gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003 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 2, 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 COPYING.  If not, write to
  18 the Free Software Foundation, 59 Temple Place - Suite 330,
  19 Boston, MA 02111-1307, USA.  */
  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"
  27 #include "varray.h"
  28 #include "ggc.h"
  29 #include "basic-block.h"
  30 #include "tree-flow.h"
  31 #include "toplev.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    Right now we maintain our free list on a per-function basis.  It may
  48    or may not make sense to maintain the free list for the duration of
  49    a compilation unit.
  50
  51    We could also use a zone allocator for these objects since they have
  52    a very well defined lifetime.  If someone wants to experiment with that
  53    this is the place to try it.
  54
  55    PHI nodes have different sizes, so we can't have a single list of all
  56    the PHI nodes as it would be too expensive to walk down that list to
  57    find a PHI of a suitable size.
  58
  59    Instead we have an array of lists of free PHI nodes.  The array is
  60    indexed by the number of PHI alternatives that PHI node can hold.
  61    Except for the last array member, which holds all remaining PHI
  62    nodes.
  63
  64    So to find a free PHI node, we compute its index into the free PHI
  65    node array and see if there are any elements with an exact match.
  66    If so, then we are done.  Otherwise, we test the next larger size
  67    up and continue until we are in the last array element.
  68
  69    We do not actually walk members of the last array element.  While it
  70    might allow us to pick up a few reusable PHI nodes, it could potentially
  71    be very expensive if the program has released a bunch of large PHI nodes,
  72    but keeps asking for even larger PHI nodes.  Experiments have shown that
  73    walking the elements of the last array entry would result in finding less
  74    than .1% additional reusable PHI nodes.
  75
  76    Note that we can never have less than two PHI argument slots.  Thus,
  77    the -2 on all the calculations below.  */
  78
  79 #define NUM_BUCKETS 10
  80 static GTY ((deletable (""))) tree free_phinodes[NUM_BUCKETS - 2];
  81 static unsigned long free_phinode_count;
  82
  83 static int ideal_phi_node_len (int);
  84 static void resize_phi_node (tree *, int);
  85
  86 #ifdef GATHER_STATISTICS
  87 unsigned int phi_nodes_reused;
  88 unsigned int phi_nodes_created;
  89 #endif
  90
  91 /* Initialize management of PHIs.  */
  92
  93 void
  94 init_phinodes (void)
  95 {
  96   int i;
  97
  98   for (i = 0; i < NUM_BUCKETS - 2; i++)
  99     free_phinodes[i] = NULL;
 100   free_phinode_count = 0;
 101 }
 102
 103 /* Finalize management of PHIs.  */
 104
 105 void
 106 fini_phinodes (void)
 107 {
 108   int i;
 109
 110   for (i = 0; i < NUM_BUCKETS - 2; i++)
 111     free_phinodes[i] = NULL;
 112   free_phinode_count = 0;
 113 }
 114
 115 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
 116
 117 #ifdef GATHER_STATISTICS
 118 void
 119 phinodes_print_statistics (void)
 120 {
 121   fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
 122   fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
 123 }
 124 #endif
 125
 126 /* Allocate a PHI node with at least LEN arguments.  If the free list
 127    happens to contain a PHI node with LEN arguments or more, return
 128    that one.  */
 129
 130 static inline tree
 131 allocate_phi_node (int len)
 132 {
 133   tree phi;
 134   int bucket = NUM_BUCKETS - 2;
 135   int size = (sizeof (struct tree_phi_node)
 136               + (len - 1) * sizeof (struct phi_arg_d));
 137
 138   if (free_phinode_count)
 139     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 140       if (free_phinodes[bucket])
 141         break;
 142
 143   /* If our free list has an element, then use it.  */
 144   if (bucket < NUM_BUCKETS - 2
 145       && PHI_ARG_CAPACITY (free_phinodes[bucket]) >= len)
 146     {
 147       free_phinode_count--;
 148       phi = free_phinodes[bucket];
 149       free_phinodes[bucket] = PHI_CHAIN (free_phinodes[bucket]);
 150 #ifdef GATHER_STATISTICS
 151       phi_nodes_reused++;
 152 #endif
 153     }
 154   else
 155     {
 156       phi = ggc_alloc (size);
 157 #ifdef GATHER_STATISTICS
 158       phi_nodes_created++;
 159       tree_node_counts[(int) phi_kind]++;
 160       tree_node_sizes[(int) phi_kind] += size;
 161 #endif
 162     }
 163
 164   return phi;
 165 }
 166
 167 /* Given LEN, the original number of requested PHI arguments, return
 168    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 169    the total size of the PHI node up to the next power of two bytes.
 170
 171    Rounding up will not result in wasting any memory since the size request
 172    will be rounded up by the GC system anyway.  [ Note this is not entirely
 173    true since the original length might have fit on one of the special
 174    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 175    PHI node later if we increase the number of arguments for the PHI.  */
 176
 177 static int
 178 ideal_phi_node_len (int len)
 179 {
 180   size_t size, new_size;
 181   int log2, new_len;
 182
 183   /* We do not support allocations of less than two PHI argument slots.  */
 184   if (len < 2)
 185     len = 2;
 186
 187   /* Compute the number of bytes of the original request.  */
 188   size = sizeof (struct tree_phi_node) + (len - 1) * sizeof (struct phi_arg_d);
 189
 190   /* Round it up to the next power of two.  */
 191   log2 = ceil_log2 (size);
 192   new_size = 1 << log2;
 193
 194   /* Now compute and return the number of PHI argument slots given an
 195      ideal size allocation.  */
 196   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 197   return new_len;
 198 }
 199
 200 /* Return a PHI node for variable VAR defined in statement STMT.
 201    STMT may be an empty statement for artificial references (e.g., default
 202    definitions created when a variable is used without a preceding
 203    definition).  */
 204
 205 static tree
 206 make_phi_node (tree var, int len)
 207 {
 208   tree phi;
 209   int capacity;
 210
 211   capacity = ideal_phi_node_len (len);
 212
 213   phi = allocate_phi_node (capacity);
 214
 215   /* We do not have to clear a part of the PHI node that stores PHI
 216      arguments, which is safe because we tell the garbage collector to
 217      scan up to num_args elements in the array of PHI arguments.  In
 218      other words, the garbage collector will not follow garbage
 219      pointers in the unused portion of the array.  */
 220   memset (phi, 0, sizeof (struct tree_phi_node) - sizeof (struct phi_arg_d));
 221   TREE_SET_CODE (phi, PHI_NODE);
 222   PHI_ARG_CAPACITY (phi) = capacity;
 223   TREE_TYPE (phi) = TREE_TYPE (var);
 224   if (TREE_CODE (var) == SSA_NAME)
 225     SET_PHI_RESULT (phi, var);
 226   else
 227     SET_PHI_RESULT (phi, make_ssa_name (var, phi));
 228
 229   return phi;
 230 }
 231
 232 /* We no longer need PHI, release it so that it may be reused.  */
 233
 234 void
 235 release_phi_node (tree phi)
 236 {
 237   int bucket;
 238   int len = PHI_ARG_CAPACITY (phi);
 239
 240   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 241   bucket -= 2;
 242   PHI_CHAIN (phi) = free_phinodes[bucket];
 243   free_phinodes[bucket] = phi;
 244   free_phinode_count++;
 245 }
 246
 247 /* Resize an existing PHI node.  The only way is up.  Return the
 248    possibly relocated phi.  */
 249
 250 static void
 251 resize_phi_node (tree *phi, int len)
 252 {
 253   int old_size;
 254   tree new_phi;
 255
 256   gcc_assert (len >= PHI_ARG_CAPACITY (*phi));
 257
 258   /* The garbage collector will not look at the PHI node beyond the
 259      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 260      portion of the PHI node currently in use.  */
 261   old_size = (sizeof (struct tree_phi_node)
 262              + (PHI_NUM_ARGS (*phi) - 1) * sizeof (struct phi_arg_d));
 263
 264   new_phi = allocate_phi_node (len);
 265
 266   memcpy (new_phi, *phi, old_size);
 267
 268   PHI_ARG_CAPACITY (new_phi) = len;
 269
 270   *phi = new_phi;
 271 }
 272
 273 /* Reserve PHI arguments for a new edge to basic block BB.  */
 274
 275 void
 276 reserve_phi_args_for_new_edge (basic_block bb)
 277 {
 278   tree *loc;
 279   int len = EDGE_COUNT (bb->preds);
 280   int cap = ideal_phi_node_len (len + 4);
 281
 282   for (loc = &(bb_ann (bb)->phi_nodes);
 283        *loc;
 284        loc = &PHI_CHAIN (*loc))
 285     {
 286       if (len > PHI_ARG_CAPACITY (*loc))
 287         {
 288           tree old_phi = *loc;
 289
 290           resize_phi_node (loc, cap);
 291
 292           /* The result of the phi is defined by this phi node.  */
 293           SSA_NAME_DEF_STMT (PHI_RESULT (*loc)) = *loc;
 294
 295           release_phi_node (old_phi);
 296         }
 297     }
 298 }
 299
 300 /* Create a new PHI node for variable VAR at basic block BB.  */
 301
 302 tree
 303 create_phi_node (tree var, basic_block bb)
 304 {
 305   tree phi;
 306
 307   phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 308
 309   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 310   PHI_CHAIN (phi) = phi_nodes (bb);
 311   bb_ann (bb)->phi_nodes = phi;
 312
 313   /* Associate BB to the PHI node.  */
 314   set_bb_for_stmt (phi, bb);
 315
 316   return phi;
 317 }
 318
 319 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 320    definition and E is the edge through which DEF reaches PHI.  The new
 321    argument is added at the end of the argument list.
 322    If PHI has reached its maximum capacity, add a few slots.  In this case,
 323    PHI points to the reallocated phi node when we return.  */
 324
 325 void
 326 add_phi_arg (tree *phi, tree def, edge e)
 327 {
 328   basic_block bb = e->dest;
 329   int i = PHI_NUM_ARGS (*phi);
 330
 331   gcc_assert (bb == bb_for_stmt (*phi));
 332
 333   /* We resize PHI nodes upon edge creation.  We should always have
 334      enough room at this point.  */
 335   gcc_assert (PHI_NUM_ARGS (*phi) < PHI_ARG_CAPACITY (*phi));
 336
 337   /* Copy propagation needs to know what object occur in abnormal
 338      PHI nodes.  This is a convenient place to record such information.  */
 339   if (e->flags & EDGE_ABNORMAL)
 340     {
 341       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 342       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (*phi)) = 1;
 343     }
 344
 345   SET_PHI_ARG_DEF (*phi, i, def);
 346   PHI_ARG_EDGE (*phi, i) = e;
 347   PHI_ARG_NONZERO (*phi, i) = false;
 348   PHI_NUM_ARGS (*phi)++;
 349 }
 350
 351 /* Remove the Ith argument from PHI's argument list.  This routine assumes
 352    ordering of alternatives in the vector is not important and implements
 353    removal by swapping the last alternative with the alternative we want to
 354    delete, then shrinking the vector.  */
 355
 356 static void
 357 remove_phi_arg_num (tree phi, int i)
 358 {
 359   int num_elem = PHI_NUM_ARGS (phi);
 360
 361   gcc_assert (i < num_elem);
 362
 363   /* If we are not at the last element, switch the last element
 364      with the element we want to delete.  */
 365   if (i != num_elem - 1)
 366     {
 367       SET_PHI_ARG_DEF (phi, i, PHI_ARG_DEF (phi, num_elem - 1));
 368       PHI_ARG_EDGE (phi, i) = PHI_ARG_EDGE (phi, num_elem - 1);
 369       PHI_ARG_NONZERO (phi, i) = PHI_ARG_NONZERO (phi, num_elem - 1);
 370     }
 371
 372   /* Shrink the vector and return.  Note that we do not have to clear
 373      PHI_ARG_DEF, PHI_ARG_EDGE, or PHI_ARG_NONZERO because the garbage
 374      collector will not look at those elements beyond the first
 375      PHI_NUM_ARGS elements of the array.  */
 376   PHI_NUM_ARGS (phi)--;
 377 }
 378
 379 /* Remove all PHI arguments associated with edge E.  */
 380
 381 void
 382 remove_phi_args (edge e)
 383 {
 384   tree phi;
 385
 386   for (phi = phi_nodes (e->dest); phi; phi = PHI_CHAIN (phi))
 387     {
 388       int index = phi_arg_from_edge (phi, e);
 389       if (index >= 0)
 390         remove_phi_arg_num (phi, index);
 391     }
 392 }
 393
 394 /* Remove PHI node PHI from basic block BB.  If PREV is non-NULL, it is
 395    used as the node immediately before PHI in the linked list.  */
 396
 397 void
 398 remove_phi_node (tree phi, tree prev, basic_block bb)
 399 {
 400   if (prev)
 401     {
 402       /* Rewire the list if we are given a PREV pointer.  */
 403       PHI_CHAIN (prev) = PHI_CHAIN (phi);
 404
 405       /* If we are deleting the PHI node, then we should release the
 406          SSA_NAME node so that it can be reused.  */
 407       release_ssa_name (PHI_RESULT (phi));
 408       release_phi_node (phi);
 409     }
 410   else if (phi == phi_nodes (bb))
 411     {
 412       /* Update the list head if removing the first element.  */
 413       bb_ann (bb)->phi_nodes = PHI_CHAIN (phi);
 414
 415       /* If we are deleting the PHI node, then we should release the
 416          SSA_NAME node so that it can be reused.  */
 417       release_ssa_name (PHI_RESULT (phi));
 418       release_phi_node (phi);
 419     }
 420   else
 421     {
 422       /* Traverse the list looking for the node to remove.  */
 423       tree prev, t;
 424       prev = NULL_TREE;
 425       for (t = phi_nodes (bb); t && t != phi; t = PHI_CHAIN (t))
 426         prev = t;
 427       if (t)
 428         remove_phi_node (t, prev, bb);
 429     }
 430 }
 431
 432
 433 /* Remove all the PHI nodes for variables in the VARS bitmap.  */
 434
 435 void
 436 remove_all_phi_nodes_for (bitmap vars)
 437 {
 438   basic_block bb;
 439
 440   FOR_EACH_BB (bb)
 441     {
 442       /* Build a new PHI list for BB without variables in VARS.  */
 443       tree phi, new_phi_list, next;
 444       tree *lastp = &new_phi_list;
 445
 446       for (phi = phi_nodes (bb); phi; phi = next)
 447         {
 448           tree var = SSA_NAME_VAR (PHI_RESULT (phi));
 449
 450           next = PHI_CHAIN (phi);
 451           /* Only add PHI nodes for variables not in VARS.  */
 452           if (!bitmap_bit_p (vars, var_ann (var)->uid))
 453             {
 454               /* If we're not removing this PHI node, then it must have
 455                  been rewritten by a previous call into the SSA rewriter.
 456                  Note that fact in PHI_REWRITTEN.  */
 457               PHI_REWRITTEN (phi) = 1;
 458
 459               *lastp = phi;
 460               lastp = &PHI_CHAIN (phi);
 461             }
 462           else
 463             {
 464               /* If we are deleting the PHI node, then we should release the
 465                  SSA_NAME node so that it can be reused.  */
 466               release_ssa_name (PHI_RESULT (phi));
 467               release_phi_node (phi);
 468             }
 469         }
 470
 471       /* Make sure the last node in the new list has no successors.  */
 472       *lastp = NULL;
 473       bb_ann (bb)->phi_nodes = new_phi_list;
 474
 475 #if defined ENABLE_CHECKING
 476       for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 477         {
 478           tree var = SSA_NAME_VAR (PHI_RESULT (phi));
 479           gcc_assert (!bitmap_bit_p (vars, var_ann (var)->uid));
 480         }
 481 #endif
 482     }
 483 }
 484
 485 /* Reverse the order of PHI nodes in the chain PHI.
 486    Return the new head of the chain (old last PHI node).  */
 487
 488 tree
 489 phi_reverse (tree phi)
 490 {
 491   tree prev = NULL_TREE, next;
 492   for (; phi; phi = next)
 493     {
 494       next = PHI_CHAIN (phi);
 495       PHI_CHAIN (phi) = prev;
 496       prev = phi;
 497     }
 498   return prev;
 499 }
 500
 501 #include "gt-tree-phinodes.h"
 502