gcc/tree-ssa-sink.c

   1 /* Code sinking for trees
   2    Copyright (C) 2001-2017 Free Software Foundation, Inc.
   3    Contributed by Daniel Berlin <dan@dberlin.org>
   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 "backend.h"
  25 #include "tree.h"
  26 #include "gimple.h"
  27 #include "cfghooks.h"
  28 #include "tree-pass.h"
  29 #include "ssa.h"
  30 #include "gimple-pretty-print.h"
  31 #include "fold-const.h"
  32 #include "stor-layout.h"
  33 #include "cfganal.h"
  34 #include "gimple-iterator.h"
  35 #include "tree-cfg.h"
  36 #include "cfgloop.h"
  37 #include "params.h"
  38
  39 /* TODO:
  40    1. Sinking store only using scalar promotion (IE without moving the RHS):
  41
  42    *q = p;
  43    p = p + 1;
  44    if (something)
  45      *q = <not p>;
  46    else
  47      y = *q;
  48
  49
  50    should become
  51    sinktemp = p;
  52    p = p + 1;
  53    if (something)
  54      *q = <not p>;
  55    else
  56    {
  57      *q = sinktemp;
  58      y = *q
  59    }
  60    Store copy propagation will take care of the store elimination above.
  61
  62
  63    2. Sinking using Partial Dead Code Elimination.  */
  64
  65
  66 static struct
  67 {
  68   /* The number of statements sunk down the flowgraph by code sinking.  */
  69   int sunk;
  70
  71 } sink_stats;
  72
  73
  74 /* Given a PHI, and one of its arguments (DEF), find the edge for
  75    that argument and return it.  If the argument occurs twice in the PHI node,
  76    we return NULL.  */
  77
  78 static basic_block
  79 find_bb_for_arg (gphi *phi, tree def)
  80 {
  81   size_t i;
  82   bool foundone = false;
  83   basic_block result = NULL;
  84   for (i = 0; i < gimple_phi_num_args (phi); i++)
  85     if (PHI_ARG_DEF (phi, i) == def)
  86       {
  87         if (foundone)
  88           return NULL;
  89         foundone = true;
  90         result = gimple_phi_arg_edge (phi, i)->src;
  91       }
  92   return result;
  93 }
  94
  95 /* When the first immediate use is in a statement, then return true if all
  96    immediate uses in IMM are in the same statement.
  97    We could also do the case where  the first immediate use is in a phi node,
  98    and all the other uses are in phis in the same basic block, but this
  99    requires some expensive checking later (you have to make sure no def/vdef
 100    in the statement occurs for multiple edges in the various phi nodes it's
 101    used in, so that you only have one place you can sink it to.  */
 102
 103 static bool
 104 all_immediate_uses_same_place (def_operand_p def_p)
 105 {
 106   tree var = DEF_FROM_PTR (def_p);
 107   imm_use_iterator imm_iter;
 108   use_operand_p use_p;
 109
 110   gimple *firstuse = NULL;
 111   FOR_EACH_IMM_USE_FAST (use_p, imm_iter, var)
 112     {
 113       if (is_gimple_debug (USE_STMT (use_p)))
 114         continue;
 115       if (firstuse == NULL)
 116         firstuse = USE_STMT (use_p);
 117       else
 118         if (firstuse != USE_STMT (use_p))
 119           return false;
 120     }
 121
 122   return true;
 123 }
 124
 125 /* Find the nearest common dominator of all of the immediate uses in IMM.  */
 126
 127 static basic_block
 128 nearest_common_dominator_of_uses (def_operand_p def_p, bool *debug_stmts)
 129 {
 130   tree var = DEF_FROM_PTR (def_p);
 131   bitmap blocks = BITMAP_ALLOC (NULL);
 132   basic_block commondom;
 133   unsigned int j;
 134   bitmap_iterator bi;
 135   imm_use_iterator imm_iter;
 136   use_operand_p use_p;
 137
 138   FOR_EACH_IMM_USE_FAST (use_p, imm_iter, var)
 139     {
 140       gimple *usestmt = USE_STMT (use_p);
 141       basic_block useblock;
 142
 143       if (gphi *phi = dyn_cast <gphi *> (usestmt))
 144         {
 145           int idx = PHI_ARG_INDEX_FROM_USE (use_p);
 146
 147           useblock = gimple_phi_arg_edge (phi, idx)->src;
 148         }
 149       else if (is_gimple_debug (usestmt))
 150         {
 151           *debug_stmts = true;
 152           continue;
 153         }
 154       else
 155         {
 156           useblock = gimple_bb (usestmt);
 157         }
 158
 159       /* Short circuit. Nothing dominates the entry block.  */
 160       if (useblock == ENTRY_BLOCK_PTR_FOR_FN (cfun))
 161         {
 162           BITMAP_FREE (blocks);
 163           return NULL;
 164         }
 165       bitmap_set_bit (blocks, useblock->index);
 166     }
 167   commondom = BASIC_BLOCK_FOR_FN (cfun, bitmap_first_set_bit (blocks));
 168   EXECUTE_IF_SET_IN_BITMAP (blocks, 0, j, bi)
 169     commondom = nearest_common_dominator (CDI_DOMINATORS, commondom,
 170                                           BASIC_BLOCK_FOR_FN (cfun, j));
 171   BITMAP_FREE (blocks);
 172   return commondom;
 173 }
 174
 175 /* Given EARLY_BB and LATE_BB, two blocks in a path through the dominator
 176    tree, return the best basic block between them (inclusive) to place
 177    statements.
 178
 179    We want the most control dependent block in the shallowest loop nest.
 180
 181    If the resulting block is in a shallower loop nest, then use it.  Else
 182    only use the resulting block if it has significantly lower execution
 183    frequency than EARLY_BB to avoid gratutious statement movement.  We
 184    consider statements with VOPS more desirable to move.
 185
 186    This pass would obviously benefit from PDO as it utilizes block
 187    frequencies.  It would also benefit from recomputing frequencies
 188    if profile data is not available since frequencies often get out
 189    of sync with reality.  */
 190
 191 static basic_block
 192 select_best_block (basic_block early_bb,
 193                    basic_block late_bb,
 194                    gimple *stmt)
 195 {
 196   basic_block best_bb = late_bb;
 197   basic_block temp_bb = late_bb;
 198   int threshold;
 199
 200   while (temp_bb != early_bb)
 201     {
 202       /* If we've moved into a lower loop nest, then that becomes
 203          our best block.  */
 204       if (bb_loop_depth (temp_bb) < bb_loop_depth (best_bb))
 205         best_bb = temp_bb;
 206
 207       /* Walk up the dominator tree, hopefully we'll find a shallower
 208          loop nest.  */
 209       temp_bb = get_immediate_dominator (CDI_DOMINATORS, temp_bb);
 210     }
 211
 212   /* If we found a shallower loop nest, then we always consider that
 213      a win.  This will always give us the most control dependent block
 214      within that loop nest.  */
 215   if (bb_loop_depth (best_bb) < bb_loop_depth (early_bb))
 216     return best_bb;
 217
 218   /* Get the sinking threshold.  If the statement to be moved has memory
 219      operands, then increase the threshold by 7% as those are even more
 220      profitable to avoid, clamping at 100%.  */
 221   threshold = PARAM_VALUE (PARAM_SINK_FREQUENCY_THRESHOLD);
 222   if (gimple_vuse (stmt) || gimple_vdef (stmt))
 223     {
 224       threshold += 7;
 225       if (threshold > 100)
 226         threshold = 100;
 227     }
 228
 229   /* If BEST_BB is at the same nesting level, then require it to have
 230      significantly lower execution frequency to avoid gratutious movement.  */
 231   if (bb_loop_depth (best_bb) == bb_loop_depth (early_bb)
 232       && best_bb->frequency < (early_bb->frequency * threshold / 100.0))
 233     return best_bb;
 234
 235   /* No better block found, so return EARLY_BB, which happens to be the
 236      statement's original block.  */
 237   return early_bb;
 238 }
 239
 240 /* Given a statement (STMT) and the basic block it is currently in (FROMBB),
 241    determine the location to sink the statement to, if any.
 242    Returns true if there is such location; in that case, TOGSI points to the
 243    statement before that STMT should be moved.  */
 244
 245 static bool
 246 statement_sink_location (gimple *stmt, basic_block frombb,
 247                          gimple_stmt_iterator *togsi)
 248 {
 249   gimple *use;
 250   use_operand_p one_use = NULL_USE_OPERAND_P;
 251   basic_block sinkbb;
 252   use_operand_p use_p;
 253   def_operand_p def_p;
 254   ssa_op_iter iter;
 255   imm_use_iterator imm_iter;
 256
 257   /* We only can sink assignments.  */
 258   if (!is_gimple_assign (stmt))
 259     return false;
 260
 261   /* We only can sink stmts with a single definition.  */
 262   def_p = single_ssa_def_operand (stmt, SSA_OP_ALL_DEFS);
 263   if (def_p == NULL_DEF_OPERAND_P)
 264     return false;
 265
 266   /* Return if there are no immediate uses of this stmt.  */
 267   if (has_zero_uses (DEF_FROM_PTR (def_p)))
 268     return false;
 269
 270   /* There are a few classes of things we can't or don't move, some because we
 271      don't have code to handle it, some because it's not profitable and some
 272      because it's not legal.
 273
 274      We can't sink things that may be global stores, at least not without
 275      calculating a lot more information, because we may cause it to no longer
 276      be seen by an external routine that needs it depending on where it gets
 277      moved to.
 278
 279      We can't sink statements that end basic blocks without splitting the
 280      incoming edge for the sink location to place it there.
 281
 282      We can't sink statements that have volatile operands.
 283
 284      We don't want to sink dead code, so anything with 0 immediate uses is not
 285      sunk.
 286
 287      Don't sink BLKmode assignments if current function has any local explicit
 288      register variables, as BLKmode assignments may involve memcpy or memset
 289      calls or, on some targets, inline expansion thereof that sometimes need
 290      to use specific hard registers.
 291
 292   */
 293   if (stmt_ends_bb_p (stmt)
 294       || gimple_has_side_effects (stmt)
 295       || gimple_has_volatile_ops (stmt)
 296       || (cfun->has_local_explicit_reg_vars
 297           && TYPE_MODE (TREE_TYPE (gimple_assign_lhs (stmt))) == BLKmode))
 298     return false;
 299
 300   if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (DEF_FROM_PTR (def_p)))
 301     return false;
 302
 303   FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_ALL_USES)
 304     {
 305       tree use = USE_FROM_PTR (use_p);
 306       if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (use))
 307         return false;
 308     }
 309
 310   use = NULL;
 311
 312   /* If stmt is a store the one and only use needs to be the VOP
 313      merging PHI node.  */
 314   if (virtual_operand_p (DEF_FROM_PTR (def_p)))
 315     {
 316       FOR_EACH_IMM_USE_FAST (use_p, imm_iter, DEF_FROM_PTR (def_p))
 317         {
 318           gimple *use_stmt = USE_STMT (use_p);
 319
 320           /* A killing definition is not a use.  */
 321           if ((gimple_has_lhs (use_stmt)
 322                && operand_equal_p (gimple_assign_lhs (stmt),
 323                                    gimple_get_lhs (use_stmt), 0))
 324               || stmt_kills_ref_p (use_stmt, gimple_assign_lhs (stmt)))
 325             {
 326               /* If use_stmt is or might be a nop assignment then USE_STMT
 327                  acts as a use as well as definition.  */
 328               if (stmt != use_stmt
 329                   && ref_maybe_used_by_stmt_p (use_stmt,
 330                                                gimple_assign_lhs (stmt)))
 331                 return false;
 332               continue;
 333             }
 334
 335           if (gimple_code (use_stmt) != GIMPLE_PHI)
 336             return false;
 337
 338           if (use
 339               && use != use_stmt)
 340             return false;
 341
 342           use = use_stmt;
 343         }
 344       if (!use)
 345         return false;
 346     }
 347   /* If all the immediate uses are not in the same place, find the nearest
 348      common dominator of all the immediate uses.  For PHI nodes, we have to
 349      find the nearest common dominator of all of the predecessor blocks, since
 350      that is where insertion would have to take place.  */
 351   else if (gimple_vuse (stmt)
 352            || !all_immediate_uses_same_place (def_p))
 353     {
 354       bool debug_stmts = false;
 355       basic_block commondom = nearest_common_dominator_of_uses (def_p,
 356                                                                 &debug_stmts);
 357
 358       if (commondom == frombb)
 359         return false;
 360
 361       /* If this is a load then do not sink past any stores.
 362          ???  This is overly simple but cheap.  We basically look
 363          for an existing load with the same VUSE in the path to one
 364          of the sink candidate blocks and we adjust commondom to the
 365          nearest to commondom.  */
 366       if (gimple_vuse (stmt))
 367         {
 368           /* Do not sink loads from hard registers.  */
 369           if (gimple_assign_single_p (stmt)
 370               && TREE_CODE (gimple_assign_rhs1 (stmt)) == VAR_DECL
 371               && DECL_HARD_REGISTER (gimple_assign_rhs1 (stmt)))
 372             return false;
 373
 374           imm_use_iterator imm_iter;
 375           use_operand_p use_p;
 376           basic_block found = NULL;
 377           FOR_EACH_IMM_USE_FAST (use_p, imm_iter, gimple_vuse (stmt))
 378             {
 379               gimple *use_stmt = USE_STMT (use_p);
 380               basic_block bb = gimple_bb (use_stmt);
 381               /* For PHI nodes the block we know sth about
 382                  is the incoming block with the use.  */
 383               if (gimple_code (use_stmt) == GIMPLE_PHI)
 384                 bb = EDGE_PRED (bb, PHI_ARG_INDEX_FROM_USE (use_p))->src;
 385               /* Any dominator of commondom would be ok with
 386                  adjusting commondom to that block.  */
 387               bb = nearest_common_dominator (CDI_DOMINATORS, bb, commondom);
 388               if (!found)
 389                 found = bb;
 390               else if (dominated_by_p (CDI_DOMINATORS, bb, found))
 391                 found = bb;
 392               /* If we can't improve, stop.  */
 393               if (found == commondom)
 394                 break;
 395             }
 396           commondom = found;
 397           if (commondom == frombb)
 398             return false;
 399         }
 400
 401       /* Our common dominator has to be dominated by frombb in order to be a
 402          trivially safe place to put this statement, since it has multiple
 403          uses.  */
 404       if (!dominated_by_p (CDI_DOMINATORS, commondom, frombb))
 405         return false;
 406
 407       commondom = select_best_block (frombb, commondom, stmt);
 408
 409       if (commondom == frombb)
 410         return false;
 411
 412       *togsi = gsi_after_labels (commondom);
 413
 414       return true;
 415     }
 416   else
 417     {
 418       FOR_EACH_IMM_USE_FAST (one_use, imm_iter, DEF_FROM_PTR (def_p))
 419         {
 420           if (is_gimple_debug (USE_STMT (one_use)))
 421             continue;
 422           break;
 423         }
 424       use = USE_STMT (one_use);
 425
 426       if (gimple_code (use) != GIMPLE_PHI)
 427         {
 428           sinkbb = gimple_bb (use);
 429           sinkbb = select_best_block (frombb, gimple_bb (use), stmt);
 430
 431           if (sinkbb == frombb)
 432             return false;
 433
 434           *togsi = gsi_for_stmt (use);
 435
 436           return true;
 437         }
 438     }
 439
 440   sinkbb = find_bb_for_arg (as_a <gphi *> (use), DEF_FROM_PTR (def_p));
 441
 442   /* This can happen if there are multiple uses in a PHI.  */
 443   if (!sinkbb)
 444     return false;
 445
 446   sinkbb = select_best_block (frombb, sinkbb, stmt);
 447   if (!sinkbb || sinkbb == frombb)
 448     return false;
 449
 450   /* If the latch block is empty, don't make it non-empty by sinking
 451      something into it.  */
 452   if (sinkbb == frombb->loop_father->latch
 453       && empty_block_p (sinkbb))
 454     return false;
 455
 456   *togsi = gsi_after_labels (sinkbb);
 457
 458   return true;
 459 }
 460
 461 /* Perform code sinking on BB */
 462
 463 static void
 464 sink_code_in_bb (basic_block bb)
 465 {
 466   basic_block son;
 467   gimple_stmt_iterator gsi;
 468   edge_iterator ei;
 469   edge e;
 470   bool last = true;
 471
 472   /* If this block doesn't dominate anything, there can't be any place to sink
 473      the statements to.  */
 474   if (first_dom_son (CDI_DOMINATORS, bb) == NULL)
 475     goto earlyout;
 476
 477   /* We can't move things across abnormal edges, so don't try.  */
 478   FOR_EACH_EDGE (e, ei, bb->succs)
 479     if (e->flags & EDGE_ABNORMAL)
 480       goto earlyout;
 481
 482   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
 483     {
 484       gimple *stmt = gsi_stmt (gsi);
 485       gimple_stmt_iterator togsi;
 486
 487       if (!statement_sink_location (stmt, bb, &togsi))
 488         {
 489           if (!gsi_end_p (gsi))
 490             gsi_prev (&gsi);
 491           last = false;
 492           continue;
 493         }
 494       if (dump_file)
 495         {
 496           fprintf (dump_file, "Sinking ");
 497           print_gimple_stmt (dump_file, stmt, 0, TDF_VOPS);
 498           fprintf (dump_file, " from bb %d to bb %d\n",
 499                    bb->index, (gsi_bb (togsi))->index);
 500         }
 501
 502       /* Update virtual operands of statements in the path we
 503          do not sink to.  */
 504       if (gimple_vdef (stmt))
 505         {
 506           imm_use_iterator iter;
 507           use_operand_p use_p;
 508           gimple *vuse_stmt;
 509
 510           FOR_EACH_IMM_USE_STMT (vuse_stmt, iter, gimple_vdef (stmt))
 511             if (gimple_code (vuse_stmt) != GIMPLE_PHI)
 512               FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
 513                 SET_USE (use_p, gimple_vuse (stmt));
 514         }
 515
 516       /* If this is the end of the basic block, we need to insert at the end
 517          of the basic block.  */
 518       if (gsi_end_p (togsi))
 519         gsi_move_to_bb_end (&gsi, gsi_bb (togsi));
 520       else
 521         gsi_move_before (&gsi, &togsi);
 522
 523       sink_stats.sunk++;
 524
 525       /* If we've just removed the last statement of the BB, the
 526          gsi_end_p() test below would fail, but gsi_prev() would have
 527          succeeded, and we want it to succeed.  So we keep track of
 528          whether we're at the last statement and pick up the new last
 529          statement.  */
 530       if (last)
 531         {
 532           gsi = gsi_last_bb (bb);
 533           continue;
 534         }
 535
 536       last = false;
 537       if (!gsi_end_p (gsi))
 538         gsi_prev (&gsi);
 539
 540     }
 541  earlyout:
 542   for (son = first_dom_son (CDI_POST_DOMINATORS, bb);
 543        son;
 544        son = next_dom_son (CDI_POST_DOMINATORS, son))
 545     {
 546       sink_code_in_bb (son);
 547     }
 548 }
 549
 550 /* Perform code sinking.
 551    This moves code down the flowgraph when we know it would be
 552    profitable to do so, or it wouldn't increase the number of
 553    executions of the statement.
 554
 555    IE given
 556
 557    a_1 = b + c;
 558    if (<something>)
 559    {
 560    }
 561    else
 562    {
 563      foo (&b, &c);
 564      a_5 = b + c;
 565    }
 566    a_6 = PHI (a_5, a_1);
 567    USE a_6.
 568
 569    we'll transform this into:
 570
 571    if (<something>)
 572    {
 573       a_1 = b + c;
 574    }
 575    else
 576    {
 577       foo (&b, &c);
 578       a_5 = b + c;
 579    }
 580    a_6 = PHI (a_5, a_1);
 581    USE a_6.
 582
 583    Note that this reduces the number of computations of a = b + c to 1
 584    when we take the else edge, instead of 2.
 585 */
 586 namespace {
 587
 588 const pass_data pass_data_sink_code =
 589 {
 590   GIMPLE_PASS, /* type */
 591   "sink", /* name */
 592   OPTGROUP_NONE, /* optinfo_flags */
 593   TV_TREE_SINK, /* tv_id */
 594   /* PROP_no_crit_edges is ensured by running split_critical_edges in
 595      pass_data_sink_code::execute ().  */
 596   ( PROP_cfg | PROP_ssa ), /* properties_required */
 597   0, /* properties_provided */
 598   0, /* properties_destroyed */
 599   0, /* todo_flags_start */
 600   TODO_update_ssa, /* todo_flags_finish */
 601 };
 602
 603 class pass_sink_code : public gimple_opt_pass
 604 {
 605 public:
 606   pass_sink_code (gcc::context *ctxt)
 607     : gimple_opt_pass (pass_data_sink_code, ctxt)
 608   {}
 609
 610   /* opt_pass methods: */
 611   virtual bool gate (function *) { return flag_tree_sink != 0; }
 612   virtual unsigned int execute (function *);
 613
 614 }; // class pass_sink_code
 615
 616 unsigned int
 617 pass_sink_code::execute (function *fun)
 618 {
 619   loop_optimizer_init (LOOPS_NORMAL);
 620   split_critical_edges ();
 621   connect_infinite_loops_to_exit ();
 622   memset (&sink_stats, 0, sizeof (sink_stats));
 623   calculate_dominance_info (CDI_DOMINATORS);
 624   calculate_dominance_info (CDI_POST_DOMINATORS);
 625   sink_code_in_bb (EXIT_BLOCK_PTR_FOR_FN (fun));
 626   statistics_counter_event (fun, "Sunk statements", sink_stats.sunk);
 627   free_dominance_info (CDI_POST_DOMINATORS);
 628   remove_fake_exit_edges ();
 629   loop_optimizer_finalize ();
 630
 631   return 0;
 632 }
 633
 634 } // anon namespace
 635
 636 gimple_opt_pass *
 637 make_pass_sink_code (gcc::context *ctxt)
 638 {
 639   return new pass_sink_code (ctxt);
 640 }