gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003, 2005, 2007, 2008, 2009, 2010
   3    Free Software Foundation, Inc.
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 3, or (at your option)
  10 any later version.
  11
  12 GCC is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "tree.h"
  26 #include "ggc.h"
  27 #include "basic-block.h"
  28 #include "tree-flow.h"
  29 #include "diagnostic-core.h"
  30 #include "gimple.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    We could also use a zone allocator for these objects since they have
  47    a very well defined lifetime.  If someone wants to experiment with that
  48    this is the place to try it.
  49
  50    PHI nodes have different sizes, so we can't have a single list of all
  51    the PHI nodes as it would be too expensive to walk down that list to
  52    find a PHI of a suitable size.
  53
  54    Instead we have an array of lists of free PHI nodes.  The array is
  55    indexed by the number of PHI alternatives that PHI node can hold.
  56    Except for the last array member, which holds all remaining PHI
  57    nodes.
  58
  59    So to find a free PHI node, we compute its index into the free PHI
  60    node array and see if there are any elements with an exact match.
  61    If so, then we are done.  Otherwise, we test the next larger size
  62    up and continue until we are in the last array element.
  63
  64    We do not actually walk members of the last array element.  While it
  65    might allow us to pick up a few reusable PHI nodes, it could potentially
  66    be very expensive if the program has released a bunch of large PHI nodes,
  67    but keeps asking for even larger PHI nodes.  Experiments have shown that
  68    walking the elements of the last array entry would result in finding less
  69    than .1% additional reusable PHI nodes.
  70
  71    Note that we can never have less than two PHI argument slots.  Thus,
  72    the -2 on all the calculations below.  */
  73
  74 #define NUM_BUCKETS 10
  75 static GTY ((deletable (""))) vec<gimple, va_gc> *free_phinodes[NUM_BUCKETS - 2];
  76 static unsigned long free_phinode_count;
  77
  78 static int ideal_phi_node_len (int);
  79
  80 unsigned int phi_nodes_reused;
  81 unsigned int phi_nodes_created;
  82
  83 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
  84
  85 void
  86 phinodes_print_statistics (void)
  87 {
  88   fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
  89   fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
  90 }
  91
  92 /* Allocate a PHI node with at least LEN arguments.  If the free list
  93    happens to contain a PHI node with LEN arguments or more, return
  94    that one.  */
  95
  96 static inline gimple
  97 allocate_phi_node (size_t len)
  98 {
  99   gimple phi;
 100   size_t bucket = NUM_BUCKETS - 2;
 101   size_t size = sizeof (struct gimple_statement_phi)
 102                 + (len - 1) * sizeof (struct phi_arg_d);
 103
 104   if (free_phinode_count)
 105     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 106       if (free_phinodes[bucket])
 107         break;
 108
 109   /* If our free list has an element, then use it.  */
 110   if (bucket < NUM_BUCKETS - 2
 111       && gimple_phi_capacity ((*free_phinodes[bucket])[0]) >= len)
 112     {
 113       free_phinode_count--;
 114       phi = free_phinodes[bucket]->pop ();
 115       if (free_phinodes[bucket]->is_empty ())
 116         vec_free (free_phinodes[bucket]);
 117       if (GATHER_STATISTICS)
 118         phi_nodes_reused++;
 119     }
 120   else
 121     {
 122       phi = ggc_alloc_gimple_statement_d (size);
 123       if (GATHER_STATISTICS)
 124         {
 125           enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
 126           phi_nodes_created++;
 127           gimple_alloc_counts[(int) kind]++;
 128           gimple_alloc_sizes[(int) kind] += size;
 129         }
 130     }
 131
 132   return phi;
 133 }
 134
 135 /* Given LEN, the original number of requested PHI arguments, return
 136    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 137    the total size of the PHI node up to the next power of two bytes.
 138
 139    Rounding up will not result in wasting any memory since the size request
 140    will be rounded up by the GC system anyway.  [ Note this is not entirely
 141    true since the original length might have fit on one of the special
 142    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 143    PHI node later if we increase the number of arguments for the PHI.  */
 144
 145 static int
 146 ideal_phi_node_len (int len)
 147 {
 148   size_t size, new_size;
 149   int log2, new_len;
 150
 151   /* We do not support allocations of less than two PHI argument slots.  */
 152   if (len < 2)
 153     len = 2;
 154
 155   /* Compute the number of bytes of the original request.  */
 156   size = sizeof (struct gimple_statement_phi)
 157          + (len - 1) * sizeof (struct phi_arg_d);
 158
 159   /* Round it up to the next power of two.  */
 160   log2 = ceil_log2 (size);
 161   new_size = 1 << log2;
 162
 163   /* Now compute and return the number of PHI argument slots given an
 164      ideal size allocation.  */
 165   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 166   return new_len;
 167 }
 168
 169 /* Return a PHI node with LEN argument slots for variable VAR.  */
 170
 171 static gimple
 172 make_phi_node (tree var, int len)
 173 {
 174   gimple phi;
 175   int capacity, i;
 176
 177   capacity = ideal_phi_node_len (len);
 178
 179   phi = allocate_phi_node (capacity);
 180
 181   /* We need to clear the entire PHI node, including the argument
 182      portion, because we represent a "missing PHI argument" by placing
 183      NULL_TREE in PHI_ARG_DEF.  */
 184   memset (phi, 0, (sizeof (struct gimple_statement_phi)
 185                    - sizeof (struct phi_arg_d)
 186                    + sizeof (struct phi_arg_d) * len));
 187   phi->gsbase.code = GIMPLE_PHI;
 188   gimple_init_singleton (phi);
 189   phi->gimple_phi.nargs = len;
 190   phi->gimple_phi.capacity = capacity;
 191   if (!var)
 192     ;
 193   else if (TREE_CODE (var) == SSA_NAME)
 194     gimple_phi_set_result (phi, var);
 195   else
 196     gimple_phi_set_result (phi, make_ssa_name (var, phi));
 197
 198   for (i = 0; i < capacity; i++)
 199     {
 200       use_operand_p  imm;
 201
 202       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
 203       imm = gimple_phi_arg_imm_use_ptr (phi, i);
 204       imm->use = gimple_phi_arg_def_ptr (phi, i);
 205       imm->prev = NULL;
 206       imm->next = NULL;
 207       imm->loc.stmt = phi;
 208     }
 209
 210   return phi;
 211 }
 212
 213 /* We no longer need PHI, release it so that it may be reused.  */
 214
 215 void
 216 release_phi_node (gimple phi)
 217 {
 218   size_t bucket;
 219   size_t len = gimple_phi_capacity (phi);
 220   size_t x;
 221
 222   for (x = 0; x < gimple_phi_num_args (phi); x++)
 223     {
 224       use_operand_p  imm;
 225       imm = gimple_phi_arg_imm_use_ptr (phi, x);
 226       delink_imm_use (imm);
 227     }
 228
 229   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 230   bucket -= 2;
 231   vec_safe_push (free_phinodes[bucket], phi);
 232   free_phinode_count++;
 233 }
 234
 235
 236 /* Resize an existing PHI node.  The only way is up.  Return the
 237    possibly relocated phi.  */
 238
 239 static gimple
 240 resize_phi_node (gimple phi, size_t len)
 241 {
 242   size_t old_size, i;
 243   gimple new_phi;
 244
 245   gcc_assert (len > gimple_phi_capacity (phi));
 246
 247   /* The garbage collector will not look at the PHI node beyond the
 248      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 249      portion of the PHI node currently in use.  */
 250   old_size = sizeof (struct gimple_statement_phi)
 251              + (gimple_phi_num_args (phi) - 1) * sizeof (struct phi_arg_d);
 252
 253   new_phi = allocate_phi_node (len);
 254
 255   memcpy (new_phi, phi, old_size);
 256
 257   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 258     {
 259       use_operand_p imm, old_imm;
 260       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 261       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
 262       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 263       relink_imm_use_stmt (imm, old_imm, new_phi);
 264     }
 265
 266   new_phi->gimple_phi.capacity = len;
 267
 268   for (i = gimple_phi_num_args (new_phi); i < len; i++)
 269     {
 270       use_operand_p imm;
 271
 272       gimple_phi_arg_set_location (new_phi, i, UNKNOWN_LOCATION);
 273       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 274       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 275       imm->prev = NULL;
 276       imm->next = NULL;
 277       imm->loc.stmt = new_phi;
 278     }
 279
 280   return new_phi;
 281 }
 282
 283 /* Reserve PHI arguments for a new edge to basic block BB.  */
 284
 285 void
 286 reserve_phi_args_for_new_edge (basic_block bb)
 287 {
 288   size_t len = EDGE_COUNT (bb->preds);
 289   size_t cap = ideal_phi_node_len (len + 4);
 290   gimple_stmt_iterator gsi;
 291
 292   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 293     {
 294       gimple stmt = gsi_stmt (gsi);
 295
 296       if (len > gimple_phi_capacity (stmt))
 297         {
 298           gimple new_phi = resize_phi_node (stmt, cap);
 299
 300           /* The result of the PHI is defined by this PHI node.  */
 301           SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
 302           gsi_set_stmt (&gsi, new_phi);
 303
 304           release_phi_node (stmt);
 305           stmt = new_phi;
 306         }
 307
 308       /* We represent a "missing PHI argument" by placing NULL_TREE in
 309          the corresponding slot.  If PHI arguments were added
 310          immediately after an edge is created, this zeroing would not
 311          be necessary, but unfortunately this is not the case.  For
 312          example, the loop optimizer duplicates several basic blocks,
 313          redirects edges, and then fixes up PHI arguments later in
 314          batch.  */
 315       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
 316
 317       stmt->gimple_phi.nargs++;
 318     }
 319 }
 320
 321 /* Adds PHI to BB.  */
 322
 323 void
 324 add_phi_node_to_bb (gimple phi, basic_block bb)
 325 {
 326   gimple_seq seq = phi_nodes (bb);
 327   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 328   if (seq == NULL)
 329     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
 330   else
 331     {
 332       gimple_seq_add_stmt (&seq, phi);
 333       gcc_assert (seq == phi_nodes (bb));
 334     }
 335
 336   /* Associate BB to the PHI node.  */
 337   gimple_set_bb (phi, bb);
 338
 339 }
 340
 341 /* Create a new PHI node for variable VAR at basic block BB.  */
 342
 343 gimple
 344 create_phi_node (tree var, basic_block bb)
 345 {
 346   gimple phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 347
 348   add_phi_node_to_bb (phi, bb);
 349   return phi;
 350 }
 351
 352
 353 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 354    definition and E is the edge through which DEF reaches PHI.  The new
 355    argument is added at the end of the argument list.
 356    If PHI has reached its maximum capacity, add a few slots.  In this case,
 357    PHI points to the reallocated phi node when we return.  */
 358
 359 void
 360 add_phi_arg (gimple phi, tree def, edge e, source_location locus)
 361 {
 362   basic_block bb = e->dest;
 363
 364   gcc_assert (bb == gimple_bb (phi));
 365
 366   /* We resize PHI nodes upon edge creation.  We should always have
 367      enough room at this point.  */
 368   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 369
 370   /* We resize PHI nodes upon edge creation.  We should always have
 371      enough room at this point.  */
 372   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 373
 374   /* Copy propagation needs to know what object occur in abnormal
 375      PHI nodes.  This is a convenient place to record such information.  */
 376   if (e->flags & EDGE_ABNORMAL)
 377     {
 378       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 379       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 380     }
 381
 382   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 383   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 384 }
 385
 386
 387 /* Remove the Ith argument from PHI's argument list.  This routine
 388    implements removal by swapping the last alternative with the
 389    alternative we want to delete and then shrinking the vector, which
 390    is consistent with how we remove an edge from the edge vector.  */
 391
 392 static void
 393 remove_phi_arg_num (gimple phi, int i)
 394 {
 395   int num_elem = gimple_phi_num_args (phi);
 396
 397   gcc_assert (i < num_elem);
 398
 399   /* Delink the item which is being removed.  */
 400   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 401
 402   /* If it is not the last element, move the last element
 403      to the element we want to delete, resetting all the links. */
 404   if (i != num_elem - 1)
 405     {
 406       use_operand_p old_p, new_p;
 407       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 408       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 409       /* Set use on new node, and link into last element's place.  */
 410       *(new_p->use) = *(old_p->use);
 411       relink_imm_use (new_p, old_p);
 412       /* Move the location as well.  */
 413       gimple_phi_arg_set_location (phi, i,
 414                                    gimple_phi_arg_location (phi, num_elem - 1));
 415     }
 416
 417   /* Shrink the vector and return.  Note that we do not have to clear
 418      PHI_ARG_DEF because the garbage collector will not look at those
 419      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 420   phi->gimple_phi.nargs--;
 421 }
 422
 423
 424 /* Remove all PHI arguments associated with edge E.  */
 425
 426 void
 427 remove_phi_args (edge e)
 428 {
 429   gimple_stmt_iterator gsi;
 430
 431   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 432     remove_phi_arg_num (gsi_stmt (gsi), e->dest_idx);
 433 }
 434
 435
 436 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 437    removal, iterator GSI is updated to point to the next PHI node in the
 438    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 439    into the free pool of SSA names.  */
 440
 441 void
 442 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 443 {
 444   gimple phi = gsi_stmt (*gsi);
 445
 446   if (release_lhs_p)
 447     insert_debug_temps_for_defs (gsi);
 448
 449   gsi_remove (gsi, false);
 450
 451   /* If we are deleting the PHI node, then we should release the
 452      SSA_NAME node so that it can be reused.  */
 453   release_phi_node (phi);
 454   if (release_lhs_p)
 455     release_ssa_name (gimple_phi_result (phi));
 456 }
 457
 458 /* Remove all the phi nodes from BB.  */
 459
 460 void
 461 remove_phi_nodes (basic_block bb)
 462 {
 463   gimple_stmt_iterator gsi;
 464
 465   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 466     remove_phi_node (&gsi, true);
 467
 468   set_phi_nodes (bb, NULL);
 469 }
 470
 471 #include "gt-tree-phinodes.h"