gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003-2021 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 "backend.h"
  24 #include "tree.h"
  25 #include "gimple.h"
  26 #include "ssa.h"
  27 #include "fold-const.h"
  28 #include "gimple-iterator.h"
  29 #include "tree-ssa.h"
  30
  31 /* Rewriting a function into SSA form can create a huge number of PHIs
  32    many of which may be thrown away shortly after their creation if jumps
  33    were threaded through PHI nodes.
  34
  35    While our garbage collection mechanisms will handle this situation, it
  36    is extremely wasteful to create nodes and throw them away, especially
  37    when the nodes can be reused.
  38
  39    For PR 8361, we can significantly reduce the number of nodes allocated
  40    and thus the total amount of memory allocated by managing PHIs a
  41    little.  This additionally helps reduce the amount of work done by the
  42    garbage collector.  Similar results have been seen on a wider variety
  43    of tests (such as the compiler itself).
  44
  45    PHI nodes have different sizes, so we can't have a single list of all
  46    the PHI nodes as it would be too expensive to walk down that list to
  47    find a PHI of a suitable size.
  48
  49    Instead we have an array of lists of free PHI nodes.  The array is
  50    indexed by the number of PHI alternatives that PHI node can hold.
  51    Except for the last array member, which holds all remaining PHI
  52    nodes.
  53
  54    So to find a free PHI node, we compute its index into the free PHI
  55    node array and see if there are any elements with an exact match.
  56    If so, then we are done.  Otherwise, we test the next larger size
  57    up and continue until we are in the last array element.
  58
  59    We do not actually walk members of the last array element.  While it
  60    might allow us to pick up a few reusable PHI nodes, it could potentially
  61    be very expensive if the program has released a bunch of large PHI nodes,
  62    but keeps asking for even larger PHI nodes.  Experiments have shown that
  63    walking the elements of the last array entry would result in finding less
  64    than .1% additional reusable PHI nodes.
  65
  66    Note that we can never have less than two PHI argument slots.  Thus,
  67    the -2 on all the calculations below.  */
  68
  69 #define NUM_BUCKETS 10
  70 static GTY ((deletable (""))) vec<gimple *, va_gc> *free_phinodes[NUM_BUCKETS - 2];
  71 static unsigned long free_phinode_count;
  72
  73 static int ideal_phi_node_len (int);
  74
  75 unsigned int phi_nodes_reused;
  76 unsigned int phi_nodes_created;
  77
  78 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
  79
  80 void
  81 phinodes_print_statistics (void)
  82 {
  83   fprintf (stderr, "%-32s" PRsa (11) "\n", "PHI nodes allocated:",
  84            SIZE_AMOUNT (phi_nodes_created));
  85   fprintf (stderr, "%-32s" PRsa (11) "\n", "PHI nodes reused:",
  86            SIZE_AMOUNT (phi_nodes_reused));
  87 }
  88
  89 /* Allocate a PHI node with at least LEN arguments.  If the free list
  90    happens to contain a PHI node with LEN arguments or more, return
  91    that one.  */
  92
  93 static inline gphi *
  94 allocate_phi_node (size_t len)
  95 {
  96   gphi *phi;
  97   size_t bucket = NUM_BUCKETS - 2;
  98   size_t size = sizeof (struct gphi)
  99                 + (len - 1) * sizeof (struct phi_arg_d);
 100
 101   if (free_phinode_count)
 102     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 103       if (free_phinodes[bucket])
 104         break;
 105
 106   /* If our free list has an element, then use it.  */
 107   if (bucket < NUM_BUCKETS - 2
 108       && gimple_phi_capacity ((*free_phinodes[bucket])[0]) >= len)
 109     {
 110       free_phinode_count--;
 111       phi = as_a <gphi *> (free_phinodes[bucket]->pop ());
 112       if (free_phinodes[bucket]->is_empty ())
 113         vec_free (free_phinodes[bucket]);
 114       if (GATHER_STATISTICS)
 115         phi_nodes_reused++;
 116     }
 117   else
 118     {
 119       phi = static_cast <gphi *> (ggc_internal_alloc (size));
 120       if (GATHER_STATISTICS)
 121         {
 122           enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
 123           phi_nodes_created++;
 124           gimple_alloc_counts[(int) kind]++;
 125           gimple_alloc_sizes[(int) kind] += size;
 126         }
 127     }
 128
 129   return phi;
 130 }
 131
 132 /* Given LEN, the original number of requested PHI arguments, return
 133    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 134    the total size of the PHI node up to the next power of two bytes.
 135
 136    Rounding up will not result in wasting any memory since the size request
 137    will be rounded up by the GC system anyway.  [ Note this is not entirely
 138    true since the original length might have fit on one of the special
 139    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 140    PHI node later if we increase the number of arguments for the PHI.  */
 141
 142 static int
 143 ideal_phi_node_len (int len)
 144 {
 145   size_t size, new_size;
 146   int log2, new_len;
 147
 148   /* We do not support allocations of less than two PHI argument slots.  */
 149   if (len < 2)
 150     len = 2;
 151
 152   /* Compute the number of bytes of the original request.  */
 153   size = sizeof (struct gphi)
 154          + (len - 1) * sizeof (struct phi_arg_d);
 155
 156   /* Round it up to the next power of two.  */
 157   log2 = ceil_log2 (size);
 158   new_size = 1 << log2;
 159
 160   /* Now compute and return the number of PHI argument slots given an
 161      ideal size allocation.  */
 162   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 163   return new_len;
 164 }
 165
 166 /* Return a PHI node with LEN argument slots for variable VAR.  */
 167
 168 static gphi *
 169 make_phi_node (tree var, int len)
 170 {
 171   gphi *phi;
 172   int capacity, i;
 173
 174   capacity = ideal_phi_node_len (len);
 175
 176   phi = allocate_phi_node (capacity);
 177
 178   /* We need to clear the entire PHI node, including the argument
 179      portion, because we represent a "missing PHI argument" by placing
 180      NULL_TREE in PHI_ARG_DEF.  */
 181   memset (phi, 0, (sizeof (struct gphi)
 182                    - sizeof (struct phi_arg_d)
 183                    + sizeof (struct phi_arg_d) * len));
 184   phi->code = GIMPLE_PHI;
 185   gimple_init_singleton (phi);
 186   phi->nargs = len;
 187   phi->capacity = capacity;
 188   if (!var)
 189     ;
 190   else if (TREE_CODE (var) == SSA_NAME)
 191     gimple_phi_set_result (phi, var);
 192   else
 193     gimple_phi_set_result (phi, make_ssa_name (var, phi));
 194
 195   for (i = 0; i < len; i++)
 196     {
 197       use_operand_p  imm;
 198
 199       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
 200       imm = gimple_phi_arg_imm_use_ptr (phi, i);
 201       imm->use = gimple_phi_arg_def_ptr (phi, i);
 202       imm->prev = NULL;
 203       imm->next = NULL;
 204       imm->loc.stmt = phi;
 205     }
 206
 207   return phi;
 208 }
 209
 210 /* We no longer need PHI, release it so that it may be reused.  */
 211
 212 static void
 213 release_phi_node (gimple *phi)
 214 {
 215   size_t bucket;
 216   size_t len = gimple_phi_capacity (phi);
 217   size_t x;
 218
 219   for (x = 0; x < gimple_phi_num_args (phi); x++)
 220     {
 221       use_operand_p  imm;
 222       imm = gimple_phi_arg_imm_use_ptr (phi, x);
 223       delink_imm_use (imm);
 224     }
 225
 226   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 227   bucket -= 2;
 228   vec_safe_push (free_phinodes[bucket], phi);
 229   free_phinode_count++;
 230 }
 231
 232
 233 /* Resize an existing PHI node.  The only way is up.  Return the
 234    possibly relocated phi.  */
 235
 236 static gphi *
 237 resize_phi_node (gphi *phi, size_t len)
 238 {
 239   size_t old_size, i;
 240   gphi *new_phi;
 241
 242   gcc_assert (len > gimple_phi_capacity (phi));
 243
 244   /* The garbage collector will not look at the PHI node beyond the
 245      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 246      portion of the PHI node currently in use.  */
 247   old_size = sizeof (struct gphi)
 248              + (gimple_phi_num_args (phi) - 1) * sizeof (struct phi_arg_d);
 249
 250   new_phi = allocate_phi_node (len);
 251
 252   memcpy (new_phi, phi, old_size);
 253   memset ((char *)new_phi + old_size, 0,
 254           (sizeof (struct gphi)
 255            - sizeof (struct phi_arg_d)
 256            + sizeof (struct phi_arg_d) * len) - old_size);
 257
 258   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 259     {
 260       use_operand_p imm, old_imm;
 261       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 262       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
 263       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 264       relink_imm_use_stmt (imm, old_imm, new_phi);
 265     }
 266
 267   new_phi->capacity = len;
 268
 269   return new_phi;
 270 }
 271
 272 /* Reserve PHI arguments for a new edge to basic block BB.  */
 273
 274 void
 275 reserve_phi_args_for_new_edge (basic_block bb)
 276 {
 277   size_t len = EDGE_COUNT (bb->preds);
 278   size_t cap = ideal_phi_node_len (len + 4);
 279   gphi_iterator gsi;
 280
 281   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 282     {
 283       gphi *stmt = gsi.phi ();
 284
 285       if (len > gimple_phi_capacity (stmt))
 286         {
 287           gphi *new_phi = resize_phi_node (stmt, cap);
 288
 289           /* The result of the PHI is defined by this PHI node.  */
 290           SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
 291           gsi_set_stmt (&gsi, new_phi);
 292
 293           release_phi_node (stmt);
 294           stmt = new_phi;
 295         }
 296
 297       stmt->nargs++;
 298
 299       /* We represent a "missing PHI argument" by placing NULL_TREE in
 300          the corresponding slot.  If PHI arguments were added
 301          immediately after an edge is created, this zeroing would not
 302          be necessary, but unfortunately this is not the case.  For
 303          example, the loop optimizer duplicates several basic blocks,
 304          redirects edges, and then fixes up PHI arguments later in
 305          batch.  */
 306       use_operand_p imm = gimple_phi_arg_imm_use_ptr (stmt, len - 1);
 307       imm->use = gimple_phi_arg_def_ptr (stmt, len - 1);
 308       imm->prev = NULL;
 309       imm->next = NULL;
 310       imm->loc.stmt = stmt;
 311       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
 312       gimple_phi_arg_set_location (stmt, len - 1, UNKNOWN_LOCATION);
 313     }
 314 }
 315
 316 /* Adds PHI to BB.  */
 317
 318 void
 319 add_phi_node_to_bb (gphi *phi, basic_block bb)
 320 {
 321   gimple_seq seq = phi_nodes (bb);
 322   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 323   if (seq == NULL)
 324     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
 325   else
 326     {
 327       gimple_seq_add_stmt (&seq, phi);
 328       gcc_assert (seq == phi_nodes (bb));
 329     }
 330
 331   /* Associate BB to the PHI node.  */
 332   gimple_set_bb (phi, bb);
 333
 334 }
 335
 336 /* Create a new PHI node for variable VAR at basic block BB.  */
 337
 338 gphi *
 339 create_phi_node (tree var, basic_block bb)
 340 {
 341   gphi *phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 342
 343   add_phi_node_to_bb (phi, bb);
 344   return phi;
 345 }
 346
 347
 348 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 349    definition and E is the edge through which DEF reaches PHI.  The new
 350    argument is added at the end of the argument list.
 351    If PHI has reached its maximum capacity, add a few slots.  In this case,
 352    PHI points to the reallocated phi node when we return.  */
 353
 354 void
 355 add_phi_arg (gphi *phi, tree def, edge e, location_t locus)
 356 {
 357   basic_block bb = e->dest;
 358
 359   gcc_assert (bb == gimple_bb (phi));
 360
 361   /* We resize PHI nodes upon edge creation.  We should always have
 362      enough room at this point.  */
 363   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 364
 365   /* We resize PHI nodes upon edge creation.  We should always have
 366      enough room at this point.  */
 367   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 368
 369   /* Copy propagation needs to know what object occur in abnormal
 370      PHI nodes.  This is a convenient place to record such information.  */
 371   if (e->flags & EDGE_ABNORMAL)
 372     {
 373       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 374       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 375     }
 376
 377   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 378   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 379 }
 380
 381
 382 /* Remove the Ith argument from PHI's argument list.  This routine
 383    implements removal by swapping the last alternative with the
 384    alternative we want to delete and then shrinking the vector, which
 385    is consistent with how we remove an edge from the edge vector.  */
 386
 387 static void
 388 remove_phi_arg_num (gphi *phi, int i)
 389 {
 390   int num_elem = gimple_phi_num_args (phi);
 391
 392   gcc_assert (i < num_elem);
 393
 394   /* Delink the item which is being removed.  */
 395   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 396
 397   /* If it is not the last element, move the last element
 398      to the element we want to delete, resetting all the links. */
 399   if (i != num_elem - 1)
 400     {
 401       use_operand_p old_p, new_p;
 402       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 403       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 404       /* Set use on new node, and link into last element's place.  */
 405       *(new_p->use) = *(old_p->use);
 406       relink_imm_use (new_p, old_p);
 407       /* Move the location as well.  */
 408       gimple_phi_arg_set_location (phi, i,
 409                                    gimple_phi_arg_location (phi, num_elem - 1));
 410     }
 411
 412   /* Shrink the vector and return.  Note that we do not have to clear
 413      PHI_ARG_DEF because the garbage collector will not look at those
 414      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 415   phi->nargs--;
 416 }
 417
 418
 419 /* Remove all PHI arguments associated with edge E.  */
 420
 421 void
 422 remove_phi_args (edge e)
 423 {
 424   gphi_iterator gsi;
 425
 426   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 427     remove_phi_arg_num (gsi.phi (),
 428                         e->dest_idx);
 429 }
 430
 431
 432 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 433    removal, iterator GSI is updated to point to the next PHI node in the
 434    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 435    into the free pool of SSA names.  */
 436
 437 void
 438 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 439 {
 440   gimple *phi = gsi_stmt (*gsi);
 441
 442   if (release_lhs_p)
 443     insert_debug_temps_for_defs (gsi);
 444
 445   gsi_remove (gsi, false);
 446
 447   /* If we are deleting the PHI node, then we should release the
 448      SSA_NAME node so that it can be reused.  */
 449   release_phi_node (phi);
 450   if (release_lhs_p)
 451     release_ssa_name (gimple_phi_result (phi));
 452 }
 453
 454 /* Remove all the phi nodes from BB.  */
 455
 456 void
 457 remove_phi_nodes (basic_block bb)
 458 {
 459   gphi_iterator gsi;
 460
 461   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 462     remove_phi_node (&gsi, true);
 463
 464   set_phi_nodes (bb, NULL);
 465 }
 466
 467 /* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
 468    NULL.  */
 469
 470 tree
 471 degenerate_phi_result (gphi *phi)
 472 {
 473   tree lhs = gimple_phi_result (phi);
 474   tree val = NULL;
 475   size_t i;
 476
 477   /* Ignoring arguments which are the same as LHS, if all the remaining
 478      arguments are the same, then the PHI is a degenerate and has the
 479      value of that common argument.  */
 480   for (i = 0; i < gimple_phi_num_args (phi); i++)
 481     {
 482       tree arg = gimple_phi_arg_def (phi, i);
 483
 484       if (arg == lhs)
 485         continue;
 486       else if (!arg)
 487         break;
 488       else if (!val)
 489         val = arg;
 490       else if (arg == val)
 491         continue;
 492       /* We bring in some of operand_equal_p not only to speed things
 493          up, but also to avoid crashing when dereferencing the type of
 494          a released SSA name.  */
 495       else if (TREE_CODE (val) != TREE_CODE (arg)
 496                || TREE_CODE (val) == SSA_NAME
 497                || !operand_equal_p (arg, val, 0))
 498         break;
 499     }
 500   return (i == gimple_phi_num_args (phi) ? val : NULL);
 501 }
 502
 503 /* Set PHI nodes of a basic block BB to SEQ.  */
 504
 505 void
 506 set_phi_nodes (basic_block bb, gimple_seq seq)
 507 {
 508   gimple_stmt_iterator i;
 509
 510   gcc_checking_assert (!(bb->flags & BB_RTL));
 511   bb->il.gimple.phi_nodes = seq;
 512   if (seq)
 513     for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (&i))
 514       gimple_set_bb (gsi_stmt (i), bb);
 515 }
 516
 517 #include "gt-tree-phinodes.h"