gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003, 2005, 2007, 2008, 2009, 2010
   3    Free Software Foundation, Inc.
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 3, or (at your option)
  10 any later version.
  11
  12 GCC is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "tree.h"
  26 #include "ggc.h"
  27 #include "basic-block.h"
  28 #include "tree-flow.h"
  29 #include "diagnostic-core.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    We could also use a zone allocator for these objects since they have
  47    a very well defined lifetime.  If someone wants to experiment with that
  48    this is the place to try it.
  49
  50    PHI nodes have different sizes, so we can't have a single list of all
  51    the PHI nodes as it would be too expensive to walk down that list to
  52    find a PHI of a suitable size.
  53
  54    Instead we have an array of lists of free PHI nodes.  The array is
  55    indexed by the number of PHI alternatives that PHI node can hold.
  56    Except for the last array member, which holds all remaining PHI
  57    nodes.
  58
  59    So to find a free PHI node, we compute its index into the free PHI
  60    node array and see if there are any elements with an exact match.
  61    If so, then we are done.  Otherwise, we test the next larger size
  62    up and continue until we are in the last array element.
  63
  64    We do not actually walk members of the last array element.  While it
  65    might allow us to pick up a few reusable PHI nodes, it could potentially
  66    be very expensive if the program has released a bunch of large PHI nodes,
  67    but keeps asking for even larger PHI nodes.  Experiments have shown that
  68    walking the elements of the last array entry would result in finding less
  69    than .1% additional reusable PHI nodes.
  70
  71    Note that we can never have less than two PHI argument slots.  Thus,
  72    the -2 on all the calculations below.  */
  73
  74 #define NUM_BUCKETS 10
  75 static GTY ((deletable (""))) VEC(gimple,gc) *free_phinodes[NUM_BUCKETS - 2];
  76 static unsigned long free_phinode_count;
  77
  78 static int ideal_phi_node_len (int);
  79
  80 unsigned int phi_nodes_reused;
  81 unsigned int phi_nodes_created;
  82
  83 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
  84
  85 void
  86 phinodes_print_statistics (void)
  87 {
  88   fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
  89   fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
  90 }
  91
  92 /* Allocate a PHI node with at least LEN arguments.  If the free list
  93    happens to contain a PHI node with LEN arguments or more, return
  94    that one.  */
  95
  96 static inline gimple
  97 allocate_phi_node (size_t len)
  98 {
  99   gimple phi;
 100   size_t bucket = NUM_BUCKETS - 2;
 101   size_t size = sizeof (struct gimple_statement_phi)
 102                 + (len - 1) * sizeof (struct phi_arg_d);
 103
 104   if (free_phinode_count)
 105     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 106       if (free_phinodes[bucket])
 107         break;
 108
 109   /* If our free list has an element, then use it.  */
 110   if (bucket < NUM_BUCKETS - 2
 111       && gimple_phi_capacity (VEC_index (gimple, free_phinodes[bucket], 0))
 112          >= len)
 113     {
 114       free_phinode_count--;
 115       phi = VEC_pop (gimple, free_phinodes[bucket]);
 116       if (VEC_empty (gimple, free_phinodes[bucket]))
 117         VEC_free (gimple, gc, free_phinodes[bucket]);
 118       if (GATHER_STATISTICS)
 119         phi_nodes_reused++;
 120     }
 121   else
 122     {
 123       phi = ggc_alloc_gimple_statement_d (size);
 124       if (GATHER_STATISTICS)
 125         {
 126           enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
 127           phi_nodes_created++;
 128           gimple_alloc_counts[(int) kind]++;
 129           gimple_alloc_sizes[(int) kind] += size;
 130         }
 131     }
 132
 133   return phi;
 134 }
 135
 136 /* Given LEN, the original number of requested PHI arguments, return
 137    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 138    the total size of the PHI node up to the next power of two bytes.
 139
 140    Rounding up will not result in wasting any memory since the size request
 141    will be rounded up by the GC system anyway.  [ Note this is not entirely
 142    true since the original length might have fit on one of the special
 143    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 144    PHI node later if we increase the number of arguments for the PHI.  */
 145
 146 static int
 147 ideal_phi_node_len (int len)
 148 {
 149   size_t size, new_size;
 150   int log2, new_len;
 151
 152   /* We do not support allocations of less than two PHI argument slots.  */
 153   if (len < 2)
 154     len = 2;
 155
 156   /* Compute the number of bytes of the original request.  */
 157   size = sizeof (struct gimple_statement_phi)
 158          + (len - 1) * sizeof (struct phi_arg_d);
 159
 160   /* Round it up to the next power of two.  */
 161   log2 = ceil_log2 (size);
 162   new_size = 1 << log2;
 163
 164   /* Now compute and return the number of PHI argument slots given an
 165      ideal size allocation.  */
 166   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 167   return new_len;
 168 }
 169
 170 /* Return a PHI node with LEN argument slots for variable VAR.  */
 171
 172 static gimple
 173 make_phi_node (tree var, int len)
 174 {
 175   gimple phi;
 176   int capacity, i;
 177
 178   capacity = ideal_phi_node_len (len);
 179
 180   phi = allocate_phi_node (capacity);
 181
 182   /* We need to clear the entire PHI node, including the argument
 183      portion, because we represent a "missing PHI argument" by placing
 184      NULL_TREE in PHI_ARG_DEF.  */
 185   memset (phi, 0, (sizeof (struct gimple_statement_phi)
 186                    - sizeof (struct phi_arg_d)
 187                    + sizeof (struct phi_arg_d) * len));
 188   phi->gsbase.code = GIMPLE_PHI;
 189   gimple_init_singleton (phi);
 190   phi->gimple_phi.nargs = len;
 191   phi->gimple_phi.capacity = capacity;
 192   if (!var)
 193     ;
 194   else if (TREE_CODE (var) == SSA_NAME)
 195     gimple_phi_set_result (phi, var);
 196   else
 197     gimple_phi_set_result (phi, make_ssa_name (var, phi));
 198
 199   for (i = 0; i < capacity; i++)
 200     {
 201       use_operand_p  imm;
 202
 203       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
 204       imm = gimple_phi_arg_imm_use_ptr (phi, i);
 205       imm->use = gimple_phi_arg_def_ptr (phi, i);
 206       imm->prev = NULL;
 207       imm->next = NULL;
 208       imm->loc.stmt = phi;
 209     }
 210
 211   return phi;
 212 }
 213
 214 /* We no longer need PHI, release it so that it may be reused.  */
 215
 216 void
 217 release_phi_node (gimple phi)
 218 {
 219   size_t bucket;
 220   size_t len = gimple_phi_capacity (phi);
 221   size_t x;
 222
 223   for (x = 0; x < gimple_phi_num_args (phi); x++)
 224     {
 225       use_operand_p  imm;
 226       imm = gimple_phi_arg_imm_use_ptr (phi, x);
 227       delink_imm_use (imm);
 228     }
 229
 230   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 231   bucket -= 2;
 232   VEC_safe_push (gimple, gc, free_phinodes[bucket], phi);
 233   free_phinode_count++;
 234 }
 235
 236
 237 /* Resize an existing PHI node.  The only way is up.  Return the
 238    possibly relocated phi.  */
 239
 240 static gimple
 241 resize_phi_node (gimple phi, size_t len)
 242 {
 243   size_t old_size, i;
 244   gimple new_phi;
 245
 246   gcc_assert (len > gimple_phi_capacity (phi));
 247
 248   /* The garbage collector will not look at the PHI node beyond the
 249      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 250      portion of the PHI node currently in use.  */
 251   old_size = sizeof (struct gimple_statement_phi)
 252              + (gimple_phi_num_args (phi) - 1) * sizeof (struct phi_arg_d);
 253
 254   new_phi = allocate_phi_node (len);
 255
 256   memcpy (new_phi, phi, old_size);
 257
 258   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 259     {
 260       use_operand_p imm, old_imm;
 261       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 262       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
 263       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 264       relink_imm_use_stmt (imm, old_imm, new_phi);
 265     }
 266
 267   new_phi->gimple_phi.capacity = len;
 268
 269   for (i = gimple_phi_num_args (new_phi); i < len; i++)
 270     {
 271       use_operand_p imm;
 272
 273       gimple_phi_arg_set_location (new_phi, i, UNKNOWN_LOCATION);
 274       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 275       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 276       imm->prev = NULL;
 277       imm->next = NULL;
 278       imm->loc.stmt = new_phi;
 279     }
 280
 281   return new_phi;
 282 }
 283
 284 /* Reserve PHI arguments for a new edge to basic block BB.  */
 285
 286 void
 287 reserve_phi_args_for_new_edge (basic_block bb)
 288 {
 289   size_t len = EDGE_COUNT (bb->preds);
 290   size_t cap = ideal_phi_node_len (len + 4);
 291   gimple_stmt_iterator gsi;
 292
 293   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 294     {
 295       gimple stmt = gsi_stmt (gsi);
 296
 297       if (len > gimple_phi_capacity (stmt))
 298         {
 299           gimple new_phi = resize_phi_node (stmt, cap);
 300
 301           /* The result of the PHI is defined by this PHI node.  */
 302           SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
 303           gsi_set_stmt (&gsi, new_phi);
 304
 305           release_phi_node (stmt);
 306           stmt = new_phi;
 307         }
 308
 309       /* We represent a "missing PHI argument" by placing NULL_TREE in
 310          the corresponding slot.  If PHI arguments were added
 311          immediately after an edge is created, this zeroing would not
 312          be necessary, but unfortunately this is not the case.  For
 313          example, the loop optimizer duplicates several basic blocks,
 314          redirects edges, and then fixes up PHI arguments later in
 315          batch.  */
 316       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
 317
 318       stmt->gimple_phi.nargs++;
 319     }
 320 }
 321
 322 /* Adds PHI to BB.  */
 323
 324 void
 325 add_phi_node_to_bb (gimple phi, basic_block bb)
 326 {
 327   gimple_seq seq = phi_nodes (bb);
 328   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 329   if (seq == NULL)
 330     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
 331   else
 332     {
 333       gimple_seq_add_stmt (&seq, phi);
 334       gcc_assert (seq == phi_nodes (bb));
 335     }
 336
 337   /* Associate BB to the PHI node.  */
 338   gimple_set_bb (phi, bb);
 339
 340 }
 341
 342 /* Create a new PHI node for variable VAR at basic block BB.  */
 343
 344 gimple
 345 create_phi_node (tree var, basic_block bb)
 346 {
 347   gimple phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 348
 349   add_phi_node_to_bb (phi, bb);
 350   return phi;
 351 }
 352
 353
 354 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 355    definition and E is the edge through which DEF reaches PHI.  The new
 356    argument is added at the end of the argument list.
 357    If PHI has reached its maximum capacity, add a few slots.  In this case,
 358    PHI points to the reallocated phi node when we return.  */
 359
 360 void
 361 add_phi_arg (gimple phi, tree def, edge e, source_location locus)
 362 {
 363   basic_block bb = e->dest;
 364
 365   gcc_assert (bb == gimple_bb (phi));
 366
 367   /* We resize PHI nodes upon edge creation.  We should always have
 368      enough room at this point.  */
 369   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 370
 371   /* We resize PHI nodes upon edge creation.  We should always have
 372      enough room at this point.  */
 373   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 374
 375   /* Copy propagation needs to know what object occur in abnormal
 376      PHI nodes.  This is a convenient place to record such information.  */
 377   if (e->flags & EDGE_ABNORMAL)
 378     {
 379       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 380       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 381     }
 382
 383   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 384   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 385 }
 386
 387
 388 /* Remove the Ith argument from PHI's argument list.  This routine
 389    implements removal by swapping the last alternative with the
 390    alternative we want to delete and then shrinking the vector, which
 391    is consistent with how we remove an edge from the edge vector.  */
 392
 393 static void
 394 remove_phi_arg_num (gimple phi, int i)
 395 {
 396   int num_elem = gimple_phi_num_args (phi);
 397
 398   gcc_assert (i < num_elem);
 399
 400   /* Delink the item which is being removed.  */
 401   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 402
 403   /* If it is not the last element, move the last element
 404      to the element we want to delete, resetting all the links. */
 405   if (i != num_elem - 1)
 406     {
 407       use_operand_p old_p, new_p;
 408       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 409       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 410       /* Set use on new node, and link into last element's place.  */
 411       *(new_p->use) = *(old_p->use);
 412       relink_imm_use (new_p, old_p);
 413       /* Move the location as well.  */
 414       gimple_phi_arg_set_location (phi, i,
 415                                    gimple_phi_arg_location (phi, num_elem - 1));
 416     }
 417
 418   /* Shrink the vector and return.  Note that we do not have to clear
 419      PHI_ARG_DEF because the garbage collector will not look at those
 420      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 421   phi->gimple_phi.nargs--;
 422 }
 423
 424
 425 /* Remove all PHI arguments associated with edge E.  */
 426
 427 void
 428 remove_phi_args (edge e)
 429 {
 430   gimple_stmt_iterator gsi;
 431
 432   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 433     remove_phi_arg_num (gsi_stmt (gsi), e->dest_idx);
 434 }
 435
 436
 437 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 438    removal, iterator GSI is updated to point to the next PHI node in the
 439    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 440    into the free pool of SSA names.  */
 441
 442 void
 443 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 444 {
 445   gimple phi = gsi_stmt (*gsi);
 446
 447   if (release_lhs_p)
 448     insert_debug_temps_for_defs (gsi);
 449
 450   gsi_remove (gsi, false);
 451
 452   /* If we are deleting the PHI node, then we should release the
 453      SSA_NAME node so that it can be reused.  */
 454   release_phi_node (phi);
 455   if (release_lhs_p)
 456     release_ssa_name (gimple_phi_result (phi));
 457 }
 458
 459 /* Remove all the phi nodes from BB.  */
 460
 461 void
 462 remove_phi_nodes (basic_block bb)
 463 {
 464   gimple_stmt_iterator gsi;
 465
 466   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 467     remove_phi_node (&gsi, true);
 468
 469   set_phi_nodes (bb, NULL);
 470 }
 471
 472 #include "gt-tree-phinodes.h"