gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003, 2005, 2007, 2008 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 #include "gimple.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 (""))) VEC(gimple,gc) *free_phinodes[NUM_BUCKETS - 2];
  81 static unsigned long free_phinode_count;
  82
  83 static int ideal_phi_node_len (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 gimple
 130 allocate_phi_node (size_t len)
 131 {
 132   gimple phi;
 133   size_t bucket = NUM_BUCKETS - 2;
 134   size_t size = sizeof (struct gimple_statement_phi)
 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       && gimple_phi_capacity (VEC_index (gimple, free_phinodes[bucket], 0))
 145          >= len)
 146     {
 147       free_phinode_count--;
 148       phi = VEC_pop (gimple, free_phinodes[bucket]);
 149       if (VEC_empty (gimple, free_phinodes[bucket]))
 150         VEC_free (gimple, gc, free_phinodes[bucket]);
 151 #ifdef GATHER_STATISTICS
 152       phi_nodes_reused++;
 153 #endif
 154     }
 155   else
 156     {
 157       phi = (gimple) ggc_alloc (size);
 158 #ifdef GATHER_STATISTICS
 159       phi_nodes_created++;
 160         {
 161           enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
 162           gimple_alloc_counts[(int) kind]++;
 163           gimple_alloc_sizes[(int) kind] += size;
 164         }
 165 #endif
 166     }
 167
 168   return phi;
 169 }
 170
 171 /* Given LEN, the original number of requested PHI arguments, return
 172    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 173    the total size of the PHI node up to the next power of two bytes.
 174
 175    Rounding up will not result in wasting any memory since the size request
 176    will be rounded up by the GC system anyway.  [ Note this is not entirely
 177    true since the original length might have fit on one of the special
 178    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 179    PHI node later if we increase the number of arguments for the PHI.  */
 180
 181 static int
 182 ideal_phi_node_len (int len)
 183 {
 184   size_t size, new_size;
 185   int log2, new_len;
 186
 187   /* We do not support allocations of less than two PHI argument slots.  */
 188   if (len < 2)
 189     len = 2;
 190
 191   /* Compute the number of bytes of the original request.  */
 192   size = sizeof (struct gimple_statement_phi)
 193          + (len - 1) * sizeof (struct phi_arg_d);
 194
 195   /* Round it up to the next power of two.  */
 196   log2 = ceil_log2 (size);
 197   new_size = 1 << log2;
 198
 199   /* Now compute and return the number of PHI argument slots given an
 200      ideal size allocation.  */
 201   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 202   return new_len;
 203 }
 204
 205 /* Return a PHI node with LEN argument slots for variable VAR.  */
 206
 207 gimple
 208 make_phi_node (tree var, int len)
 209 {
 210   gimple phi;
 211   int capacity, i;
 212
 213   capacity = ideal_phi_node_len (len);
 214
 215   phi = allocate_phi_node (capacity);
 216
 217   /* We need to clear the entire PHI node, including the argument
 218      portion, because we represent a "missing PHI argument" by placing
 219      NULL_TREE in PHI_ARG_DEF.  */
 220   memset (phi, 0, (sizeof (struct gimple_statement_phi)
 221                    - sizeof (struct phi_arg_d)
 222                    + sizeof (struct phi_arg_d) * len));
 223   phi->gsbase.code = GIMPLE_PHI;
 224   phi->gimple_phi.nargs = len;
 225   phi->gimple_phi.capacity = capacity;
 226   if (TREE_CODE (var) == SSA_NAME)
 227     gimple_phi_set_result (phi, var);
 228   else
 229     gimple_phi_set_result (phi, make_ssa_name (var, phi));
 230
 231   for (i = 0; i < capacity; i++)
 232     {
 233       use_operand_p  imm;
 234
 235       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
 236       imm = gimple_phi_arg_imm_use_ptr (phi, i);
 237       imm->use = gimple_phi_arg_def_ptr (phi, i);
 238       imm->prev = NULL;
 239       imm->next = NULL;
 240       imm->loc.stmt = phi;
 241     }
 242
 243   return phi;
 244 }
 245
 246 /* We no longer need PHI, release it so that it may be reused.  */
 247
 248 void
 249 release_phi_node (gimple phi)
 250 {
 251   size_t bucket;
 252   size_t len = gimple_phi_capacity (phi);
 253   size_t x;
 254
 255   for (x = 0; x < gimple_phi_num_args (phi); x++)
 256     {
 257       use_operand_p  imm;
 258       imm = gimple_phi_arg_imm_use_ptr (phi, x);
 259       delink_imm_use (imm);
 260     }
 261
 262   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 263   bucket -= 2;
 264   VEC_safe_push (gimple, gc, free_phinodes[bucket], phi);
 265   free_phinode_count++;
 266 }
 267
 268
 269 /* Resize an existing PHI node.  The only way is up.  Return the
 270    possibly relocated phi.  */
 271
 272 static void
 273 resize_phi_node (gimple *phi, size_t len)
 274 {
 275   size_t old_size, i;
 276   gimple new_phi;
 277
 278   gcc_assert (len > gimple_phi_capacity (*phi));
 279
 280   /* The garbage collector will not look at the PHI node beyond the
 281      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 282      portion of the PHI node currently in use.  */
 283   old_size = sizeof (struct gimple_statement_phi)
 284              + (gimple_phi_num_args (*phi) - 1) * sizeof (struct phi_arg_d);
 285
 286   new_phi = allocate_phi_node (len);
 287
 288   memcpy (new_phi, *phi, old_size);
 289
 290   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 291     {
 292       use_operand_p imm, old_imm;
 293       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 294       old_imm = gimple_phi_arg_imm_use_ptr (*phi, i);
 295       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 296       relink_imm_use_stmt (imm, old_imm, new_phi);
 297     }
 298
 299   new_phi->gimple_phi.capacity = len;
 300
 301   for (i = gimple_phi_num_args (new_phi); i < len; i++)
 302     {
 303       use_operand_p imm;
 304
 305       gimple_phi_arg_set_location (new_phi, i, UNKNOWN_LOCATION);
 306       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 307       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 308       imm->prev = NULL;
 309       imm->next = NULL;
 310       imm->loc.stmt = new_phi;
 311     }
 312
 313   *phi = new_phi;
 314 }
 315
 316 /* Reserve PHI arguments for a new edge to basic block BB.  */
 317
 318 void
 319 reserve_phi_args_for_new_edge (basic_block bb)
 320 {
 321   size_t len = EDGE_COUNT (bb->preds);
 322   size_t cap = ideal_phi_node_len (len + 4);
 323   gimple_stmt_iterator gsi;
 324
 325   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 326     {
 327       gimple *loc = gsi_stmt_ptr (&gsi);
 328
 329       if (len > gimple_phi_capacity (*loc))
 330         {
 331           gimple old_phi = *loc;
 332
 333           resize_phi_node (loc, cap);
 334
 335           /* The result of the PHI is defined by this PHI node.  */
 336           SSA_NAME_DEF_STMT (gimple_phi_result (*loc)) = *loc;
 337
 338           release_phi_node (old_phi);
 339         }
 340
 341       /* We represent a "missing PHI argument" by placing NULL_TREE in
 342          the corresponding slot.  If PHI arguments were added
 343          immediately after an edge is created, this zeroing would not
 344          be necessary, but unfortunately this is not the case.  For
 345          example, the loop optimizer duplicates several basic blocks,
 346          redirects edges, and then fixes up PHI arguments later in
 347          batch.  */
 348       SET_PHI_ARG_DEF (*loc, len - 1, NULL_TREE);
 349
 350       (*loc)->gimple_phi.nargs++;
 351     }
 352 }
 353
 354 /* Adds PHI to BB.  */
 355
 356 void
 357 add_phi_node_to_bb (gimple phi, basic_block bb)
 358 {
 359   gimple_stmt_iterator gsi;
 360   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 361   if (phi_nodes (bb) == NULL)
 362     set_phi_nodes (bb, gimple_seq_alloc ());
 363
 364   gsi = gsi_last (phi_nodes (bb));
 365   gsi_insert_after (&gsi, phi, GSI_NEW_STMT);
 366
 367   /* Associate BB to the PHI node.  */
 368   gimple_set_bb (phi, bb);
 369
 370 }
 371
 372 /* Create a new PHI node for variable VAR at basic block BB.  */
 373
 374 gimple
 375 create_phi_node (tree var, basic_block bb)
 376 {
 377   gimple phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 378
 379   add_phi_node_to_bb (phi, bb);
 380   return phi;
 381 }
 382
 383
 384 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 385    definition and E is the edge through which DEF reaches PHI.  The new
 386    argument is added at the end of the argument list.
 387    If PHI has reached its maximum capacity, add a few slots.  In this case,
 388    PHI points to the reallocated phi node when we return.  */
 389
 390 void
 391 add_phi_arg (gimple phi, tree def, edge e, source_location locus)
 392 {
 393   basic_block bb = e->dest;
 394
 395   gcc_assert (bb == gimple_bb (phi));
 396
 397   /* We resize PHI nodes upon edge creation.  We should always have
 398      enough room at this point.  */
 399   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 400
 401   /* We resize PHI nodes upon edge creation.  We should always have
 402      enough room at this point.  */
 403   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 404
 405   /* Copy propagation needs to know what object occur in abnormal
 406      PHI nodes.  This is a convenient place to record such information.  */
 407   if (e->flags & EDGE_ABNORMAL)
 408     {
 409       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 410       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 411     }
 412
 413   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 414   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 415 }
 416
 417
 418 /* Remove the Ith argument from PHI's argument list.  This routine
 419    implements removal by swapping the last alternative with the
 420    alternative we want to delete and then shrinking the vector, which
 421    is consistent with how we remove an edge from the edge vector.  */
 422
 423 static void
 424 remove_phi_arg_num (gimple phi, int i)
 425 {
 426   int num_elem = gimple_phi_num_args (phi);
 427
 428   gcc_assert (i < num_elem);
 429
 430   /* Delink the item which is being removed.  */
 431   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 432
 433   /* If it is not the last element, move the last element
 434      to the element we want to delete, resetting all the links. */
 435   if (i != num_elem - 1)
 436     {
 437       use_operand_p old_p, new_p;
 438       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 439       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 440       /* Set use on new node, and link into last element's place.  */
 441       *(new_p->use) = *(old_p->use);
 442       relink_imm_use (new_p, old_p);
 443       /* Move the location as well.  */
 444       gimple_phi_arg_set_location (phi, i,
 445                                    gimple_phi_arg_location (phi, num_elem - 1));
 446     }
 447
 448   /* Shrink the vector and return.  Note that we do not have to clear
 449      PHI_ARG_DEF because the garbage collector will not look at those
 450      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 451   phi->gimple_phi.nargs--;
 452 }
 453
 454
 455 /* Remove all PHI arguments associated with edge E.  */
 456
 457 void
 458 remove_phi_args (edge e)
 459 {
 460   gimple_stmt_iterator gsi;
 461
 462   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 463     remove_phi_arg_num (gsi_stmt (gsi), e->dest_idx);
 464 }
 465
 466
 467 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 468    removal, iterator GSI is updated to point to the next PHI node in the
 469    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 470    into the free pool of SSA names.  */
 471
 472 void
 473 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 474 {
 475   gimple phi = gsi_stmt (*gsi);
 476   gsi_remove (gsi, false);
 477
 478   /* If we are deleting the PHI node, then we should release the
 479      SSA_NAME node so that it can be reused.  */
 480   release_phi_node (phi);
 481   if (release_lhs_p)
 482     release_ssa_name (gimple_phi_result (phi));
 483 }
 484
 485 /* Remove all the phi nodes from BB.  */
 486
 487 void
 488 remove_phi_nodes (basic_block bb)
 489 {
 490   gimple_stmt_iterator gsi;
 491
 492   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 493     remove_phi_node (&gsi, true);
 494
 495   set_phi_nodes (bb, NULL);
 496 }
 497
 498 #include "gt-tree-phinodes.h"