gcc/tree-phinodes.cc

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003-2024 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   /* Immediately return the memory to the allocator when we would
 227      only ever re-use it for a smaller size allocation.  */
 228   if (len - 2 >= NUM_BUCKETS - 2)
 229     {
 230       ggc_free (phi);
 231       return;
 232     }
 233
 234   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 235   bucket -= 2;
 236   vec_safe_push (free_phinodes[bucket], phi);
 237   free_phinode_count++;
 238 }
 239
 240
 241 /* Resize an existing PHI node.  The only way is up.  Return the
 242    possibly relocated phi.  */
 243
 244 static gphi *
 245 resize_phi_node (gphi *phi, size_t len)
 246 {
 247   size_t old_size, i;
 248   gphi *new_phi;
 249
 250   gcc_assert (len > gimple_phi_capacity (phi));
 251
 252   /* The garbage collector will not look at the PHI node beyond the
 253      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 254      portion of the PHI node currently in use.  */
 255   old_size = sizeof (struct gphi)
 256              + (gimple_phi_num_args (phi) - 1) * sizeof (struct phi_arg_d);
 257
 258   new_phi = allocate_phi_node (len);
 259
 260   memcpy (new_phi, phi, old_size);
 261   memset ((char *)new_phi + old_size, 0,
 262           (sizeof (struct gphi)
 263            - sizeof (struct phi_arg_d)
 264            + sizeof (struct phi_arg_d) * len) - old_size);
 265
 266   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 267     {
 268       use_operand_p imm, old_imm;
 269       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 270       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
 271       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 272       relink_imm_use_stmt (imm, old_imm, new_phi);
 273     }
 274
 275   new_phi->capacity = len;
 276
 277   return new_phi;
 278 }
 279
 280 /* Reserve PHI arguments for a new edge to basic block BB.  */
 281
 282 void
 283 reserve_phi_args_for_new_edge (basic_block bb)
 284 {
 285   size_t len = EDGE_COUNT (bb->preds);
 286   size_t cap = ideal_phi_node_len (len + 4);
 287   gphi_iterator gsi;
 288
 289   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 290     {
 291       gphi *stmt = gsi.phi ();
 292
 293       if (len > gimple_phi_capacity (stmt))
 294         {
 295           gphi *new_phi = resize_phi_node (stmt, cap);
 296
 297           /* The result of the PHI is defined by this PHI node.  */
 298           SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
 299           gsi_set_stmt (&gsi, new_phi);
 300
 301           release_phi_node (stmt);
 302           stmt = new_phi;
 303         }
 304
 305       stmt->nargs++;
 306
 307       /* We represent a "missing PHI argument" by placing NULL_TREE in
 308          the corresponding slot.  If PHI arguments were added
 309          immediately after an edge is created, this zeroing would not
 310          be necessary, but unfortunately this is not the case.  For
 311          example, the loop optimizer duplicates several basic blocks,
 312          redirects edges, and then fixes up PHI arguments later in
 313          batch.  */
 314       use_operand_p imm = gimple_phi_arg_imm_use_ptr (stmt, len - 1);
 315       imm->use = gimple_phi_arg_def_ptr (stmt, len - 1);
 316       imm->prev = NULL;
 317       imm->next = NULL;
 318       imm->loc.stmt = stmt;
 319       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
 320       gimple_phi_arg_set_location (stmt, len - 1, UNKNOWN_LOCATION);
 321     }
 322 }
 323
 324 /* Adds PHI to BB.  */
 325
 326 static void
 327 add_phi_node_to_bb (gphi *phi, basic_block bb)
 328 {
 329   gimple_seq seq = phi_nodes (bb);
 330   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 331   if (seq == NULL)
 332     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
 333   else
 334     {
 335       gimple_seq_add_stmt (&seq, phi);
 336       gcc_assert (seq == phi_nodes (bb));
 337     }
 338
 339   /* Associate BB to the PHI node.  */
 340   gimple_set_bb (phi, bb);
 341 }
 342
 343 /* Create a new PHI node for variable VAR at basic block BB.  */
 344
 345 gphi *
 346 create_phi_node (tree var, basic_block bb)
 347 {
 348   gphi *phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 349
 350   add_phi_node_to_bb (phi, bb);
 351   return phi;
 352 }
 353
 354
 355 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 356    definition and E is the edge through which DEF reaches PHI.  The new
 357    argument is added at the end of the argument list.
 358    If PHI has reached its maximum capacity, add a few slots.  In this case,
 359    PHI points to the reallocated phi node when we return.  */
 360
 361 void
 362 add_phi_arg (gphi *phi, tree def, edge e, location_t locus)
 363 {
 364   basic_block bb = e->dest;
 365
 366   gcc_assert (bb == gimple_bb (phi));
 367
 368   /* We resize PHI nodes upon edge creation.  We should always have
 369      enough room at this point.  */
 370   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 371
 372   /* We resize PHI nodes upon edge creation.  We should always have
 373      enough room at this point.  */
 374   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 375
 376   /* Copy propagation needs to know what object occur in abnormal
 377      PHI nodes.  This is a convenient place to record such information.  */
 378   if (e->flags & EDGE_ABNORMAL)
 379     {
 380       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 381       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 382     }
 383
 384   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 385   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 386 }
 387
 388
 389 /* Remove the Ith argument from PHI's argument list.  This routine
 390    implements removal by swapping the last alternative with the
 391    alternative we want to delete and then shrinking the vector, which
 392    is consistent with how we remove an edge from the edge vector.  */
 393
 394 static void
 395 remove_phi_arg_num (gphi *phi, int i)
 396 {
 397   int num_elem = gimple_phi_num_args (phi);
 398
 399   gcc_assert (i < num_elem);
 400
 401   /* Delink the item which is being removed.  */
 402   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 403
 404   /* If it is not the last element, move the last element
 405      to the element we want to delete, resetting all the links. */
 406   if (i != num_elem - 1)
 407     {
 408       use_operand_p old_p, new_p;
 409       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 410       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 411       /* Set use on new node, and link into last element's place.  */
 412       *(new_p->use) = *(old_p->use);
 413       relink_imm_use (new_p, old_p);
 414       /* Move the location as well.  */
 415       gimple_phi_arg_set_location (phi, i,
 416                                    gimple_phi_arg_location (phi, num_elem - 1));
 417     }
 418
 419   /* Shrink the vector and return.  Note that we do not have to clear
 420      PHI_ARG_DEF because the garbage collector will not look at those
 421      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 422   phi->nargs--;
 423 }
 424
 425
 426 /* Remove all PHI arguments associated with edge E.  */
 427
 428 void
 429 remove_phi_args (edge e)
 430 {
 431   gphi_iterator gsi;
 432
 433   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 434     remove_phi_arg_num (gsi.phi (),
 435                         e->dest_idx);
 436 }
 437
 438
 439 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 440    removal, iterator GSI is updated to point to the next PHI node in the
 441    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 442    into the free pool of SSA names.  */
 443
 444 void
 445 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 446 {
 447   gimple *phi = gsi_stmt (*gsi);
 448
 449   if (release_lhs_p)
 450     insert_debug_temps_for_defs (gsi);
 451
 452   gsi_remove (gsi, false);
 453
 454   /* If we are deleting the PHI node, then we should release the
 455      SSA_NAME node so that it can be reused.  */
 456   if (release_lhs_p)
 457     release_ssa_name (gimple_phi_result (phi));
 458   release_phi_node (phi);
 459 }
 460
 461 /* Remove all the phi nodes from BB.  */
 462
 463 void
 464 remove_phi_nodes (basic_block bb)
 465 {
 466   gphi_iterator gsi;
 467
 468   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 469     remove_phi_node (&gsi, true);
 470
 471   set_phi_nodes (bb, NULL);
 472 }
 473
 474 /* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
 475    NULL.  */
 476
 477 tree
 478 degenerate_phi_result (gphi *phi)
 479 {
 480   tree lhs = gimple_phi_result (phi);
 481   tree val = NULL;
 482   size_t i;
 483
 484   /* Ignoring arguments which are the same as LHS, if all the remaining
 485      arguments are the same, then the PHI is a degenerate and has the
 486      value of that common argument.  */
 487   for (i = 0; i < gimple_phi_num_args (phi); i++)
 488     {
 489       tree arg = gimple_phi_arg_def (phi, i);
 490
 491       if (arg == lhs)
 492         continue;
 493       else if (!arg)
 494         break;
 495       else if (!val)
 496         val = arg;
 497       else if (arg == val)
 498         continue;
 499       /* We bring in some of operand_equal_p not only to speed things
 500          up, but also to avoid crashing when dereferencing the type of
 501          a released SSA name.  */
 502       else if (TREE_CODE (val) != TREE_CODE (arg)
 503                || TREE_CODE (val) == SSA_NAME
 504                || !operand_equal_p (arg, val, 0))
 505         break;
 506     }
 507   return (i == gimple_phi_num_args (phi) ? val : NULL);
 508 }
 509
 510 /* Set PHI nodes of a basic block BB to SEQ.  */
 511
 512 void
 513 set_phi_nodes (basic_block bb, gimple_seq seq)
 514 {
 515   gimple_stmt_iterator i;
 516
 517   gcc_checking_assert (!(bb->flags & BB_RTL));
 518   bb->il.gimple.phi_nodes = seq;
 519   if (seq)
 520     for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (&i))
 521       gimple_set_bb (gsi_stmt (i), bb);
 522 }
 523
 524 #include "gt-tree-phinodes.h"