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_statement_phi *
 101 allocate_phi_node (size_t len)
 102 {
 103   gimple_statement_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_statement_phi *> (
 127         ggc_internal_alloc (size));
 128       if (GATHER_STATISTICS)
 129         {
 130           enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
 131           phi_nodes_created++;
 132           gimple_alloc_counts[(int) kind]++;
 133           gimple_alloc_sizes[(int) kind] += size;
 134         }
 135     }
 136
 137   return phi;
 138 }
 139
 140 /* Given LEN, the original number of requested PHI arguments, return
 141    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 142    the total size of the PHI node up to the next power of two bytes.
 143
 144    Rounding up will not result in wasting any memory since the size request
 145    will be rounded up by the GC system anyway.  [ Note this is not entirely
 146    true since the original length might have fit on one of the special
 147    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 148    PHI node later if we increase the number of arguments for the PHI.  */
 149
 150 static int
 151 ideal_phi_node_len (int len)
 152 {
 153   size_t size, new_size;
 154   int log2, new_len;
 155
 156   /* We do not support allocations of less than two PHI argument slots.  */
 157   if (len < 2)
 158     len = 2;
 159
 160   /* Compute the number of bytes of the original request.  */
 161   size = sizeof (struct gimple_statement_phi)
 162          + (len - 1) * sizeof (struct phi_arg_d);
 163
 164   /* Round it up to the next power of two.  */
 165   log2 = ceil_log2 (size);
 166   new_size = 1 << log2;
 167
 168   /* Now compute and return the number of PHI argument slots given an
 169      ideal size allocation.  */
 170   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 171   return new_len;
 172 }
 173
 174 /* Return a PHI node with LEN argument slots for variable VAR.  */
 175
 176 static gimple
 177 make_phi_node (tree var, int len)
 178 {
 179   gimple_statement_phi *phi;
 180   int capacity, i;
 181
 182   capacity = ideal_phi_node_len (len);
 183
 184   phi = allocate_phi_node (capacity);
 185
 186   /* We need to clear the entire PHI node, including the argument
 187      portion, because we represent a "missing PHI argument" by placing
 188      NULL_TREE in PHI_ARG_DEF.  */
 189   memset (phi, 0, (sizeof (struct gimple_statement_phi)
 190                    - sizeof (struct phi_arg_d)
 191                    + sizeof (struct phi_arg_d) * len));
 192   phi->code = GIMPLE_PHI;
 193   gimple_init_singleton (phi);
 194   phi->nargs = len;
 195   phi->capacity = capacity;
 196   if (!var)
 197     ;
 198   else if (TREE_CODE (var) == SSA_NAME)
 199     gimple_phi_set_result (phi, var);
 200   else
 201     gimple_phi_set_result (phi, make_ssa_name (var, phi));
 202
 203   for (i = 0; i < capacity; i++)
 204     {
 205       use_operand_p  imm;
 206
 207       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
 208       imm = gimple_phi_arg_imm_use_ptr (phi, i);
 209       imm->use = gimple_phi_arg_def_ptr (phi, i);
 210       imm->prev = NULL;
 211       imm->next = NULL;
 212       imm->loc.stmt = phi;
 213     }
 214
 215   return phi;
 216 }
 217
 218 /* We no longer need PHI, release it so that it may be reused.  */
 219
 220 void
 221 release_phi_node (gimple phi)
 222 {
 223   size_t bucket;
 224   size_t len = gimple_phi_capacity (phi);
 225   size_t x;
 226
 227   for (x = 0; x < gimple_phi_num_args (phi); x++)
 228     {
 229       use_operand_p  imm;
 230       imm = gimple_phi_arg_imm_use_ptr (phi, x);
 231       delink_imm_use (imm);
 232     }
 233
 234   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 235   bucket -= 2;
 236   vec_safe_push (free_phinodes[bucket], 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
 244 static gimple_statement_phi *
 245 resize_phi_node (gimple_statement_phi *phi, size_t len)
 246 {
 247   size_t old_size, i;
 248   gimple_statement_phi *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 gimple_statement_phi)
 256              + (gimple_phi_num_args (phi) - 1) * sizeof (struct phi_arg_d);
 257
 258   new_phi = allocate_phi_node (len);
 259
 260   memcpy (new_phi, phi, old_size);
 261
 262   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 263     {
 264       use_operand_p imm, old_imm;
 265       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 266       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
 267       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 268       relink_imm_use_stmt (imm, old_imm, new_phi);
 269     }
 270
 271   new_phi->capacity = len;
 272
 273   for (i = gimple_phi_num_args (new_phi); i < len; i++)
 274     {
 275       use_operand_p imm;
 276
 277       gimple_phi_arg_set_location (new_phi, i, UNKNOWN_LOCATION);
 278       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 279       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 280       imm->prev = NULL;
 281       imm->next = NULL;
 282       imm->loc.stmt = new_phi;
 283     }
 284
 285   return new_phi;
 286 }
 287
 288 /* Reserve PHI arguments for a new edge to basic block BB.  */
 289
 290 void
 291 reserve_phi_args_for_new_edge (basic_block bb)
 292 {
 293   size_t len = EDGE_COUNT (bb->preds);
 294   size_t cap = ideal_phi_node_len (len + 4);
 295   gimple_stmt_iterator gsi;
 296
 297   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 298     {
 299       gimple_statement_phi *stmt =
 300         as_a <gimple_statement_phi *> (gsi_stmt (gsi));
 301
 302       if (len > gimple_phi_capacity (stmt))
 303         {
 304           gimple_statement_phi *new_phi = resize_phi_node (stmt, cap);
 305
 306           /* The result of the PHI is defined by this PHI node.  */
 307           SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
 308           gsi_set_stmt (&gsi, new_phi);
 309
 310           release_phi_node (stmt);
 311           stmt = new_phi;
 312         }
 313
 314       /* We represent a "missing PHI argument" by placing NULL_TREE in
 315          the corresponding slot.  If PHI arguments were added
 316          immediately after an edge is created, this zeroing would not
 317          be necessary, but unfortunately this is not the case.  For
 318          example, the loop optimizer duplicates several basic blocks,
 319          redirects edges, and then fixes up PHI arguments later in
 320          batch.  */
 321       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
 322       gimple_phi_arg_set_location (stmt, len - 1, UNKNOWN_LOCATION);
 323
 324       stmt->nargs++;
 325     }
 326 }
 327
 328 /* Adds PHI to BB.  */
 329
 330 void
 331 add_phi_node_to_bb (gimple phi, basic_block bb)
 332 {
 333   gimple_seq seq = phi_nodes (bb);
 334   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 335   if (seq == NULL)
 336     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
 337   else
 338     {
 339       gimple_seq_add_stmt (&seq, phi);
 340       gcc_assert (seq == phi_nodes (bb));
 341     }
 342
 343   /* Associate BB to the PHI node.  */
 344   gimple_set_bb (phi, bb);
 345
 346 }
 347
 348 /* Create a new PHI node for variable VAR at basic block BB.  */
 349
 350 gimple
 351 create_phi_node (tree var, basic_block bb)
 352 {
 353   gimple phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 354
 355   add_phi_node_to_bb (phi, bb);
 356   return phi;
 357 }
 358
 359
 360 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 361    definition and E is the edge through which DEF reaches PHI.  The new
 362    argument is added at the end of the argument list.
 363    If PHI has reached its maximum capacity, add a few slots.  In this case,
 364    PHI points to the reallocated phi node when we return.  */
 365
 366 void
 367 add_phi_arg (gimple phi, tree def, edge e, source_location locus)
 368 {
 369   basic_block bb = e->dest;
 370
 371   gcc_assert (bb == gimple_bb (phi));
 372
 373   /* We resize PHI nodes upon edge creation.  We should always have
 374      enough room at this point.  */
 375   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 376
 377   /* We resize PHI nodes upon edge creation.  We should always have
 378      enough room at this point.  */
 379   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 380
 381   /* Copy propagation needs to know what object occur in abnormal
 382      PHI nodes.  This is a convenient place to record such information.  */
 383   if (e->flags & EDGE_ABNORMAL)
 384     {
 385       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 386       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 387     }
 388
 389   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 390   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 391 }
 392
 393
 394 /* Remove the Ith argument from PHI's argument list.  This routine
 395    implements removal by swapping the last alternative with the
 396    alternative we want to delete and then shrinking the vector, which
 397    is consistent with how we remove an edge from the edge vector.  */
 398
 399 static void
 400 remove_phi_arg_num (gimple_statement_phi *phi, int i)
 401 {
 402   int num_elem = gimple_phi_num_args (phi);
 403
 404   gcc_assert (i < num_elem);
 405
 406   /* Delink the item which is being removed.  */
 407   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 408
 409   /* If it is not the last element, move the last element
 410      to the element we want to delete, resetting all the links. */
 411   if (i != num_elem - 1)
 412     {
 413       use_operand_p old_p, new_p;
 414       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 415       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 416       /* Set use on new node, and link into last element's place.  */
 417       *(new_p->use) = *(old_p->use);
 418       relink_imm_use (new_p, old_p);
 419       /* Move the location as well.  */
 420       gimple_phi_arg_set_location (phi, i,
 421                                    gimple_phi_arg_location (phi, num_elem - 1));
 422     }
 423
 424   /* Shrink the vector and return.  Note that we do not have to clear
 425      PHI_ARG_DEF because the garbage collector will not look at those
 426      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 427   phi->nargs--;
 428 }
 429
 430
 431 /* Remove all PHI arguments associated with edge E.  */
 432
 433 void
 434 remove_phi_args (edge e)
 435 {
 436   gimple_stmt_iterator gsi;
 437
 438   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 439     remove_phi_arg_num (as_a <gimple_statement_phi *> (gsi_stmt (gsi)),
 440                         e->dest_idx);
 441 }
 442
 443
 444 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 445    removal, iterator GSI is updated to point to the next PHI node in the
 446    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 447    into the free pool of SSA names.  */
 448
 449 void
 450 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 451 {
 452   gimple phi = gsi_stmt (*gsi);
 453
 454   if (release_lhs_p)
 455     insert_debug_temps_for_defs (gsi);
 456
 457   gsi_remove (gsi, false);
 458
 459   /* If we are deleting the PHI node, then we should release the
 460      SSA_NAME node so that it can be reused.  */
 461   release_phi_node (phi);
 462   if (release_lhs_p)
 463     release_ssa_name (gimple_phi_result (phi));
 464 }
 465
 466 /* Remove all the phi nodes from BB.  */
 467
 468 void
 469 remove_phi_nodes (basic_block bb)
 470 {
 471   gimple_stmt_iterator gsi;
 472
 473   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 474     remove_phi_node (&gsi, true);
 475
 476   set_phi_nodes (bb, NULL);
 477 }
 478
 479 /* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
 480    NULL.  */
 481
 482 tree
 483 degenerate_phi_result (gimple phi)
 484 {
 485   tree lhs = gimple_phi_result (phi);
 486   tree val = NULL;
 487   size_t i;
 488
 489   /* Ignoring arguments which are the same as LHS, if all the remaining
 490      arguments are the same, then the PHI is a degenerate and has the
 491      value of that common argument.  */
 492   for (i = 0; i < gimple_phi_num_args (phi); i++)
 493     {
 494       tree arg = gimple_phi_arg_def (phi, i);
 495
 496       if (arg == lhs)
 497         continue;
 498       else if (!arg)
 499         break;
 500       else if (!val)
 501         val = arg;
 502       else if (arg == val)
 503         continue;
 504       /* We bring in some of operand_equal_p not only to speed things
 505          up, but also to avoid crashing when dereferencing the type of
 506          a released SSA name.  */
 507       else if (TREE_CODE (val) != TREE_CODE (arg)
 508                || TREE_CODE (val) == SSA_NAME
 509                || !operand_equal_p (arg, val, 0))
 510         break;
 511     }
 512   return (i == gimple_phi_num_args (phi) ? val : NULL);
 513 }
 514
 515 /* Set PHI nodes of a basic block BB to SEQ.  */
 516
 517 void
 518 set_phi_nodes (basic_block bb, gimple_seq seq)
 519 {
 520   gimple_stmt_iterator i;
 521
 522   gcc_checking_assert (!(bb->flags & BB_RTL));
 523   bb->il.gimple.phi_nodes = seq;
 524   if (seq)
 525     for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (&i))
 526       gimple_set_bb (gsi_stmt (i), bb);
 527 }
 528
 529 #include "gt-tree-phinodes.h"