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 "predict.h"
  26 #include "vec.h"
  27 #include "hashtab.h"
  28 #include "hash-set.h"
  29 #include "machmode.h"
  30 #include "hard-reg-set.h"
  31 #include "input.h"
  32 #include "function.h"
  33 #include "basic-block.h"
  34 #include "tree-ssa-alias.h"
  35 #include "internal-fn.h"
  36 #include "gimple-expr.h"
  37 #include "is-a.h"
  38 #include "gimple.h"
  39 #include "gimple-iterator.h"
  40 #include "gimple-ssa.h"
  41 #include "tree-phinodes.h"
  42 #include "ssa-iterators.h"
  43 #include "stringpool.h"
  44 #include "tree-ssanames.h"
  45 #include "tree-ssa.h"
  46 #include "diagnostic-core.h"
  47
  48 /* Rewriting a function into SSA form can create a huge number of PHIs
  49    many of which may be thrown away shortly after their creation if jumps
  50    were threaded through PHI nodes.
  51
  52    While our garbage collection mechanisms will handle this situation, it
  53    is extremely wasteful to create nodes and throw them away, especially
  54    when the nodes can be reused.
  55
  56    For PR 8361, we can significantly reduce the number of nodes allocated
  57    and thus the total amount of memory allocated by managing PHIs a
  58    little.  This additionally helps reduce the amount of work done by the
  59    garbage collector.  Similar results have been seen on a wider variety
  60    of tests (such as the compiler itself).
  61
  62    PHI nodes have different sizes, so we can't have a single list of all
  63    the PHI nodes as it would be too expensive to walk down that list to
  64    find a PHI of a suitable size.
  65
  66    Instead we have an array of lists of free PHI nodes.  The array is
  67    indexed by the number of PHI alternatives that PHI node can hold.
  68    Except for the last array member, which holds all remaining PHI
  69    nodes.
  70
  71    So to find a free PHI node, we compute its index into the free PHI
  72    node array and see if there are any elements with an exact match.
  73    If so, then we are done.  Otherwise, we test the next larger size
  74    up and continue until we are in the last array element.
  75
  76    We do not actually walk members of the last array element.  While it
  77    might allow us to pick up a few reusable PHI nodes, it could potentially
  78    be very expensive if the program has released a bunch of large PHI nodes,
  79    but keeps asking for even larger PHI nodes.  Experiments have shown that
  80    walking the elements of the last array entry would result in finding less
  81    than .1% additional reusable PHI nodes.
  82
  83    Note that we can never have less than two PHI argument slots.  Thus,
  84    the -2 on all the calculations below.  */
  85
  86 #define NUM_BUCKETS 10
  87 static GTY ((deletable (""))) vec<gimple, va_gc> *free_phinodes[NUM_BUCKETS - 2];
  88 static unsigned long free_phinode_count;
  89
  90 static int ideal_phi_node_len (int);
  91
  92 unsigned int phi_nodes_reused;
  93 unsigned int phi_nodes_created;
  94
  95 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
  96
  97 void
  98 phinodes_print_statistics (void)
  99 {
 100   fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
 101   fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
 102 }
 103
 104 /* Allocate a PHI node with at least LEN arguments.  If the free list
 105    happens to contain a PHI node with LEN arguments or more, return
 106    that one.  */
 107
 108 static inline gimple_statement_phi *
 109 allocate_phi_node (size_t len)
 110 {
 111   gimple_statement_phi *phi;
 112   size_t bucket = NUM_BUCKETS - 2;
 113   size_t size = sizeof (struct gimple_statement_phi)
 114                 + (len - 1) * sizeof (struct phi_arg_d);
 115
 116   if (free_phinode_count)
 117     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 118       if (free_phinodes[bucket])
 119         break;
 120
 121   /* If our free list has an element, then use it.  */
 122   if (bucket < NUM_BUCKETS - 2
 123       && gimple_phi_capacity ((*free_phinodes[bucket])[0]) >= len)
 124     {
 125       free_phinode_count--;
 126       phi = as_a <gimple_statement_phi *> (free_phinodes[bucket]->pop ());
 127       if (free_phinodes[bucket]->is_empty ())
 128         vec_free (free_phinodes[bucket]);
 129       if (GATHER_STATISTICS)
 130         phi_nodes_reused++;
 131     }
 132   else
 133     {
 134       phi = static_cast <gimple_statement_phi *> (
 135         ggc_internal_alloc (size));
 136       if (GATHER_STATISTICS)
 137         {
 138           enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
 139           phi_nodes_created++;
 140           gimple_alloc_counts[(int) kind]++;
 141           gimple_alloc_sizes[(int) kind] += size;
 142         }
 143     }
 144
 145   return phi;
 146 }
 147
 148 /* Given LEN, the original number of requested PHI arguments, return
 149    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 150    the total size of the PHI node up to the next power of two bytes.
 151
 152    Rounding up will not result in wasting any memory since the size request
 153    will be rounded up by the GC system anyway.  [ Note this is not entirely
 154    true since the original length might have fit on one of the special
 155    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 156    PHI node later if we increase the number of arguments for the PHI.  */
 157
 158 static int
 159 ideal_phi_node_len (int len)
 160 {
 161   size_t size, new_size;
 162   int log2, new_len;
 163
 164   /* We do not support allocations of less than two PHI argument slots.  */
 165   if (len < 2)
 166     len = 2;
 167
 168   /* Compute the number of bytes of the original request.  */
 169   size = sizeof (struct gimple_statement_phi)
 170          + (len - 1) * sizeof (struct phi_arg_d);
 171
 172   /* Round it up to the next power of two.  */
 173   log2 = ceil_log2 (size);
 174   new_size = 1 << log2;
 175
 176   /* Now compute and return the number of PHI argument slots given an
 177      ideal size allocation.  */
 178   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 179   return new_len;
 180 }
 181
 182 /* Return a PHI node with LEN argument slots for variable VAR.  */
 183
 184 static gimple
 185 make_phi_node (tree var, int len)
 186 {
 187   gimple_statement_phi *phi;
 188   int capacity, i;
 189
 190   capacity = ideal_phi_node_len (len);
 191
 192   phi = allocate_phi_node (capacity);
 193
 194   /* We need to clear the entire PHI node, including the argument
 195      portion, because we represent a "missing PHI argument" by placing
 196      NULL_TREE in PHI_ARG_DEF.  */
 197   memset (phi, 0, (sizeof (struct gimple_statement_phi)
 198                    - sizeof (struct phi_arg_d)
 199                    + sizeof (struct phi_arg_d) * len));
 200   phi->code = GIMPLE_PHI;
 201   gimple_init_singleton (phi);
 202   phi->nargs = len;
 203   phi->capacity = capacity;
 204   if (!var)
 205     ;
 206   else if (TREE_CODE (var) == SSA_NAME)
 207     gimple_phi_set_result (phi, var);
 208   else
 209     gimple_phi_set_result (phi, make_ssa_name (var, phi));
 210
 211   for (i = 0; i < capacity; i++)
 212     {
 213       use_operand_p  imm;
 214
 215       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
 216       imm = gimple_phi_arg_imm_use_ptr (phi, i);
 217       imm->use = gimple_phi_arg_def_ptr (phi, i);
 218       imm->prev = NULL;
 219       imm->next = NULL;
 220       imm->loc.stmt = phi;
 221     }
 222
 223   return phi;
 224 }
 225
 226 /* We no longer need PHI, release it so that it may be reused.  */
 227
 228 void
 229 release_phi_node (gimple phi)
 230 {
 231   size_t bucket;
 232   size_t len = gimple_phi_capacity (phi);
 233   size_t x;
 234
 235   for (x = 0; x < gimple_phi_num_args (phi); x++)
 236     {
 237       use_operand_p  imm;
 238       imm = gimple_phi_arg_imm_use_ptr (phi, x);
 239       delink_imm_use (imm);
 240     }
 241
 242   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 243   bucket -= 2;
 244   vec_safe_push (free_phinodes[bucket], phi);
 245   free_phinode_count++;
 246 }
 247
 248
 249 /* Resize an existing PHI node.  The only way is up.  Return the
 250    possibly relocated phi.  */
 251
 252 static gimple_statement_phi *
 253 resize_phi_node (gimple_statement_phi *phi, size_t len)
 254 {
 255   size_t old_size, i;
 256   gimple_statement_phi *new_phi;
 257
 258   gcc_assert (len > gimple_phi_capacity (phi));
 259
 260   /* The garbage collector will not look at the PHI node beyond the
 261      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 262      portion of the PHI node currently in use.  */
 263   old_size = sizeof (struct gimple_statement_phi)
 264              + (gimple_phi_num_args (phi) - 1) * sizeof (struct phi_arg_d);
 265
 266   new_phi = allocate_phi_node (len);
 267
 268   memcpy (new_phi, phi, old_size);
 269
 270   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 271     {
 272       use_operand_p imm, old_imm;
 273       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 274       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
 275       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 276       relink_imm_use_stmt (imm, old_imm, new_phi);
 277     }
 278
 279   new_phi->capacity = len;
 280
 281   for (i = gimple_phi_num_args (new_phi); i < len; i++)
 282     {
 283       use_operand_p imm;
 284
 285       gimple_phi_arg_set_location (new_phi, i, UNKNOWN_LOCATION);
 286       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 287       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 288       imm->prev = NULL;
 289       imm->next = NULL;
 290       imm->loc.stmt = new_phi;
 291     }
 292
 293   return new_phi;
 294 }
 295
 296 /* Reserve PHI arguments for a new edge to basic block BB.  */
 297
 298 void
 299 reserve_phi_args_for_new_edge (basic_block bb)
 300 {
 301   size_t len = EDGE_COUNT (bb->preds);
 302   size_t cap = ideal_phi_node_len (len + 4);
 303   gimple_stmt_iterator gsi;
 304
 305   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 306     {
 307       gimple_statement_phi *stmt =
 308         as_a <gimple_statement_phi *> (gsi_stmt (gsi));
 309
 310       if (len > gimple_phi_capacity (stmt))
 311         {
 312           gimple_statement_phi *new_phi = resize_phi_node (stmt, cap);
 313
 314           /* The result of the PHI is defined by this PHI node.  */
 315           SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
 316           gsi_set_stmt (&gsi, new_phi);
 317
 318           release_phi_node (stmt);
 319           stmt = new_phi;
 320         }
 321
 322       /* We represent a "missing PHI argument" by placing NULL_TREE in
 323          the corresponding slot.  If PHI arguments were added
 324          immediately after an edge is created, this zeroing would not
 325          be necessary, but unfortunately this is not the case.  For
 326          example, the loop optimizer duplicates several basic blocks,
 327          redirects edges, and then fixes up PHI arguments later in
 328          batch.  */
 329       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
 330       gimple_phi_arg_set_location (stmt, len - 1, UNKNOWN_LOCATION);
 331
 332       stmt->nargs++;
 333     }
 334 }
 335
 336 /* Adds PHI to BB.  */
 337
 338 void
 339 add_phi_node_to_bb (gimple phi, basic_block bb)
 340 {
 341   gimple_seq seq = phi_nodes (bb);
 342   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 343   if (seq == NULL)
 344     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
 345   else
 346     {
 347       gimple_seq_add_stmt (&seq, phi);
 348       gcc_assert (seq == phi_nodes (bb));
 349     }
 350
 351   /* Associate BB to the PHI node.  */
 352   gimple_set_bb (phi, bb);
 353
 354 }
 355
 356 /* Create a new PHI node for variable VAR at basic block BB.  */
 357
 358 gimple
 359 create_phi_node (tree var, basic_block bb)
 360 {
 361   gimple phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 362
 363   add_phi_node_to_bb (phi, bb);
 364   return phi;
 365 }
 366
 367
 368 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 369    definition and E is the edge through which DEF reaches PHI.  The new
 370    argument is added at the end of the argument list.
 371    If PHI has reached its maximum capacity, add a few slots.  In this case,
 372    PHI points to the reallocated phi node when we return.  */
 373
 374 void
 375 add_phi_arg (gimple phi, tree def, edge e, source_location locus)
 376 {
 377   basic_block bb = e->dest;
 378
 379   gcc_assert (bb == gimple_bb (phi));
 380
 381   /* We resize PHI nodes upon edge creation.  We should always have
 382      enough room at this point.  */
 383   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 384
 385   /* We resize PHI nodes upon edge creation.  We should always have
 386      enough room at this point.  */
 387   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 388
 389   /* Copy propagation needs to know what object occur in abnormal
 390      PHI nodes.  This is a convenient place to record such information.  */
 391   if (e->flags & EDGE_ABNORMAL)
 392     {
 393       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 394       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 395     }
 396
 397   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 398   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 399 }
 400
 401
 402 /* Remove the Ith argument from PHI's argument list.  This routine
 403    implements removal by swapping the last alternative with the
 404    alternative we want to delete and then shrinking the vector, which
 405    is consistent with how we remove an edge from the edge vector.  */
 406
 407 static void
 408 remove_phi_arg_num (gimple_statement_phi *phi, int i)
 409 {
 410   int num_elem = gimple_phi_num_args (phi);
 411
 412   gcc_assert (i < num_elem);
 413
 414   /* Delink the item which is being removed.  */
 415   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 416
 417   /* If it is not the last element, move the last element
 418      to the element we want to delete, resetting all the links. */
 419   if (i != num_elem - 1)
 420     {
 421       use_operand_p old_p, new_p;
 422       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 423       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 424       /* Set use on new node, and link into last element's place.  */
 425       *(new_p->use) = *(old_p->use);
 426       relink_imm_use (new_p, old_p);
 427       /* Move the location as well.  */
 428       gimple_phi_arg_set_location (phi, i,
 429                                    gimple_phi_arg_location (phi, num_elem - 1));
 430     }
 431
 432   /* Shrink the vector and return.  Note that we do not have to clear
 433      PHI_ARG_DEF because the garbage collector will not look at those
 434      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 435   phi->nargs--;
 436 }
 437
 438
 439 /* Remove all PHI arguments associated with edge E.  */
 440
 441 void
 442 remove_phi_args (edge e)
 443 {
 444   gimple_stmt_iterator gsi;
 445
 446   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 447     remove_phi_arg_num (as_a <gimple_statement_phi *> (gsi_stmt (gsi)),
 448                         e->dest_idx);
 449 }
 450
 451
 452 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 453    removal, iterator GSI is updated to point to the next PHI node in the
 454    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 455    into the free pool of SSA names.  */
 456
 457 void
 458 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 459 {
 460   gimple phi = gsi_stmt (*gsi);
 461
 462   if (release_lhs_p)
 463     insert_debug_temps_for_defs (gsi);
 464
 465   gsi_remove (gsi, false);
 466
 467   /* If we are deleting the PHI node, then we should release the
 468      SSA_NAME node so that it can be reused.  */
 469   release_phi_node (phi);
 470   if (release_lhs_p)
 471     release_ssa_name (gimple_phi_result (phi));
 472 }
 473
 474 /* Remove all the phi nodes from BB.  */
 475
 476 void
 477 remove_phi_nodes (basic_block bb)
 478 {
 479   gimple_stmt_iterator gsi;
 480
 481   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 482     remove_phi_node (&gsi, true);
 483
 484   set_phi_nodes (bb, NULL);
 485 }
 486
 487 /* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
 488    NULL.  */
 489
 490 tree
 491 degenerate_phi_result (gimple phi)
 492 {
 493   tree lhs = gimple_phi_result (phi);
 494   tree val = NULL;
 495   size_t i;
 496
 497   /* Ignoring arguments which are the same as LHS, if all the remaining
 498      arguments are the same, then the PHI is a degenerate and has the
 499      value of that common argument.  */
 500   for (i = 0; i < gimple_phi_num_args (phi); i++)
 501     {
 502       tree arg = gimple_phi_arg_def (phi, i);
 503
 504       if (arg == lhs)
 505         continue;
 506       else if (!arg)
 507         break;
 508       else if (!val)
 509         val = arg;
 510       else if (arg == val)
 511         continue;
 512       /* We bring in some of operand_equal_p not only to speed things
 513          up, but also to avoid crashing when dereferencing the type of
 514          a released SSA name.  */
 515       else if (TREE_CODE (val) != TREE_CODE (arg)
 516                || TREE_CODE (val) == SSA_NAME
 517                || !operand_equal_p (arg, val, 0))
 518         break;
 519     }
 520   return (i == gimple_phi_num_args (phi) ? val : NULL);
 521 }
 522
 523 /* Set PHI nodes of a basic block BB to SEQ.  */
 524
 525 void
 526 set_phi_nodes (basic_block bb, gimple_seq seq)
 527 {
 528   gimple_stmt_iterator i;
 529
 530   gcc_checking_assert (!(bb->flags & BB_RTL));
 531   bb->il.gimple.phi_nodes = seq;
 532   if (seq)
 533     for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (&i))
 534       gimple_set_bb (gsi_stmt (i), bb);
 535 }
 536
 537 #include "gt-tree-phinodes.h"