gcc/tree-phinodes.c

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