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