 274       use_operand_p imm;
 275
 276       gimple_phi_arg_set_location (new_phi, i, UNKNOWN_LOCATION);
 277       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 278       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 279       imm->prev = NULL;
 280       imm->next = NULL;
 281       imm->loc.stmt = new_phi;
 282     }
 283
 284   return new_phi;
 285 }
 286
 287 /* Reserve PHI arguments for a new edge to basic block BB.  */
 288
 289 void
 290 reserve_phi_args_for_new_edge (basic_block bb)
 291 {
 292   size_t len = EDGE_COUNT (bb->preds);
 293   size_t cap = ideal_phi_node_len (len + 4);
 294   gimple_phi_iterator gsi;
 295
 296   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 297     {
 298       gimple_phi stmt = gsi.phi ();
 299
 300       if (len > gimple_phi_capacity (stmt))
 301         {
 302           gimple_phi new_phi = resize_phi_node (stmt, cap);
 303
 304           /* The result of the PHI is defined by this PHI node.  */
 305           SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
 306           gsi_set_stmt (&gsi, new_phi);
 307
 308           release_phi_node (stmt);
 309           stmt = new_phi;
 310         }
 311
 312       /* We represent a "missing PHI argument" by placing NULL_TREE in
 313          the corresponding slot.  If PHI arguments were added
 314          immediately after an edge is created, this zeroing would not
 315          be necessary, but unfortunately this is not the case.  For
 316          example, the loop optimizer duplicates several basic blocks,
 317          redirects edges, and then fixes up PHI arguments later in
 318          batch.  */
 319       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
 320       gimple_phi_arg_set_location (stmt, len - 1, UNKNOWN_LOCATION);
 321
 322       stmt->nargs++;
 323     }
 324 }
 325
 326 /* Adds PHI to BB.  */
 327
 328 void
 329 add_phi_node_to_bb (gimple_phi phi, basic_block bb)
 330 {
 331   gimple_seq seq = phi_nodes (bb);
 332   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 333   if (seq == NULL)
 334     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
 335   else
 336     {
 337       gimple_seq_add_stmt (&seq, phi);
 338       gcc_assert (seq == phi_nodes (bb));
 339     }
 340
 341   /* Associate BB to the PHI node.  */
 342   gimple_set_bb (phi, bb);
 343
 344 }
 345
 346 /* Create a new PHI node for variable VAR at basic block BB.  */
 347
 348 gimple_phi
 349 create_phi_node (tree var, basic_block bb)
 350 {
 351   gimple_phi phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 352
 353   add_phi_node_to_bb (phi, bb);
 354   return phi;
 355 }
 356
 357
 358 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 359    definition and E is the edge through which DEF reaches PHI.  The new
 360    argument is added at the end of the argument list.
 361    If PHI has reached its maximum capacity, add a few slots.  In this case,
 362    PHI points to the reallocated phi node when we return.  */
 363
 364 void
 365 add_phi_arg (gimple_phi phi, tree def, edge e, source_location locus)
 366 {
 367   basic_block bb = e->dest;
 368
 369   gcc_assert (bb == gimple_bb (phi));
 370
 371   /* We resize PHI nodes upon edge creation.  We should always have
 372      enough room at this point.  */
 373   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 374
 375   /* We resize PHI nodes upon edge creation.  We should always have
 376      enough room at this point.  */
 377   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 378
 379   /* Copy propagation needs to know what object occur in abnormal
 380      PHI nodes.  This is a convenient place to record such information.  */
 381   if (e->flags & EDGE_ABNORMAL)
 382     {
 383       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 384       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 385     }
 386
 387   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 388   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 389 }
 390
 391
 392 /* Remove the Ith argument from PHI's argument list.  This routine
 393    implements removal by swapping the last alternative with the
 394    alternative we want to delete and then shrinking the vector, which
 395    is consistent with how we remove an edge from the edge vector.  */
 396
 397 static void
 398 remove_phi_arg_num (gimple_phi phi, int i)
 399 {
 400   int num_elem = gimple_phi_num_args (phi);
 401
 402   gcc_assert (i < num_elem);
 403
 404   /* Delink the item which is being removed.  */
 405   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 406
 407   /* If it is not the last element, move the last element
 408      to the element we want to delete, resetting all the links. */
 409   if (i != num_elem - 1)
 410     {
 411       use_operand_p old_p, new_p;
 412       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 413       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 414       /* Set use on new node, and link into last element's place.  */
 415       *(new_p->use) = *(old_p->use);
 416       relink_imm_use (new_p, old_p);
 417       /* Move the location as well.  */
 418       gimple_phi_arg_set_location (phi, i,
 419                                    gimple_phi_arg_location (phi, num_elem - 1));
 420     }
 421
 422   /* Shrink the vector and return.  Note that we do not have to clear
 423      PHI_ARG_DEF because the garbage collector will not look at those
 424      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 425   phi->nargs--;
 426 }
 427
 428
 429 /* Remove all PHI arguments associated with edge E.  */
 430
 431 void
 432 remove_phi_args (edge e)
 433 {
 434   gimple_phi_iterator gsi;
 435
 436   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 437     remove_phi_arg_num (gsi.phi (),
 438                         e->dest_idx);
 439 }
 440
 441
 442 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 443    removal, iterator GSI is updated to point to the next PHI node in the
 444    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 445    into the free pool of SSA names.  */
 446
 447 void
 448 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 449 {
 450   gimple phi = gsi_stmt (*gsi);
 451
 452   if (release_lhs_p)
 453     insert_debug_temps_for_defs (gsi);
 454
 455   gsi_remove (gsi, false);
 456
 457   /* If we are deleting the PHI node, then we should release the
 458      SSA_NAME node so that it can be reused.  */
 459   release_phi_node (phi);
 460   if (release_lhs_p)
 461     release_ssa_name (gimple_phi_result (phi));
 462 }
 463
 464 /* Remove all the phi nodes from BB.  */
 465
 466 void
 467 remove_phi_nodes (basic_block bb)
 468 {
 469   gimple_phi_iterator gsi;
 470
 471   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 472     remove_phi_node (&gsi, true);
 473
 474   set_phi_nodes (bb, NULL);
 475 }
 476
 477 /* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
 478    NULL.  */
 479
 480 tree
 481 degenerate_phi_result (gimple phi)
 482 {
 483   tree lhs = gimple_phi_result (phi);
 484   tree val = NULL;
 485   size_t i;
 486
 487   /* Ignoring arguments which are the same as LHS, if all the remaining
 488      arguments are the same, then the PHI is a degenerate and has the
 489      value of that common argument.  */
 490   for (i = 0; i < gimple_phi_num_args (phi); i++)
 491     {
 492       tree arg = gimple_phi_arg_def (phi, i);
 493
 494       if (arg == lhs)
 495         continue;
 496       else if (!arg)
 497         break;
 498       else if (!val)
 499         val = arg;
 500       else if (arg == val)
 501         continue;
 502       /* We bring in some of operand_equal_p not only to speed things
 503          up, but also to avoid crashing when dereferencing the type of
 504          a released SSA name.  */
 505       else if (TREE_CODE (val) != TREE_CODE (arg)
 506                || TREE_CODE (val) == SSA_NAME
 507                || !operand_equal_p (arg, val, 0))
 508         break;
 509     }
 510   return (i == gimple_phi_num_args (phi) ? val : NULL);
 511 }
 512
 513 /* Set PHI nodes of a basic block BB to SEQ.  */
 514
 515 void
 516 set_phi_nodes (basic_block bb, gimple_seq seq)
 517 {
 518   gimple_stmt_iterator i;
 519
 520   gcc_checking_assert (!(bb->flags & BB_RTL));
 521   bb->il.gimple.phi_nodes = seq;
 522   if (seq)
 523     for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (&i))
 524       gimple_set_bb (gsi_stmt (i), bb);
 525 }
 526
 527 #include "gt-tree-phinodes.h"