gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003, 2005, 2007, 2008 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 "rtl.h"        /* FIXME: Only for ceil_log2, of all things...  */
  26 #include "ggc.h"
  27 #include "basic-block.h"
  28 #include "tree-flow.h"
  29 #include "toplev.h"
  30 #include "gimple.h"
  31
  32 /* Rewriting a function into SSA form can create a huge number of PHIs
  33    many of which may be thrown away shortly after their creation if jumps
  34    were threaded through PHI nodes.
  35
  36    While our garbage collection mechanisms will handle this situation, it
  37    is extremely wasteful to create nodes and throw them away, especially
  38    when the nodes can be reused.
  39
  40    For PR 8361, we can significantly reduce the number of nodes allocated
  41    and thus the total amount of memory allocated by managing PHIs a
  42    little.  This additionally helps reduce the amount of work done by the
  43    garbage collector.  Similar results have been seen on a wider variety
  44    of tests (such as the compiler itself).
  45
  46    Right now we maintain our free list on a per-function basis.  It may
  47    or may not make sense to maintain the free list for the duration of
  48    a compilation unit.
  49
  50    We could also use a zone allocator for these objects since they have
  51    a very well defined lifetime.  If someone wants to experiment with that
  52    this is the place to try it.
  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,gc) *free_phinodes[NUM_BUCKETS - 2];
  80 static unsigned long free_phinode_count;
  81
  82 static int ideal_phi_node_len (int);
  83
  84 #ifdef GATHER_STATISTICS
  85 unsigned int phi_nodes_reused;
  86 unsigned int phi_nodes_created;
  87 #endif
  88
  89 /* Initialize management of PHIs.  */
  90
  91 void
  92 init_phinodes (void)
  93 {
  94   int i;
  95
  96   for (i = 0; i < NUM_BUCKETS - 2; i++)
  97     free_phinodes[i] = NULL;
  98   free_phinode_count = 0;
  99 }
 100
 101 /* Finalize management of PHIs.  */
 102
 103 void
 104 fini_phinodes (void)
 105 {
 106   int i;
 107
 108   for (i = 0; i < NUM_BUCKETS - 2; i++)
 109     free_phinodes[i] = NULL;
 110   free_phinode_count = 0;
 111 }
 112
 113 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
 114
 115 #ifdef GATHER_STATISTICS
 116 void
 117 phinodes_print_statistics (void)
 118 {
 119   fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
 120   fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
 121 }
 122 #endif
 123
 124 /* Allocate a PHI node with at least LEN arguments.  If the free list
 125    happens to contain a PHI node with LEN arguments or more, return
 126    that one.  */
 127
 128 static inline gimple
 129 allocate_phi_node (size_t len)
 130 {
 131   gimple phi;
 132   size_t bucket = NUM_BUCKETS - 2;
 133   size_t size = sizeof (struct gimple_statement_phi)
 134                 + (len - 1) * sizeof (struct phi_arg_d);
 135
 136   if (free_phinode_count)
 137     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 138       if (free_phinodes[bucket])
 139         break;
 140
 141   /* If our free list has an element, then use it.  */
 142   if (bucket < NUM_BUCKETS - 2
 143       && gimple_phi_capacity (VEC_index (gimple, free_phinodes[bucket], 0))
 144          >= len)
 145     {
 146       free_phinode_count--;
 147       phi = VEC_pop (gimple, free_phinodes[bucket]);
 148       if (VEC_empty (gimple, free_phinodes[bucket]))
 149         VEC_free (gimple, gc, free_phinodes[bucket]);
 150 #ifdef GATHER_STATISTICS
 151       phi_nodes_reused++;
 152 #endif
 153     }
 154   else
 155     {
 156       phi = ggc_alloc_gimple_statement_d (size);
 157 #ifdef GATHER_STATISTICS
 158       phi_nodes_created++;
 159         {
 160           enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
 161           gimple_alloc_counts[(int) kind]++;
 162           gimple_alloc_sizes[(int) kind] += size;
 163         }
 164 #endif
 165     }
 166
 167   return phi;
 168 }
 169
 170 /* Given LEN, the original number of requested PHI arguments, return
 171    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 172    the total size of the PHI node up to the next power of two bytes.
 173
 174    Rounding up will not result in wasting any memory since the size request
 175    will be rounded up by the GC system anyway.  [ Note this is not entirely
 176    true since the original length might have fit on one of the special
 177    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 178    PHI node later if we increase the number of arguments for the PHI.  */
 179
 180 static int
 181 ideal_phi_node_len (int len)
 182 {
 183   size_t size, new_size;
 184   int log2, new_len;
 185
 186   /* We do not support allocations of less than two PHI argument slots.  */
 187   if (len < 2)
 188     len = 2;
 189
 190   /* Compute the number of bytes of the original request.  */
 191   size = sizeof (struct gimple_statement_phi)
 192          + (len - 1) * sizeof (struct phi_arg_d);
 193
 194   /* Round it up to the next power of two.  */
 195   log2 = ceil_log2 (size);
 196   new_size = 1 << log2;
 197
 198   /* Now compute and return the number of PHI argument slots given an
 199      ideal size allocation.  */
 200   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 201   return new_len;
 202 }
 203
 204 /* Return a PHI node with LEN argument slots for variable VAR.  */
 205
 206 gimple
 207 make_phi_node (tree var, int len)
 208 {
 209   gimple phi;
 210   int capacity, i;
 211
 212   capacity = ideal_phi_node_len (len);
 213
 214   phi = allocate_phi_node (capacity);
 215
 216   /* We need to clear the entire PHI node, including the argument
 217      portion, because we represent a "missing PHI argument" by placing
 218      NULL_TREE in PHI_ARG_DEF.  */
 219   memset (phi, 0, (sizeof (struct gimple_statement_phi)
 220                    - sizeof (struct phi_arg_d)
 221                    + sizeof (struct phi_arg_d) * len));
 222   phi->gsbase.code = GIMPLE_PHI;
 223   phi->gimple_phi.nargs = len;
 224   phi->gimple_phi.capacity = capacity;
 225   if (TREE_CODE (var) == SSA_NAME)
 226     gimple_phi_set_result (phi, var);
 227   else
 228     gimple_phi_set_result (phi, make_ssa_name (var, phi));
 229
 230   for (i = 0; i < capacity; i++)
 231     {
 232       use_operand_p  imm;
 233
 234       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
 235       imm = gimple_phi_arg_imm_use_ptr (phi, i);
 236       imm->use = gimple_phi_arg_def_ptr (phi, i);
 237       imm->prev = NULL;
 238       imm->next = NULL;
 239       imm->loc.stmt = phi;
 240     }
 241
 242   return phi;
 243 }
 244
 245 /* We no longer need PHI, release it so that it may be reused.  */
 246
 247 void
 248 release_phi_node (gimple phi)
 249 {
 250   size_t bucket;
 251   size_t len = gimple_phi_capacity (phi);
 252   size_t x;
 253
 254   for (x = 0; x < gimple_phi_num_args (phi); x++)
 255     {
 256       use_operand_p  imm;
 257       imm = gimple_phi_arg_imm_use_ptr (phi, x);
 258       delink_imm_use (imm);
 259     }
 260
 261   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 262   bucket -= 2;
 263   VEC_safe_push (gimple, gc, free_phinodes[bucket], phi);
 264   free_phinode_count++;
 265 }
 266
 267
 268 /* Resize an existing PHI node.  The only way is up.  Return the
 269    possibly relocated phi.  */
 270
 271 static void
 272 resize_phi_node (gimple *phi, size_t len)
 273 {
 274   size_t old_size, i;
 275   gimple new_phi;
 276
 277   gcc_assert (len > gimple_phi_capacity (*phi));
 278
 279   /* The garbage collector will not look at the PHI node beyond the
 280      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 281      portion of the PHI node currently in use.  */
 282   old_size = sizeof (struct gimple_statement_phi)
 283              + (gimple_phi_num_args (*phi) - 1) * sizeof (struct phi_arg_d);
 284
 285   new_phi = allocate_phi_node (len);
 286
 287   memcpy (new_phi, *phi, old_size);
 288
 289   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 290     {
 291       use_operand_p imm, old_imm;
 292       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 293       old_imm = gimple_phi_arg_imm_use_ptr (*phi, i);
 294       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 295       relink_imm_use_stmt (imm, old_imm, new_phi);
 296     }
 297
 298   new_phi->gimple_phi.capacity = len;
 299
 300   for (i = gimple_phi_num_args (new_phi); i < len; i++)
 301     {
 302       use_operand_p imm;
 303
 304       gimple_phi_arg_set_location (new_phi, i, UNKNOWN_LOCATION);
 305       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 306       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 307       imm->prev = NULL;
 308       imm->next = NULL;
 309       imm->loc.stmt = new_phi;
 310     }
 311
 312   *phi = new_phi;
 313 }
 314
 315 /* Reserve PHI arguments for a new edge to basic block BB.  */
 316
 317 void
 318 reserve_phi_args_for_new_edge (basic_block bb)
 319 {
 320   size_t len = EDGE_COUNT (bb->preds);
 321   size_t cap = ideal_phi_node_len (len + 4);
 322   gimple_stmt_iterator gsi;
 323
 324   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 325     {
 326       gimple *loc = gsi_stmt_ptr (&gsi);
 327
 328       if (len > gimple_phi_capacity (*loc))
 329         {
 330           gimple old_phi = *loc;
 331
 332           resize_phi_node (loc, cap);
 333
 334           /* The result of the PHI is defined by this PHI node.  */
 335           SSA_NAME_DEF_STMT (gimple_phi_result (*loc)) = *loc;
 336
 337           release_phi_node (old_phi);
 338         }
 339
 340       /* We represent a "missing PHI argument" by placing NULL_TREE in
 341          the corresponding slot.  If PHI arguments were added
 342          immediately after an edge is created, this zeroing would not
 343          be necessary, but unfortunately this is not the case.  For
 344          example, the loop optimizer duplicates several basic blocks,
 345          redirects edges, and then fixes up PHI arguments later in
 346          batch.  */
 347       SET_PHI_ARG_DEF (*loc, len - 1, NULL_TREE);
 348
 349       (*loc)->gimple_phi.nargs++;
 350     }
 351 }
 352
 353 /* Adds PHI to BB.  */
 354
 355 void
 356 add_phi_node_to_bb (gimple phi, basic_block bb)
 357 {
 358   gimple_stmt_iterator gsi;
 359   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 360   if (phi_nodes (bb) == NULL)
 361     set_phi_nodes (bb, gimple_seq_alloc ());
 362
 363   gsi = gsi_last (phi_nodes (bb));
 364   gsi_insert_after (&gsi, phi, GSI_NEW_STMT);
 365
 366   /* Associate BB to the PHI node.  */
 367   gimple_set_bb (phi, bb);
 368
 369 }
 370
 371 /* Create a new PHI node for variable VAR at basic block BB.  */
 372
 373 gimple
 374 create_phi_node (tree var, basic_block bb)
 375 {
 376   gimple phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 377
 378   add_phi_node_to_bb (phi, bb);
 379   return phi;
 380 }
 381
 382
 383 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 384    definition and E is the edge through which DEF reaches PHI.  The new
 385    argument is added at the end of the argument list.
 386    If PHI has reached its maximum capacity, add a few slots.  In this case,
 387    PHI points to the reallocated phi node when we return.  */
 388
 389 void
 390 add_phi_arg (gimple phi, tree def, edge e, source_location locus)
 391 {
 392   basic_block bb = e->dest;
 393
 394   gcc_assert (bb == gimple_bb (phi));
 395
 396   /* We resize PHI nodes upon edge creation.  We should always have
 397      enough room at this point.  */
 398   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 399
 400   /* We resize PHI nodes upon edge creation.  We should always have
 401      enough room at this point.  */
 402   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 403
 404   /* Copy propagation needs to know what object occur in abnormal
 405      PHI nodes.  This is a convenient place to record such information.  */
 406   if (e->flags & EDGE_ABNORMAL)
 407     {
 408       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 409       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 410     }
 411
 412   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 413   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 414 }
 415
 416
 417 /* Remove the Ith argument from PHI's argument list.  This routine
 418    implements removal by swapping the last alternative with the
 419    alternative we want to delete and then shrinking the vector, which
 420    is consistent with how we remove an edge from the edge vector.  */
 421
 422 static void
 423 remove_phi_arg_num (gimple phi, int i)
 424 {
 425   int num_elem = gimple_phi_num_args (phi);
 426
 427   gcc_assert (i < num_elem);
 428
 429   /* Delink the item which is being removed.  */
 430   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 431
 432   /* If it is not the last element, move the last element
 433      to the element we want to delete, resetting all the links. */
 434   if (i != num_elem - 1)
 435     {
 436       use_operand_p old_p, new_p;
 437       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 438       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 439       /* Set use on new node, and link into last element's place.  */
 440       *(new_p->use) = *(old_p->use);
 441       relink_imm_use (new_p, old_p);
 442       /* Move the location as well.  */
 443       gimple_phi_arg_set_location (phi, i,
 444                                    gimple_phi_arg_location (phi, num_elem - 1));
 445     }
 446
 447   /* Shrink the vector and return.  Note that we do not have to clear
 448      PHI_ARG_DEF because the garbage collector will not look at those
 449      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 450   phi->gimple_phi.nargs--;
 451 }
 452
 453
 454 /* Remove all PHI arguments associated with edge E.  */
 455
 456 void
 457 remove_phi_args (edge e)
 458 {
 459   gimple_stmt_iterator gsi;
 460
 461   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 462     remove_phi_arg_num (gsi_stmt (gsi), e->dest_idx);
 463 }
 464
 465
 466 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 467    removal, iterator GSI is updated to point to the next PHI node in the
 468    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 469    into the free pool of SSA names.  */
 470
 471 void
 472 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 473 {
 474   gimple phi = gsi_stmt (*gsi);
 475
 476   if (release_lhs_p)
 477     insert_debug_temps_for_defs (gsi);
 478
 479   gsi_remove (gsi, false);
 480
 481   /* If we are deleting the PHI node, then we should release the
 482      SSA_NAME node so that it can be reused.  */
 483   release_phi_node (phi);
 484   if (release_lhs_p)
 485     release_ssa_name (gimple_phi_result (phi));
 486 }
 487
 488 /* Remove all the phi nodes from BB.  */
 489
 490 void
 491 remove_phi_nodes (basic_block bb)
 492 {
 493   gimple_stmt_iterator gsi;
 494
 495   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 496     remove_phi_node (&gsi, true);
 497
 498   set_phi_nodes (bb, NULL);
 499 }
 500
 501 #include "gt-tree-phinodes.h"