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
 210   len = ideal_phi_node_len (len);
 211
 212   phi = allocate_phi_node (len);
 213
 214   /* We do not have to clear a part of the PHI node that stores PHI
 215      arguments, which is safe because we tell the garbage collector to
 216      scan up to num_args elements in the array of PHI arguments.  In
 217      other words, the garbage collector will not follow garbage
 218      pointers in the unused portion of the array.  */
 219   memset (phi, 0, sizeof (struct tree_phi_node) - sizeof (struct phi_arg_d));
 220   TREE_SET_CODE (phi, PHI_NODE);
 221   PHI_ARG_CAPACITY (phi) = len;
 222   TREE_TYPE (phi) = TREE_TYPE (var);
 223   if (TREE_CODE (var) == SSA_NAME)
 224     SET_PHI_RESULT (phi, var);
 225   else
 226     SET_PHI_RESULT (phi, make_ssa_name (var, phi));
 227
 228   return phi;
 229 }
 230
 231 /* We no longer need PHI, release it so that it may be reused.  */
 232
 233 void
 234 release_phi_node (tree phi)
 235 {
 236   int bucket;
 237   int len = PHI_ARG_CAPACITY (phi);
 238
 239   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 240   bucket -= 2;
 241   PHI_CHAIN (phi) = free_phinodes[bucket];
 242   free_phinodes[bucket] = phi;
 243   free_phinode_count++;
 244 }
 245
 246 /* Resize an existing PHI node.  The only way is up.  Return the
 247    possibly relocated phi.  */
 248
 249 static void
 250 resize_phi_node (tree *phi, int len)
 251 {
 252   int old_size;
 253   tree new_phi;
 254
 255   gcc_assert (len >= PHI_ARG_CAPACITY (*phi));
 256
 257   /* The garbage collector will not look at the PHI node beyond the
 258      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 259      portion of the PHI node currently in use.  */
 260   old_size = (sizeof (struct tree_phi_node)
 261              + (PHI_NUM_ARGS (*phi) - 1) * sizeof (struct phi_arg_d));
 262
 263   new_phi = allocate_phi_node (len);
 264
 265   memcpy (new_phi, *phi, old_size);
 266
 267   PHI_ARG_CAPACITY (new_phi) = len;
 268
 269   *phi = new_phi;
 270 }
 271
 272 /* Create a new PHI node for variable VAR at basic block BB.  */
 273
 274 tree
 275 create_phi_node (tree var, basic_block bb)
 276 {
 277   tree phi;
 278
 279   phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 280
 281   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 282   PHI_CHAIN (phi) = phi_nodes (bb);
 283   bb_ann (bb)->phi_nodes = phi;
 284
 285   /* Associate BB to the PHI node.  */
 286   set_bb_for_stmt (phi, bb);
 287
 288   return phi;
 289 }
 290
 291 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 292    definition and E is the edge through which DEF reaches PHI.  The new
 293    argument is added at the end of the argument list.
 294    If PHI has reached its maximum capacity, add a few slots.  In this case,
 295    PHI points to the reallocated phi node when we return.  */
 296
 297 void
 298 add_phi_arg (tree *phi, tree def, edge e)
 299 {
 300   basic_block bb = e->dest;
 301   int i = PHI_NUM_ARGS (*phi);
 302
 303   gcc_assert (bb == bb_for_stmt (*phi));
 304
 305   if (i >= PHI_ARG_CAPACITY (*phi))
 306     {
 307       tree old_phi = *phi;
 308
 309       /* Resize the phi.  Unfortunately, this will relocate it.  */
 310       resize_phi_node (phi, ideal_phi_node_len (i + 4));
 311
 312       /* resize_phi_node will necessarily relocate the phi.  */
 313       gcc_assert (*phi != old_phi);
 314
 315       /* The result of the phi is defined by this phi node.  */
 316       SSA_NAME_DEF_STMT (PHI_RESULT (*phi)) = *phi;
 317
 318       release_phi_node (old_phi);
 319
 320       /* Update the list head if replacing the first listed phi.  */
 321       if (phi_nodes (bb) == old_phi)
 322         bb_ann (bb)->phi_nodes = *phi;
 323       else
 324         {
 325           /* Traverse the list looking for the phi node to chain to.  */
 326           tree p;
 327
 328           for (p = phi_nodes (bb);
 329                p && PHI_CHAIN (p) != old_phi;
 330                p = PHI_CHAIN (p))
 331             ;
 332
 333           gcc_assert (p);
 334           PHI_CHAIN (p) = *phi;
 335         }
 336     }
 337
 338   /* Copy propagation needs to know what object occur in abnormal
 339      PHI nodes.  This is a convenient place to record such information.  */
 340   if (e->flags & EDGE_ABNORMAL)
 341     {
 342       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 343       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (*phi)) = 1;
 344     }
 345
 346   SET_PHI_ARG_DEF (*phi, i, def);
 347   PHI_ARG_EDGE (*phi, i) = e;
 348   PHI_ARG_NONZERO (*phi, i) = false;
 349   PHI_NUM_ARGS (*phi)++;
 350 }
 351
 352 /* Remove a PHI argument from PHI.  BLOCK is the predecessor block where
 353    the PHI argument is coming from.  */
 354
 355 void
 356 remove_phi_arg (tree phi, basic_block block)
 357 {
 358   int i, num_elem = PHI_NUM_ARGS (phi);
 359
 360   for (i = 0; i < num_elem; i++)
 361     {
 362       basic_block src_bb;
 363
 364       src_bb = PHI_ARG_EDGE (phi, i)->src;
 365
 366       if (src_bb == block)
 367         {
 368           remove_phi_arg_num (phi, i);
 369           return;
 370         }
 371     }
 372 }
 373
 374
 375 /* Remove the Ith argument from PHI's argument list.  This routine assumes
 376    ordering of alternatives in the vector is not important and implements
 377    removal by swapping the last alternative with the alternative we want to
 378    delete, then shrinking the vector.  */
 379
 380 void
 381 remove_phi_arg_num (tree phi, int i)
 382 {
 383   int num_elem = PHI_NUM_ARGS (phi);
 384
 385   gcc_assert (i < num_elem);
 386
 387   /* If we are not at the last element, switch the last element
 388      with the element we want to delete.  */
 389   if (i != num_elem - 1)
 390     {
 391       SET_PHI_ARG_DEF (phi, i, PHI_ARG_DEF (phi, num_elem - 1));
 392       PHI_ARG_EDGE (phi, i) = PHI_ARG_EDGE (phi, num_elem - 1);
 393       PHI_ARG_NONZERO (phi, i) = PHI_ARG_NONZERO (phi, num_elem - 1);
 394     }
 395
 396   /* Shrink the vector and return.  Note that we do not have to clear
 397      PHI_ARG_DEF, PHI_ARG_EDGE, or PHI_ARG_NONZERO because the garbage
 398      collector will not look at those elements beyond the first
 399      PHI_NUM_ARGS elements of the array.  */
 400   PHI_NUM_ARGS (phi)--;
 401 }
 402
 403 /* Remove PHI node PHI from basic block BB.  If PREV is non-NULL, it is
 404    used as the node immediately before PHI in the linked list.  */
 405
 406 void
 407 remove_phi_node (tree phi, tree prev, basic_block bb)
 408 {
 409   if (prev)
 410     {
 411       /* Rewire the list if we are given a PREV pointer.  */
 412       PHI_CHAIN (prev) = PHI_CHAIN (phi);
 413
 414       /* If we are deleting the PHI node, then we should release the
 415          SSA_NAME node so that it can be reused.  */
 416       release_ssa_name (PHI_RESULT (phi));
 417       release_phi_node (phi);
 418     }
 419   else if (phi == phi_nodes (bb))
 420     {
 421       /* Update the list head if removing the first element.  */
 422       bb_ann (bb)->phi_nodes = PHI_CHAIN (phi);
 423
 424       /* If we are deleting the PHI node, then we should release the
 425          SSA_NAME node so that it can be reused.  */
 426       release_ssa_name (PHI_RESULT (phi));
 427       release_phi_node (phi);
 428     }
 429   else
 430     {
 431       /* Traverse the list looking for the node to remove.  */
 432       tree prev, t;
 433       prev = NULL_TREE;
 434       for (t = phi_nodes (bb); t && t != phi; t = PHI_CHAIN (t))
 435         prev = t;
 436       if (t)
 437         remove_phi_node (t, prev, bb);
 438     }
 439 }
 440
 441
 442 /* Remove all the PHI nodes for variables in the VARS bitmap.  */
 443
 444 void
 445 remove_all_phi_nodes_for (bitmap vars)
 446 {
 447   basic_block bb;
 448
 449   FOR_EACH_BB (bb)
 450     {
 451       /* Build a new PHI list for BB without variables in VARS.  */
 452       tree phi, new_phi_list, next;
 453       tree *lastp = &new_phi_list;
 454
 455       for (phi = phi_nodes (bb); phi; phi = next)
 456         {
 457           tree var = SSA_NAME_VAR (PHI_RESULT (phi));
 458
 459           next = PHI_CHAIN (phi);
 460           /* Only add PHI nodes for variables not in VARS.  */
 461           if (!bitmap_bit_p (vars, var_ann (var)->uid))
 462             {
 463               /* If we're not removing this PHI node, then it must have
 464                  been rewritten by a previous call into the SSA rewriter.
 465                  Note that fact in PHI_REWRITTEN.  */
 466               PHI_REWRITTEN (phi) = 1;
 467
 468               *lastp = phi;
 469               lastp = &PHI_CHAIN (phi);
 470             }
 471           else
 472             {
 473               /* If we are deleting the PHI node, then we should release the
 474                  SSA_NAME node so that it can be reused.  */
 475               release_ssa_name (PHI_RESULT (phi));
 476               release_phi_node (phi);
 477             }
 478         }
 479
 480       /* Make sure the last node in the new list has no successors.  */
 481       *lastp = NULL;
 482       bb_ann (bb)->phi_nodes = new_phi_list;
 483
 484 #if defined ENABLE_CHECKING
 485       for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 486         {
 487           tree var = SSA_NAME_VAR (PHI_RESULT (phi));
 488           gcc_assert (!bitmap_bit_p (vars, var_ann (var)->uid));
 489         }
 490 #endif
 491     }
 492 }
 493
 494
 495 #include "gt-tree-phinodes.h"
 496