gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003-2013 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 "ggc.h"
  26 #include "basic-block.h"
  27 #include "gimple.h"
  28 #include "gimple-iterator.h"
  29 #include "gimple-ssa.h"
  30 #include "tree-phinodes.h"
  31 #include "ssa-iterators.h"
  32 #include "stringpool.h"
  33 #include "tree-ssanames.h"
  34 #include "tree-ssa.h"
  35 #include "diagnostic-core.h"
  36
  37 /* Rewriting a function into SSA form can create a huge number of PHIs
  38    many of which may be thrown away shortly after their creation if jumps
  39    were threaded through PHI nodes.
  40
  41    While our garbage collection mechanisms will handle this situation, it
  42    is extremely wasteful to create nodes and throw them away, especially
  43    when the nodes can be reused.
  44
  45    For PR 8361, we can significantly reduce the number of nodes allocated
  46    and thus the total amount of memory allocated by managing PHIs a
  47    little.  This additionally helps reduce the amount of work done by the
  48    garbage collector.  Similar results have been seen on a wider variety
  49    of tests (such as the compiler itself).
  50
  51    PHI nodes have different sizes, so we can't have a single list of all
  52    the PHI nodes as it would be too expensive to walk down that list to
  53    find a PHI of a suitable size.
  54
  55    Instead we have an array of lists of free PHI nodes.  The array is
  56    indexed by the number of PHI alternatives that PHI node can hold.
  57    Except for the last array member, which holds all remaining PHI
  58    nodes.
  59
  60    So to find a free PHI node, we compute its index into the free PHI
  61    node array and see if there are any elements with an exact match.
  62    If so, then we are done.  Otherwise, we test the next larger size
  63    up and continue until we are in the last array element.
  64
  65    We do not actually walk members of the last array element.  While it
  66    might allow us to pick up a few reusable PHI nodes, it could potentially
  67    be very expensive if the program has released a bunch of large PHI nodes,
  68    but keeps asking for even larger PHI nodes.  Experiments have shown that
  69    walking the elements of the last array entry would result in finding less
  70    than .1% additional reusable PHI nodes.
  71
  72    Note that we can never have less than two PHI argument slots.  Thus,
  73    the -2 on all the calculations below.  */
  74
  75 #define NUM_BUCKETS 10
  76 static GTY ((deletable (""))) vec<gimple, va_gc> *free_phinodes[NUM_BUCKETS - 2];
  77 static unsigned long free_phinode_count;
  78
  79 static int ideal_phi_node_len (int);
  80
  81 unsigned int phi_nodes_reused;
  82 unsigned int phi_nodes_created;
  83
  84 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
  85
  86 void
  87 phinodes_print_statistics (void)
  88 {
  89   fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
  90   fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
  91 }
  92
  93 /* Allocate a PHI node with at least LEN arguments.  If the free list
  94    happens to contain a PHI node with LEN arguments or more, return
  95    that one.  */
  96
  97 static inline gimple_statement_phi *
  98 allocate_phi_node (size_t len)
  99 {
 100   gimple_statement_phi *phi;
 101   size_t bucket = NUM_BUCKETS - 2;
 102   size_t size = sizeof (struct gimple_statement_phi)
 103                 + (len - 1) * sizeof (struct phi_arg_d);
 104
 105   if (free_phinode_count)
 106     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 107       if (free_phinodes[bucket])
 108         break;
 109
 110   /* If our free list has an element, then use it.  */
 111   if (bucket < NUM_BUCKETS - 2
 112       && gimple_phi_capacity ((*free_phinodes[bucket])[0]) >= len)
 113     {
 114       free_phinode_count--;
 115       phi = as_a <gimple_statement_phi> (free_phinodes[bucket]->pop ());
 116       if (free_phinodes[bucket]->is_empty ())
 117         vec_free (free_phinodes[bucket]);
 118       if (GATHER_STATISTICS)
 119         phi_nodes_reused++;
 120     }
 121   else
 122     {
 123       phi = static_cast <gimple_statement_phi *> (
 124         ggc_internal_alloc_stat (size MEM_STAT_INFO));
 125       if (GATHER_STATISTICS)
 126         {
 127           enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
 128           phi_nodes_created++;
 129           gimple_alloc_counts[(int) kind]++;
 130           gimple_alloc_sizes[(int) kind] += size;
 131         }
 132     }
 133
 134   return phi;
 135 }
 136
 137 /* Given LEN, the original number of requested PHI arguments, return
 138    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 139    the total size of the PHI node up to the next power of two bytes.
 140
 141    Rounding up will not result in wasting any memory since the size request
 142    will be rounded up by the GC system anyway.  [ Note this is not entirely
 143    true since the original length might have fit on one of the special
 144    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 145    PHI node later if we increase the number of arguments for the PHI.  */
 146
 147 static int
 148 ideal_phi_node_len (int len)
 149 {
 150   size_t size, new_size;
 151   int log2, new_len;
 152
 153   /* We do not support allocations of less than two PHI argument slots.  */
 154   if (len < 2)
 155     len = 2;
 156
 157   /* Compute the number of bytes of the original request.  */
 158   size = sizeof (struct gimple_statement_phi)
 159          + (len - 1) * sizeof (struct phi_arg_d);
 160
 161   /* Round it up to the next power of two.  */
 162   log2 = ceil_log2 (size);
 163   new_size = 1 << log2;
 164
 165   /* Now compute and return the number of PHI argument slots given an
 166      ideal size allocation.  */
 167   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 168   return new_len;
 169 }
 170
 171 /* Return a PHI node with LEN argument slots for variable VAR.  */
 172
 173 static gimple
 174 make_phi_node (tree var, int len)
 175 {
 176   gimple_statement_phi *phi;
 177   int capacity, i;
 178
 179   capacity = ideal_phi_node_len (len);
 180
 181   phi = allocate_phi_node (capacity);
 182
 183   /* We need to clear the entire PHI node, including the argument
 184      portion, because we represent a "missing PHI argument" by placing
 185      NULL_TREE in PHI_ARG_DEF.  */
 186   memset (phi, 0, (sizeof (struct gimple_statement_phi)
 187                    - sizeof (struct phi_arg_d)
 188                    + sizeof (struct phi_arg_d) * len));
 189   phi->code = GIMPLE_PHI;
 190   gimple_init_singleton (phi);
 191   phi->nargs = len;
 192   phi->capacity = capacity;
 193   if (!var)
 194     ;
 195   else if (TREE_CODE (var) == SSA_NAME)
 196     gimple_phi_set_result (phi, var);
 197   else
 198     gimple_phi_set_result (phi, make_ssa_name (var, phi));
 199
 200   for (i = 0; i < capacity; i++)
 201     {
 202       use_operand_p  imm;
 203
 204       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
 205       imm = gimple_phi_arg_imm_use_ptr (phi, i);
 206       imm->use = gimple_phi_arg_def_ptr (phi, i);
 207       imm->prev = NULL;
 208       imm->next = NULL;
 209       imm->loc.stmt = phi;
 210     }
 211
 212   return phi;
 213 }
 214
 215 /* We no longer need PHI, release it so that it may be reused.  */
 216
 217 void
 218 release_phi_node (gimple phi)
 219 {
 220   size_t bucket;
 221   size_t len = gimple_phi_capacity (phi);
 222   size_t x;
 223
 224   for (x = 0; x < gimple_phi_num_args (phi); x++)
 225     {
 226       use_operand_p  imm;
 227       imm = gimple_phi_arg_imm_use_ptr (phi, x);
 228       delink_imm_use (imm);
 229     }
 230
 231   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 232   bucket -= 2;
 233   vec_safe_push (free_phinodes[bucket], phi);
 234   free_phinode_count++;
 235 }
 236
 237
 238 /* Resize an existing PHI node.  The only way is up.  Return the
 239    possibly relocated phi.  */
 240
 241 static gimple_statement_phi *
 242 resize_phi_node (gimple_statement_phi *phi, size_t len)
 243 {
 244   size_t old_size, i;
 245   gimple_statement_phi *new_phi;
 246
 247   gcc_assert (len > gimple_phi_capacity (phi));
 248
 249   /* The garbage collector will not look at the PHI node beyond the
 250      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 251      portion of the PHI node currently in use.  */
 252   old_size = sizeof (struct gimple_statement_phi)
 253              + (gimple_phi_num_args (phi) - 1) * sizeof (struct phi_arg_d);
 254
 255   new_phi = allocate_phi_node (len);
 256
 257   memcpy (new_phi, phi, old_size);
 258
 259   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 260     {
 261       use_operand_p imm, old_imm;
 262       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 263       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
 264       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 265       relink_imm_use_stmt (imm, old_imm, new_phi);
 266     }
 267
 268   new_phi->capacity = len;
 269
 270   for (i = gimple_phi_num_args (new_phi); i < len; i++)
 271     {
 272       use_operand_p imm;
 273
 274       gimple_phi_arg_set_location (new_phi, i, UNKNOWN_LOCATION);
 275       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 276       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 277       imm->prev = NULL;
 278       imm->next = NULL;
 279       imm->loc.stmt = new_phi;
 280     }
 281
 282   return new_phi;
 283 }
 284
 285 /* Reserve PHI arguments for a new edge to basic block BB.  */
 286
 287 void
 288 reserve_phi_args_for_new_edge (basic_block bb)
 289 {
 290   size_t len = EDGE_COUNT (bb->preds);
 291   size_t cap = ideal_phi_node_len (len + 4);
 292   gimple_stmt_iterator gsi;
 293
 294   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 295     {
 296       gimple_statement_phi *stmt =
 297         as_a <gimple_statement_phi> (gsi_stmt (gsi));
 298
 299       if (len > gimple_phi_capacity (stmt))
 300         {
 301           gimple_statement_phi *new_phi = resize_phi_node (stmt, cap);
 302
 303           /* The result of the PHI is defined by this PHI node.  */
 304           SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
 305           gsi_set_stmt (&gsi, new_phi);
 306
 307           release_phi_node (stmt);
 308           stmt = new_phi;
 309         }
 310
 311       /* We represent a "missing PHI argument" by placing NULL_TREE in
 312          the corresponding slot.  If PHI arguments were added
 313          immediately after an edge is created, this zeroing would not
 314          be necessary, but unfortunately this is not the case.  For
 315          example, the loop optimizer duplicates several basic blocks,
 316          redirects edges, and then fixes up PHI arguments later in
 317          batch.  */
 318       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
 319       gimple_phi_arg_set_location (stmt, len - 1, UNKNOWN_LOCATION);
 320
 321       stmt->nargs++;
 322     }
 323 }
 324
 325 /* Adds PHI to BB.  */
 326
 327 void
 328 add_phi_node_to_bb (gimple phi, basic_block bb)
 329 {
 330   gimple_seq seq = phi_nodes (bb);
 331   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 332   if (seq == NULL)
 333     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
 334   else
 335     {
 336       gimple_seq_add_stmt (&seq, phi);
 337       gcc_assert (seq == phi_nodes (bb));
 338     }
 339
 340   /* Associate BB to the PHI node.  */
 341   gimple_set_bb (phi, bb);
 342
 343 }
 344
 345 /* Create a new PHI node for variable VAR at basic block BB.  */
 346
 347 gimple
 348 create_phi_node (tree var, basic_block bb)
 349 {
 350   gimple phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 351
 352   add_phi_node_to_bb (phi, bb);
 353   return phi;
 354 }
 355
 356
 357 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 358    definition and E is the edge through which DEF reaches PHI.  The new
 359    argument is added at the end of the argument list.
 360    If PHI has reached its maximum capacity, add a few slots.  In this case,
 361    PHI points to the reallocated phi node when we return.  */
 362
 363 void
 364 add_phi_arg (gimple phi, tree def, edge e, source_location locus)
 365 {
 366   basic_block bb = e->dest;
 367
 368   gcc_assert (bb == gimple_bb (phi));
 369
 370   /* We resize PHI nodes upon edge creation.  We should always have
 371      enough room at this point.  */
 372   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 373
 374   /* We resize PHI nodes upon edge creation.  We should always have
 375      enough room at this point.  */
 376   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 377
 378   /* Copy propagation needs to know what object occur in abnormal
 379      PHI nodes.  This is a convenient place to record such information.  */
 380   if (e->flags & EDGE_ABNORMAL)
 381     {
 382       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 383       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 384     }
 385
 386   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 387   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 388 }
 389
 390
 391 /* Remove the Ith argument from PHI's argument list.  This routine
 392    implements removal by swapping the last alternative with the
 393    alternative we want to delete and then shrinking the vector, which
 394    is consistent with how we remove an edge from the edge vector.  */
 395
 396 static void
 397 remove_phi_arg_num (gimple_statement_phi *phi, int i)
 398 {
 399   int num_elem = gimple_phi_num_args (phi);
 400
 401   gcc_assert (i < num_elem);
 402
 403   /* Delink the item which is being removed.  */
 404   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 405
 406   /* If it is not the last element, move the last element
 407      to the element we want to delete, resetting all the links. */
 408   if (i != num_elem - 1)
 409     {
 410       use_operand_p old_p, new_p;
 411       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 412       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 413       /* Set use on new node, and link into last element's place.  */
 414       *(new_p->use) = *(old_p->use);
 415       relink_imm_use (new_p, old_p);
 416       /* Move the location as well.  */
 417       gimple_phi_arg_set_location (phi, i,
 418                                    gimple_phi_arg_location (phi, num_elem - 1));
 419     }
 420
 421   /* Shrink the vector and return.  Note that we do not have to clear
 422      PHI_ARG_DEF because the garbage collector will not look at those
 423      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 424   phi->nargs--;
 425 }
 426
 427
 428 /* Remove all PHI arguments associated with edge E.  */
 429
 430 void
 431 remove_phi_args (edge e)
 432 {
 433   gimple_stmt_iterator gsi;
 434
 435   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 436     remove_phi_arg_num (as_a <gimple_statement_phi> (gsi_stmt (gsi)),
 437                         e->dest_idx);
 438 }
 439
 440
 441 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 442    removal, iterator GSI is updated to point to the next PHI node in the
 443    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 444    into the free pool of SSA names.  */
 445
 446 void
 447 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 448 {
 449   gimple phi = gsi_stmt (*gsi);
 450
 451   if (release_lhs_p)
 452     insert_debug_temps_for_defs (gsi);
 453
 454   gsi_remove (gsi, false);
 455
 456   /* If we are deleting the PHI node, then we should release the
 457      SSA_NAME node so that it can be reused.  */
 458   release_phi_node (phi);
 459   if (release_lhs_p)
 460     release_ssa_name (gimple_phi_result (phi));
 461 }
 462
 463 /* Remove all the phi nodes from BB.  */
 464
 465 void
 466 remove_phi_nodes (basic_block bb)
 467 {
 468   gimple_stmt_iterator gsi;
 469
 470   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 471     remove_phi_node (&gsi, true);
 472
 473   set_phi_nodes (bb, NULL);
 474 }
 475
 476 /* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
 477    NULL.  */
 478
 479 tree
 480 degenerate_phi_result (gimple phi)
 481 {
 482   tree lhs = gimple_phi_result (phi);
 483   tree val = NULL;
 484   size_t i;
 485
 486   /* Ignoring arguments which are the same as LHS, if all the remaining
 487      arguments are the same, then the PHI is a degenerate and has the
 488      value of that common argument.  */
 489   for (i = 0; i < gimple_phi_num_args (phi); i++)
 490     {
 491       tree arg = gimple_phi_arg_def (phi, i);
 492
 493       if (arg == lhs)
 494         continue;
 495       else if (!arg)
 496         break;
 497       else if (!val)
 498         val = arg;
 499       else if (arg == val)
 500         continue;
 501       /* We bring in some of operand_equal_p not only to speed things
 502          up, but also to avoid crashing when dereferencing the type of
 503          a released SSA name.  */
 504       else if (TREE_CODE (val) != TREE_CODE (arg)
 505                || TREE_CODE (val) == SSA_NAME
 506                || !operand_equal_p (arg, val, 0))
 507         break;
 508     }
 509   return (i == gimple_phi_num_args (phi) ? val : NULL);
 510 }
 511
 512 /* Set PHI nodes of a basic block BB to SEQ.  */
 513
 514 void
 515 set_phi_nodes (basic_block bb, gimple_seq seq)
 516 {
 517   gimple_stmt_iterator i;
 518
 519   gcc_checking_assert (!(bb->flags & BB_RTL));
 520   bb->il.gimple.phi_nodes = seq;
 521   if (seq)
 522     for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (&i))
 523       gimple_set_bb (gsi_stmt (i), bb);
 524 }
 525
 526 #include "gt-tree-phinodes.h"