gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003, 2005 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, 51 Franklin Street, Fifth Floor,
  19 Boston, MA 02110-1301, 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
 201 /* Return a PHI node with LEN argument slots for variable VAR.  */
 202
 203 static tree
 204 make_phi_node (tree var, int len)
 205 {
 206   tree phi;
 207   int capacity, i;
 208
 209   capacity = ideal_phi_node_len (len);
 210
 211   phi = allocate_phi_node (capacity);
 212
 213   /* We need to clear the entire PHI node, including the argument
 214      portion, because we represent a "missing PHI argument" by placing
 215      NULL_TREE in PHI_ARG_DEF.  */
 216   memset (phi, 0, (sizeof (struct tree_phi_node) - sizeof (struct phi_arg_d)
 217                    + sizeof (struct phi_arg_d) * len));
 218   TREE_SET_CODE (phi, PHI_NODE);
 219   PHI_NUM_ARGS (phi) = len;
 220   PHI_ARG_CAPACITY (phi) = capacity;
 221   TREE_TYPE (phi) = TREE_TYPE (var);
 222   if (TREE_CODE (var) == SSA_NAME)
 223     SET_PHI_RESULT (phi, var);
 224   else
 225     SET_PHI_RESULT (phi, make_ssa_name (var, phi));
 226
 227   for (i = 0; i < capacity; i++)
 228     {
 229       use_operand_p  imm;
 230       imm = &(PHI_ARG_IMM_USE_NODE (phi, i));
 231       imm->use = &(PHI_ARG_DEF_TREE (phi, i));
 232       imm->prev = NULL;
 233       imm->next = NULL;
 234       imm->stmt = phi;
 235     }
 236   return phi;
 237 }
 238
 239 /* We no longer need PHI, release it so that it may be reused.  */
 240
 241 void
 242 release_phi_node (tree phi)
 243 {
 244   int bucket;
 245   int len = PHI_ARG_CAPACITY (phi);
 246   int x;
 247
 248   for (x = 0; x < PHI_NUM_ARGS (phi); x++)
 249     {
 250       use_operand_p  imm;
 251       imm = &(PHI_ARG_IMM_USE_NODE (phi, x));
 252       delink_imm_use (imm);
 253     }
 254
 255   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 256   bucket -= 2;
 257   PHI_CHAIN (phi) = free_phinodes[bucket];
 258   free_phinodes[bucket] = phi;
 259   free_phinode_count++;
 260 }
 261
 262 /* Resize an existing PHI node.  The only way is up.  Return the
 263    possibly relocated phi.  */
 264
 265 static void
 266 resize_phi_node (tree *phi, int len)
 267 {
 268   int old_size, i;
 269   tree new_phi;
 270
 271   gcc_assert (len > PHI_ARG_CAPACITY (*phi));
 272
 273   /* The garbage collector will not look at the PHI node beyond the
 274      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 275      portion of the PHI node currently in use.  */
 276   old_size = (sizeof (struct tree_phi_node)
 277              + (PHI_NUM_ARGS (*phi) - 1) * sizeof (struct phi_arg_d));
 278
 279   new_phi = allocate_phi_node (len);
 280
 281   memcpy (new_phi, *phi, old_size);
 282
 283   for (i = 0; i < PHI_NUM_ARGS (new_phi); i++)
 284     {
 285       use_operand_p imm, old_imm;
 286       imm = &(PHI_ARG_IMM_USE_NODE (new_phi, i));
 287       old_imm = &(PHI_ARG_IMM_USE_NODE (*phi, i));
 288       imm->use = &(PHI_ARG_DEF_TREE (new_phi, i));
 289       relink_imm_use_stmt (imm, old_imm, new_phi);
 290     }
 291
 292   PHI_ARG_CAPACITY (new_phi) = len;
 293
 294   for (i = PHI_NUM_ARGS (new_phi); i < len; i++)
 295     {
 296       use_operand_p imm;
 297       imm = &(PHI_ARG_IMM_USE_NODE (new_phi, i));
 298       imm->use = &(PHI_ARG_DEF_TREE (new_phi, i));
 299       imm->prev = NULL;
 300       imm->next = NULL;
 301       imm->stmt = new_phi;
 302     }
 303
 304
 305   *phi = new_phi;
 306 }
 307
 308 /* Reserve PHI arguments for a new edge to basic block BB.  */
 309
 310 void
 311 reserve_phi_args_for_new_edge (basic_block bb)
 312 {
 313   tree *loc;
 314   int len = EDGE_COUNT (bb->preds);
 315   int cap = ideal_phi_node_len (len + 4);
 316
 317   for (loc = &(bb->phi_nodes);
 318        *loc;
 319        loc = &PHI_CHAIN (*loc))
 320     {
 321       if (len > PHI_ARG_CAPACITY (*loc))
 322         {
 323           tree old_phi = *loc;
 324
 325           resize_phi_node (loc, cap);
 326
 327           /* The result of the phi is defined by this phi node.  */
 328           SSA_NAME_DEF_STMT (PHI_RESULT (*loc)) = *loc;
 329
 330           release_phi_node (old_phi);
 331         }
 332
 333       /* We represent a "missing PHI argument" by placing NULL_TREE in
 334          the corresponding slot.  If PHI arguments were added
 335          immediately after an edge is created, this zeroing would not
 336          be necessary, but unfortunately this is not the case.  For
 337          example, the loop optimizer duplicates several basic blocks,
 338          redirects edges, and then fixes up PHI arguments later in
 339          batch.  */
 340       SET_PHI_ARG_DEF (*loc, len - 1, NULL_TREE);
 341
 342       PHI_NUM_ARGS (*loc)++;
 343     }
 344 }
 345
 346 /* Create a new PHI node for variable VAR at basic block BB.  */
 347
 348 tree
 349 create_phi_node (tree var, basic_block bb)
 350 {
 351   tree phi;
 352
 353   phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 354
 355   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 356   PHI_CHAIN (phi) = phi_nodes (bb);
 357   bb->phi_nodes = phi;
 358
 359   /* Associate BB to the PHI node.  */
 360   set_bb_for_stmt (phi, bb);
 361
 362   return phi;
 363 }
 364
 365 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 366    definition and E is the edge through which DEF reaches PHI.  The new
 367    argument is added at the end of the argument list.
 368    If PHI has reached its maximum capacity, add a few slots.  In this case,
 369    PHI points to the reallocated phi node when we return.  */
 370
 371 void
 372 add_phi_arg (tree phi, tree def, edge e)
 373 {
 374   basic_block bb = e->dest;
 375
 376   gcc_assert (bb == bb_for_stmt (phi));
 377
 378   /* We resize PHI nodes upon edge creation.  We should always have
 379      enough room at this point.  */
 380   gcc_assert (PHI_NUM_ARGS (phi) <= PHI_ARG_CAPACITY (phi));
 381
 382   /* We resize PHI nodes upon edge creation.  We should always have
 383      enough room at this point.  */
 384   gcc_assert (e->dest_idx < (unsigned int) PHI_NUM_ARGS (phi));
 385
 386   /* Copy propagation needs to know what object occur in abnormal
 387      PHI nodes.  This is a convenient place to record such information.  */
 388   if (e->flags & EDGE_ABNORMAL)
 389     {
 390       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 391       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 392     }
 393
 394   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 395 }
 396
 397 /* Remove the Ith argument from PHI's argument list.  This routine
 398    implements removal by swapping the last alternative with the
 399    alternative we want to delete and then shrinking the vector, which
 400    is consistent with how we remove an edge from the edge vector.  */
 401
 402 static void
 403 remove_phi_arg_num (tree phi, int i)
 404 {
 405   int num_elem = PHI_NUM_ARGS (phi);
 406
 407   gcc_assert (i < num_elem);
 408
 409
 410   /* Delink the item which is being removed.  */
 411   delink_imm_use (&(PHI_ARG_IMM_USE_NODE (phi, i)));
 412
 413   /* If it is not the last element, move the last element
 414      to the element we want to delete, resetting all the links. */
 415   if (i != num_elem - 1)
 416     {
 417       use_operand_p old_p, new_p;
 418       old_p = &PHI_ARG_IMM_USE_NODE (phi, num_elem - 1);
 419       new_p = &PHI_ARG_IMM_USE_NODE (phi, i);
 420       /* Set use on new node, and link into last element's place.  */
 421       *(new_p->use) = *(old_p->use);
 422       relink_imm_use (new_p, old_p);
 423     }
 424
 425   /* Shrink the vector and return.  Note that we do not have to clear
 426      PHI_ARG_DEF because the garbage collector will not look at those
 427      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 428   PHI_NUM_ARGS (phi)--;
 429 }
 430
 431 /* Remove all PHI arguments associated with edge E.  */
 432
 433 void
 434 remove_phi_args (edge e)
 435 {
 436   tree phi;
 437
 438   for (phi = phi_nodes (e->dest); phi; phi = PHI_CHAIN (phi))
 439     remove_phi_arg_num (phi, e->dest_idx);
 440 }
 441
 442 /* Remove PHI node PHI from basic block BB.  If PREV is non-NULL, it is
 443    used as the node immediately before PHI in the linked list.  */
 444
 445 void
 446 remove_phi_node (tree phi, tree prev)
 447 {
 448   tree *loc;
 449
 450   if (prev)
 451     {
 452       loc = &PHI_CHAIN (prev);
 453     }
 454   else
 455     {
 456       for (loc = &(bb_for_stmt (phi)->phi_nodes);
 457            *loc != phi;
 458            loc = &PHI_CHAIN (*loc))
 459         ;
 460     }
 461
 462   /* Remove PHI from the chain.  */
 463   *loc = PHI_CHAIN (phi);
 464
 465   /* If we are deleting the PHI node, then we should release the
 466      SSA_NAME node so that it can be reused.  */
 467   release_phi_node (phi);
 468   release_ssa_name (PHI_RESULT (phi));
 469 }
 470
 471
 472 /* Reverse the order of PHI nodes in the chain PHI.
 473    Return the new head of the chain (old last PHI node).  */
 474
 475 tree
 476 phi_reverse (tree phi)
 477 {
 478   tree prev = NULL_TREE, next;
 479   for (; phi; phi = next)
 480     {
 481       next = PHI_CHAIN (phi);
 482       PHI_CHAIN (phi) = prev;
 483       prev = phi;
 484     }
 485   return prev;
 486 }
 487
 488 #include "gt-tree-phinodes.h"