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