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 /* Given LEN, the original number of requested PHI arguments, return
 127    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 128    the total size of the PHI node up to the next power of two bytes.
 129
 130    Rounding up will not result in wasting any memory since the size request
 131    will be rounded up by the GC system anyway.  [ Note this is not entirely
 132    true since the original length might have fit on one of the special
 133    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 134    PHI node later if we increase the number of arguments for the PHI.  */
 135
 136 static int
 137 ideal_phi_node_len (int len)
 138 {
 139   size_t size, new_size;
 140   int log2, new_len;
 141
 142   /* We do not support allocations of less than two PHI argument slots.  */
 143   if (len < 2)
 144     len = 2;
 145
 146   /* Compute the number of bytes of the original request.  */
 147   size = sizeof (struct tree_phi_node) + (len - 1) * sizeof (struct phi_arg_d);
 148
 149   /* Round it up to the next power of two.  */
 150   log2 = ceil_log2 (size);
 151   new_size = 1 << log2;
 152
 153   /* Now compute and return the number of PHI argument slots given an
 154      ideal size allocation.  */
 155   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 156   return new_len;
 157 }
 158
 159 /* Return a PHI node for variable VAR defined in statement STMT.
 160    STMT may be an empty statement for artificial references (e.g., default
 161    definitions created when a variable is used without a preceding
 162    definition).  */
 163
 164 tree
 165 make_phi_node (tree var, int len)
 166 {
 167   tree phi;
 168   int size;
 169   int bucket = NUM_BUCKETS - 2;
 170
 171   len = ideal_phi_node_len (len);
 172
 173   size = sizeof (struct tree_phi_node) + (len - 1) * sizeof (struct phi_arg_d);
 174
 175   if (free_phinode_count)
 176     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 177       if (free_phinodes[bucket])
 178         break;
 179
 180   /* If our free list has an element, then use it.  */
 181   if (bucket < NUM_BUCKETS - 2
 182       && PHI_ARG_CAPACITY (free_phinodes[bucket]) >= len)
 183     {
 184       free_phinode_count--;
 185       phi = free_phinodes[bucket];
 186       free_phinodes[bucket] = PHI_CHAIN (free_phinodes[bucket]);
 187 #ifdef GATHER_STATISTICS
 188       phi_nodes_reused++;
 189 #endif
 190     }
 191   else
 192     {
 193       phi = ggc_alloc (size);
 194 #ifdef GATHER_STATISTICS
 195       phi_nodes_created++;
 196       tree_node_counts[(int) phi_kind]++;
 197       tree_node_sizes[(int) phi_kind] += size;
 198 #endif
 199
 200     }
 201
 202   /* We do not have to clear a part of the PHI node that stores PHI
 203      arguments, which is safe because we tell the garbage collector to
 204      scan up to num_args elements in the array of PHI arguments.  In
 205      other words, the garbage collector will not follow garbage
 206      pointers in the unused portion of the array.  */
 207   memset (phi, 0, sizeof (struct tree_phi_node) - sizeof (struct phi_arg_d));
 208   TREE_SET_CODE (phi, PHI_NODE);
 209   PHI_ARG_CAPACITY (phi) = len;
 210   TREE_TYPE (phi) = TREE_TYPE (var);
 211   if (TREE_CODE (var) == SSA_NAME)
 212     SET_PHI_RESULT (phi, var);
 213   else
 214     SET_PHI_RESULT (phi, make_ssa_name (var, phi));
 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 (tree phi)
 223 {
 224   int bucket;
 225   int len = PHI_ARG_CAPACITY (phi);
 226
 227   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 228   bucket -= 2;
 229   PHI_CHAIN (phi) = free_phinodes[bucket];
 230   free_phinodes[bucket] = phi;
 231   free_phinode_count++;
 232 }
 233
 234 /* Resize an existing PHI node.  The only way is up.  Return the
 235    possibly relocated phi.  */
 236
 237 static void
 238 resize_phi_node (tree *phi, int len)
 239 {
 240   int size, old_size;
 241   tree new_phi;
 242   int bucket = NUM_BUCKETS - 2;
 243
 244   gcc_assert (len >= PHI_ARG_CAPACITY (*phi));
 245
 246   /* Note that OLD_SIZE is guaranteed to be smaller than SIZE.  */
 247   old_size = (sizeof (struct tree_phi_node)
 248              + (PHI_ARG_CAPACITY (*phi) - 1) * sizeof (struct phi_arg_d));
 249   size = sizeof (struct tree_phi_node) + (len - 1) * sizeof (struct phi_arg_d);
 250
 251   if (free_phinode_count)
 252     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 253       if (free_phinodes[bucket])
 254         break;
 255
 256   /* If our free list has an element, then use it.  */
 257   if (bucket < NUM_BUCKETS - 2
 258       && PHI_ARG_CAPACITY (free_phinodes[bucket]) >= len)
 259     {
 260       free_phinode_count--;
 261       new_phi = free_phinodes[bucket];
 262       free_phinodes[bucket] = PHI_CHAIN (free_phinodes[bucket]);
 263 #ifdef GATHER_STATISTICS
 264       phi_nodes_reused++;
 265 #endif
 266     }
 267   else
 268     {
 269       new_phi = ggc_alloc (size);
 270 #ifdef GATHER_STATISTICS
 271       phi_nodes_created++;
 272       tree_node_counts[(int) phi_kind]++;
 273       tree_node_sizes[(int) phi_kind] += size;
 274 #endif
 275     }
 276
 277   memcpy (new_phi, *phi, old_size);
 278
 279   PHI_ARG_CAPACITY (new_phi) = len;
 280
 281   *phi = new_phi;
 282 }
 283
 284 /* Create a new PHI node for variable VAR at basic block BB.  */
 285
 286 tree
 287 create_phi_node (tree var, basic_block bb)
 288 {
 289   tree phi;
 290
 291   phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 292
 293   /* This is a new phi node, so note that is has not yet been
 294      rewritten.  */
 295   PHI_REWRITTEN (phi) = 0;
 296
 297   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 298   PHI_CHAIN (phi) = phi_nodes (bb);
 299   bb_ann (bb)->phi_nodes = phi;
 300
 301   /* Associate BB to the PHI node.  */
 302   set_bb_for_stmt (phi, bb);
 303
 304   return phi;
 305 }
 306
 307 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 308    definition and E is the edge through which DEF reaches PHI.  The new
 309    argument is added at the end of the argument list.
 310    If PHI has reached its maximum capacity, add a few slots.  In this case,
 311    PHI points to the reallocated phi node when we return.  */
 312
 313 void
 314 add_phi_arg (tree *phi, tree def, edge e)
 315 {
 316   int i = PHI_NUM_ARGS (*phi);
 317
 318   if (i >= PHI_ARG_CAPACITY (*phi))
 319     {
 320       tree old_phi = *phi;
 321       basic_block bb;
 322
 323       /* Resize the phi.  Unfortunately, this will relocate it.  */
 324       resize_phi_node (phi, ideal_phi_node_len (i + 4));
 325
 326       /* resize_phi_node will necessarily relocate the phi.  */
 327       gcc_assert (*phi != old_phi);
 328
 329       /* The result of the phi is defined by this phi node.  */
 330       SSA_NAME_DEF_STMT (PHI_RESULT (*phi)) = *phi;
 331
 332       /* Extract the basic block for the PHI from the PHI's annotation
 333          rather than the edge.  This works better as the edge's
 334          destination may not currently be the block with the PHI node
 335          if we are in the process of threading the edge to a new
 336          destination.  */
 337       bb = bb_for_stmt (*phi);
 338
 339       release_phi_node (old_phi);
 340
 341       /* Update the list head if replacing the first listed phi.  */
 342       if (phi_nodes (bb) == old_phi)
 343         bb_ann (bb)->phi_nodes = *phi;
 344       else
 345         {
 346           /* Traverse the list looking for the phi node to chain to.  */
 347           tree p;
 348
 349           for (p = phi_nodes (bb);
 350                p && PHI_CHAIN (p) != old_phi;
 351                p = PHI_CHAIN (p))
 352             ;
 353
 354           gcc_assert (p);
 355           PHI_CHAIN (p) = *phi;
 356         }
 357     }
 358
 359   /* Copy propagation needs to know what object occur in abnormal
 360      PHI nodes.  This is a convenient place to record such information.  */
 361   if (e->flags & EDGE_ABNORMAL)
 362     {
 363       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 364       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (*phi)) = 1;
 365     }
 366
 367   SET_PHI_ARG_DEF (*phi, i, def);
 368   PHI_ARG_EDGE (*phi, i) = e;
 369   PHI_ARG_NONZERO (*phi, i) = false;
 370   PHI_NUM_ARGS (*phi)++;
 371 }
 372
 373 /* Remove a PHI argument from PHI.  BLOCK is the predecessor block where
 374    the PHI argument is coming from.  */
 375
 376 void
 377 remove_phi_arg (tree phi, basic_block block)
 378 {
 379   int i, num_elem = PHI_NUM_ARGS (phi);
 380
 381   for (i = 0; i < num_elem; i++)
 382     {
 383       basic_block src_bb;
 384
 385       src_bb = PHI_ARG_EDGE (phi, i)->src;
 386
 387       if (src_bb == block)
 388         {
 389           remove_phi_arg_num (phi, i);
 390           return;
 391         }
 392     }
 393 }
 394
 395
 396 /* Remove the Ith argument from PHI's argument list.  This routine assumes
 397    ordering of alternatives in the vector is not important and implements
 398    removal by swapping the last alternative with the alternative we want to
 399    delete, then shrinking the vector.  */
 400
 401 void
 402 remove_phi_arg_num (tree phi, int i)
 403 {
 404   int num_elem = PHI_NUM_ARGS (phi);
 405
 406   gcc_assert (i < num_elem);
 407
 408   /* If we are not at the last element, switch the last element
 409      with the element we want to delete.  */
 410   if (i != num_elem - 1)
 411     {
 412       SET_PHI_ARG_DEF (phi, i, PHI_ARG_DEF (phi, num_elem - 1));
 413       PHI_ARG_EDGE (phi, i) = PHI_ARG_EDGE (phi, num_elem - 1);
 414       PHI_ARG_NONZERO (phi, i) = PHI_ARG_NONZERO (phi, num_elem - 1);
 415     }
 416
 417   /* Shrink the vector and return.  */
 418   SET_PHI_ARG_DEF (phi, num_elem - 1, NULL_TREE);
 419   PHI_ARG_EDGE (phi, num_elem - 1) = NULL;
 420   PHI_ARG_NONZERO (phi, num_elem - 1) = false;
 421   PHI_NUM_ARGS (phi)--;
 422 }
 423
 424 /* Remove PHI node PHI from basic block BB.  If PREV is non-NULL, it is
 425    used as the node immediately before PHI in the linked list.  */
 426
 427 void
 428 remove_phi_node (tree phi, tree prev, basic_block bb)
 429 {
 430   if (prev)
 431     {
 432       /* Rewire the list if we are given a PREV pointer.  */
 433       PHI_CHAIN (prev) = PHI_CHAIN (phi);
 434
 435       /* If we are deleting the PHI node, then we should release the
 436          SSA_NAME node so that it can be reused.  */
 437       release_ssa_name (PHI_RESULT (phi));
 438       release_phi_node (phi);
 439     }
 440   else if (phi == phi_nodes (bb))
 441     {
 442       /* Update the list head if removing the first element.  */
 443       bb_ann (bb)->phi_nodes = PHI_CHAIN (phi);
 444
 445       /* If we are deleting the PHI node, then we should release the
 446          SSA_NAME node so that it can be reused.  */
 447       release_ssa_name (PHI_RESULT (phi));
 448       release_phi_node (phi);
 449     }
 450   else
 451     {
 452       /* Traverse the list looking for the node to remove.  */
 453       tree prev, t;
 454       prev = NULL_TREE;
 455       for (t = phi_nodes (bb); t && t != phi; t = PHI_CHAIN (t))
 456         prev = t;
 457       if (t)
 458         remove_phi_node (t, prev, bb);
 459     }
 460 }
 461
 462
 463 /* Remove all the PHI nodes for variables in the VARS bitmap.  */
 464
 465 void
 466 remove_all_phi_nodes_for (bitmap vars)
 467 {
 468   basic_block bb;
 469
 470   FOR_EACH_BB (bb)
 471     {
 472       /* Build a new PHI list for BB without variables in VARS.  */
 473       tree phi, new_phi_list, last_phi, next;
 474
 475       last_phi = new_phi_list = NULL_TREE;
 476       for (phi = phi_nodes (bb), next = NULL; phi; phi = next)
 477         {
 478           tree var = SSA_NAME_VAR (PHI_RESULT (phi));
 479
 480           next = PHI_CHAIN (phi);
 481           /* Only add PHI nodes for variables not in VARS.  */
 482           if (!bitmap_bit_p (vars, var_ann (var)->uid))
 483             {
 484               /* If we're not removing this PHI node, then it must have
 485                  been rewritten by a previous call into the SSA rewriter.
 486                  Note that fact in PHI_REWRITTEN.  */
 487               PHI_REWRITTEN (phi) = 1;
 488
 489               if (new_phi_list == NULL_TREE)
 490                 new_phi_list = last_phi = phi;
 491               else
 492                 {
 493                   PHI_CHAIN (last_phi) = phi;
 494                   last_phi = phi;
 495                 }
 496             }
 497           else
 498             {
 499               /* If we are deleting the PHI node, then we should release the
 500                  SSA_NAME node so that it can be reused.  */
 501               release_ssa_name (PHI_RESULT (phi));
 502               release_phi_node (phi);
 503             }
 504         }
 505
 506       /* Make sure the last node in the new list has no successors.  */
 507       if (last_phi)
 508         PHI_CHAIN (last_phi) = NULL_TREE;
 509       bb_ann (bb)->phi_nodes = new_phi_list;
 510
 511 #if defined ENABLE_CHECKING
 512       for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 513         {
 514           tree var = SSA_NAME_VAR (PHI_RESULT (phi));
 515           gcc_assert (!bitmap_bit_p (vars, var_ann (var)->uid));
 516         }
 517 #endif
 518     }
 519 }
 520
 521
 522 #include "gt-tree-phinodes.h"
 523