gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003, 2005, 2007 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 3, 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 COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 #include "config.h"
  21 #include "system.h"
  22 #include "coretypes.h"
  23 #include "tm.h"
  24 #include "tree.h"
  25 #include "rtl.h"
  26 #include "varray.h"
  27 #include "ggc.h"
  28 #include "basic-block.h"
  29 #include "tree-flow.h"
  30 #include "toplev.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    Right now we maintain our free list on a per-function basis.  It may
  47    or may not make sense to maintain the free list for the duration of
  48    a compilation unit.
  49
  50    We could also use a zone allocator for these objects since they have
  51    a very well defined lifetime.  If someone wants to experiment with that
  52    this is the place to try it.
  53
  54    PHI nodes have different sizes, so we can't have a single list of all
  55    the PHI nodes as it would be too expensive to walk down that list to
  56    find a PHI of a suitable size.
  57
  58    Instead we have an array of lists of free PHI nodes.  The array is
  59    indexed by the number of PHI alternatives that PHI node can hold.
  60    Except for the last array member, which holds all remaining PHI
  61    nodes.
  62
  63    So to find a free PHI node, we compute its index into the free PHI
  64    node array and see if there are any elements with an exact match.
  65    If so, then we are done.  Otherwise, we test the next larger size
  66    up and continue until we are in the last array element.
  67
  68    We do not actually walk members of the last array element.  While it
  69    might allow us to pick up a few reusable PHI nodes, it could potentially
  70    be very expensive if the program has released a bunch of large PHI nodes,
  71    but keeps asking for even larger PHI nodes.  Experiments have shown that
  72    walking the elements of the last array entry would result in finding less
  73    than .1% additional reusable PHI nodes.
  74
  75    Note that we can never have less than two PHI argument slots.  Thus,
  76    the -2 on all the calculations below.  */
  77
  78 #define NUM_BUCKETS 10
  79 static GTY ((deletable (""))) tree free_phinodes[NUM_BUCKETS - 2];
  80 static unsigned long free_phinode_count;
  81
  82 static int ideal_phi_node_len (int);
  83 static void resize_phi_node (tree *, int);
  84
  85 #ifdef GATHER_STATISTICS
  86 unsigned int phi_nodes_reused;
  87 unsigned int phi_nodes_created;
  88 #endif
  89
  90 /* Initialize management of PHIs.  */
  91
  92 void
  93 init_phinodes (void)
  94 {
  95   int i;
  96
  97   for (i = 0; i < NUM_BUCKETS - 2; i++)
  98     free_phinodes[i] = NULL;
  99   free_phinode_count = 0;
 100 }
 101
 102 /* Finalize management of PHIs.  */
 103
 104 void
 105 fini_phinodes (void)
 106 {
 107   int i;
 108
 109   for (i = 0; i < NUM_BUCKETS - 2; i++)
 110     free_phinodes[i] = NULL;
 111   free_phinode_count = 0;
 112 }
 113
 114 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
 115
 116 #ifdef GATHER_STATISTICS
 117 void
 118 phinodes_print_statistics (void)
 119 {
 120   fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
 121   fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
 122 }
 123 #endif
 124
 125 /* Allocate a PHI node with at least LEN arguments.  If the free list
 126    happens to contain a PHI node with LEN arguments or more, return
 127    that one.  */
 128
 129 static inline tree
 130 allocate_phi_node (int len)
 131 {
 132   tree phi;
 133   int bucket = NUM_BUCKETS - 2;
 134   int size = (sizeof (struct tree_phi_node)
 135               + (len - 1) * sizeof (struct phi_arg_d));
 136
 137   if (free_phinode_count)
 138     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 139       if (free_phinodes[bucket])
 140         break;
 141
 142   /* If our free list has an element, then use it.  */
 143   if (bucket < NUM_BUCKETS - 2
 144       && PHI_ARG_CAPACITY (free_phinodes[bucket]) >= len)
 145     {
 146       free_phinode_count--;
 147       phi = free_phinodes[bucket];
 148       free_phinodes[bucket] = PHI_CHAIN (free_phinodes[bucket]);
 149 #ifdef GATHER_STATISTICS
 150       phi_nodes_reused++;
 151 #endif
 152     }
 153   else
 154     {
 155       phi = ggc_alloc (size);
 156 #ifdef GATHER_STATISTICS
 157       phi_nodes_created++;
 158       tree_node_counts[(int) phi_kind]++;
 159       tree_node_sizes[(int) phi_kind] += size;
 160 #endif
 161     }
 162
 163   return phi;
 164 }
 165
 166 /* Given LEN, the original number of requested PHI arguments, return
 167    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 168    the total size of the PHI node up to the next power of two bytes.
 169
 170    Rounding up will not result in wasting any memory since the size request
 171    will be rounded up by the GC system anyway.  [ Note this is not entirely
 172    true since the original length might have fit on one of the special
 173    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 174    PHI node later if we increase the number of arguments for the PHI.  */
 175
 176 static int
 177 ideal_phi_node_len (int len)
 178 {
 179   size_t size, new_size;
 180   int log2, new_len;
 181
 182   /* We do not support allocations of less than two PHI argument slots.  */
 183   if (len < 2)
 184     len = 2;
 185
 186   /* Compute the number of bytes of the original request.  */
 187   size = sizeof (struct tree_phi_node) + (len - 1) * sizeof (struct phi_arg_d);
 188
 189   /* Round it up to the next power of two.  */
 190   log2 = ceil_log2 (size);
 191   new_size = 1 << log2;
 192
 193   /* Now compute and return the number of PHI argument slots given an
 194      ideal size allocation.  */
 195   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 196   return new_len;
 197 }
 198
 199
 200 /* Return a PHI node with LEN argument slots for variable VAR.  */
 201
 202 static tree
 203 make_phi_node (tree var, int len)
 204 {
 205   tree phi;
 206   int capacity, i;
 207
 208   capacity = ideal_phi_node_len (len);
 209
 210   phi = allocate_phi_node (capacity);
 211
 212   /* We need to clear the entire PHI node, including the argument
 213      portion, because we represent a "missing PHI argument" by placing
 214      NULL_TREE in PHI_ARG_DEF.  */
 215   memset (phi, 0, (sizeof (struct tree_phi_node) - sizeof (struct phi_arg_d)
 216                    + sizeof (struct phi_arg_d) * len));
 217   TREE_SET_CODE (phi, PHI_NODE);
 218   PHI_NUM_ARGS (phi) = len;
 219   PHI_ARG_CAPACITY (phi) = capacity;
 220   if (TREE_CODE (var) == SSA_NAME)
 221     SET_PHI_RESULT (phi, var);
 222   else
 223     SET_PHI_RESULT (phi, make_ssa_name (var, phi));
 224
 225   for (i = 0; i < capacity; i++)
 226     {
 227       use_operand_p  imm;
 228       imm = &(PHI_ARG_IMM_USE_NODE (phi, i));
 229       imm->use = &(PHI_ARG_DEF_TREE (phi, i));
 230       imm->prev = NULL;
 231       imm->next = NULL;
 232       imm->stmt = phi;
 233     }
 234
 235   return phi;
 236 }
 237
 238 /* We no longer need PHI, release it so that it may be reused.  */
 239
 240 void
 241 release_phi_node (tree phi)
 242 {
 243   int bucket;
 244   int len = PHI_ARG_CAPACITY (phi);
 245   int x;
 246
 247   for (x = 0; x < PHI_NUM_ARGS (phi); x++)
 248     {
 249       use_operand_p  imm;
 250       imm = &(PHI_ARG_IMM_USE_NODE (phi, x));
 251       delink_imm_use (imm);
 252     }
 253
 254   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 255   bucket -= 2;
 256   PHI_CHAIN (phi) = free_phinodes[bucket];
 257   free_phinodes[bucket] = phi;
 258   free_phinode_count++;
 259 }
 260
 261 /* Resize an existing PHI node.  The only way is up.  Return the
 262    possibly relocated phi.  */
 263
 264 static void
 265 resize_phi_node (tree *phi, int len)
 266 {
 267   int old_size, i;
 268   tree new_phi;
 269
 270   gcc_assert (len > PHI_ARG_CAPACITY (*phi));
 271
 272   /* The garbage collector will not look at the PHI node beyond the
 273      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 274      portion of the PHI node currently in use.  */
 275   old_size = (sizeof (struct tree_phi_node)
 276              + (PHI_NUM_ARGS (*phi) - 1) * sizeof (struct phi_arg_d));
 277
 278   new_phi = allocate_phi_node (len);
 279
 280   memcpy (new_phi, *phi, old_size);
 281
 282   for (i = 0; i < PHI_NUM_ARGS (new_phi); i++)
 283     {
 284       use_operand_p imm, old_imm;
 285       imm = &(PHI_ARG_IMM_USE_NODE (new_phi, i));
 286       old_imm = &(PHI_ARG_IMM_USE_NODE (*phi, i));
 287       imm->use = &(PHI_ARG_DEF_TREE (new_phi, i));
 288       relink_imm_use_stmt (imm, old_imm, new_phi);
 289     }
 290
 291   PHI_ARG_CAPACITY (new_phi) = len;
 292
 293   for (i = PHI_NUM_ARGS (new_phi); i < len; i++)
 294     {
 295       use_operand_p imm;
 296       imm = &(PHI_ARG_IMM_USE_NODE (new_phi, i));
 297       imm->use = &(PHI_ARG_DEF_TREE (new_phi, i));
 298       imm->prev = NULL;
 299       imm->next = NULL;
 300       imm->stmt = new_phi;
 301     }
 302
 303   *phi = new_phi;
 304 }
 305
 306 /* Reserve PHI arguments for a new edge to basic block BB.  */
 307
 308 void
 309 reserve_phi_args_for_new_edge (basic_block bb)
 310 {
 311   tree *loc;
 312   int len = EDGE_COUNT (bb->preds);
 313   int cap = ideal_phi_node_len (len + 4);
 314
 315   for (loc = phi_nodes_ptr (bb);
 316        *loc;
 317        loc = &PHI_CHAIN (*loc))
 318     {
 319       if (len > PHI_ARG_CAPACITY (*loc))
 320         {
 321           tree old_phi = *loc;
 322
 323           resize_phi_node (loc, cap);
 324
 325           /* The result of the phi is defined by this phi node.  */
 326           SSA_NAME_DEF_STMT (PHI_RESULT (*loc)) = *loc;
 327
 328           release_phi_node (old_phi);
 329         }
 330
 331       /* We represent a "missing PHI argument" by placing NULL_TREE in
 332          the corresponding slot.  If PHI arguments were added
 333          immediately after an edge is created, this zeroing would not
 334          be necessary, but unfortunately this is not the case.  For
 335          example, the loop optimizer duplicates several basic blocks,
 336          redirects edges, and then fixes up PHI arguments later in
 337          batch.  */
 338       SET_PHI_ARG_DEF (*loc, len - 1, NULL_TREE);
 339
 340       PHI_NUM_ARGS (*loc)++;
 341     }
 342 }
 343
 344
 345 /* Create a new PHI node for variable VAR at basic block BB.  */
 346
 347 tree
 348 create_phi_node (tree var, basic_block bb)
 349 {
 350   tree phi;
 351
 352   phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 353
 354   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 355   PHI_CHAIN (phi) = phi_nodes (bb);
 356   set_phi_nodes (bb, phi);
 357
 358   /* Associate BB to the PHI node.  */
 359   set_bb_for_stmt (phi, bb);
 360
 361   return phi;
 362 }
 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
 398 /* Remove the Ith argument from PHI's argument list.  This routine
 399    implements removal by swapping the last alternative with the
 400    alternative we want to delete and then shrinking the vector, which
 401    is consistent with how we remove an edge from the edge vector.  */
 402
 403 static void
 404 remove_phi_arg_num (tree phi, int i)
 405 {
 406   int num_elem = PHI_NUM_ARGS (phi);
 407
 408   gcc_assert (i < num_elem);
 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
 432 /* Remove all PHI arguments associated with edge E.  */
 433
 434 void
 435 remove_phi_args (edge e)
 436 {
 437   tree phi;
 438
 439   for (phi = phi_nodes (e->dest); phi; phi = PHI_CHAIN (phi))
 440     remove_phi_arg_num (phi, e->dest_idx);
 441 }
 442
 443
 444 /* Remove PHI node PHI from basic block BB.  If PREV is non-NULL, it is
 445    used as the node immediately before PHI in the linked list.  If
 446    RELEASE_LHS_P is true, the LHS of this PHI node is released into
 447    the free pool of SSA names.  */
 448
 449 void
 450 remove_phi_node (tree phi, tree prev, bool release_lhs_p)
 451 {
 452   tree *loc;
 453
 454   if (prev)
 455     {
 456       loc = &PHI_CHAIN (prev);
 457     }
 458   else
 459     {
 460       for (loc = phi_nodes_ptr (bb_for_stmt (phi));
 461            *loc != phi;
 462            loc = &PHI_CHAIN (*loc))
 463         ;
 464     }
 465
 466   /* Remove PHI from the chain.  */
 467   *loc = PHI_CHAIN (phi);
 468
 469   /* If we are deleting the PHI node, then we should release the
 470      SSA_NAME node so that it can be reused.  */
 471   release_phi_node (phi);
 472   if (release_lhs_p)
 473     release_ssa_name (PHI_RESULT (phi));
 474 }
 475
 476
 477 /* Reverse the order of PHI nodes in the chain PHI.
 478    Return the new head of the chain (old last PHI node).  */
 479
 480 tree
 481 phi_reverse (tree phi)
 482 {
 483   tree prev = NULL_TREE, next;
 484   for (; phi; phi = next)
 485     {
 486       next = PHI_CHAIN (phi);
 487       PHI_CHAIN (phi) = prev;
 488       prev = phi;
 489     }
 490   return prev;
 491 }
 492
 493 #include "gt-tree-phinodes.h"