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, bool *zero_uses_p)
 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   *zero_uses_p = false;
 258
 259   /* We only can sink assignments and non-looping const/pure calls.  */
 260   int cf;
 261   if (!is_gimple_assign (stmt)
 262       && (!is_gimple_call (stmt)
 263           || !((cf = gimple_call_flags (stmt)) & (ECF_CONST|ECF_PURE))
 264           || (cf & ECF_LOOPING_CONST_OR_PURE)))
 265     return false;
 266
 267   /* We only can sink stmts with a single definition.  */
 268   def_p = single_ssa_def_operand (stmt, SSA_OP_ALL_DEFS);
 269   if (def_p == NULL_DEF_OPERAND_P)
 270     return false;
 271
 272   /* There are a few classes of things we can't or don't move, some because we
 273      don't have code to handle it, some because it's not profitable and some
 274      because it's not legal.
 275
 276      We can't sink things that may be global stores, at least not without
 277      calculating a lot more information, because we may cause it to no longer
 278      be seen by an external routine that needs it depending on where it gets
 279      moved to.
 280
 281      We can't sink statements that end basic blocks without splitting the
 282      incoming edge for the sink location to place it there.
 283
 284      We can't sink statements that have volatile operands.
 285
 286      We don't want to sink dead code, so anything with 0 immediate uses is not
 287      sunk.
 288
 289      Don't sink BLKmode assignments if current function has any local explicit
 290      register variables, as BLKmode assignments may involve memcpy or memset
 291      calls or, on some targets, inline expansion thereof that sometimes need
 292      to use specific hard registers.
 293
 294   */
 295   if (stmt_ends_bb_p (stmt)
 296       || gimple_has_side_effects (stmt)
 297       || (cfun->has_local_explicit_reg_vars
 298           && TYPE_MODE (TREE_TYPE (gimple_get_lhs (stmt))) == BLKmode))
 299     return false;
 300
 301   /* Return if there are no immediate uses of this stmt.  */
 302   if (has_zero_uses (DEF_FROM_PTR (def_p)))
 303     {
 304       *zero_uses_p = true;
 305       return false;
 306     }
 307
 308   if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (DEF_FROM_PTR (def_p)))
 309     return false;
 310
 311   FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_ALL_USES)
 312     {
 313       tree use = USE_FROM_PTR (use_p);
 314       if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (use))
 315         return false;
 316     }
 317
 318   use = NULL;
 319
 320   /* If stmt is a store the one and only use needs to be the VOP
 321      merging PHI node.  */
 322   if (virtual_operand_p (DEF_FROM_PTR (def_p)))
 323     {
 324       FOR_EACH_IMM_USE_FAST (use_p, imm_iter, DEF_FROM_PTR (def_p))
 325         {
 326           gimple *use_stmt = USE_STMT (use_p);
 327
 328           /* A killing definition is not a use.  */
 329           if ((gimple_has_lhs (use_stmt)
 330                && operand_equal_p (gimple_get_lhs (stmt),
 331                                    gimple_get_lhs (use_stmt), 0))
 332               || stmt_kills_ref_p (use_stmt, gimple_get_lhs (stmt)))
 333             {
 334               /* If use_stmt is or might be a nop assignment then USE_STMT
 335                  acts as a use as well as definition.  */
 336               if (stmt != use_stmt
 337                   && ref_maybe_used_by_stmt_p (use_stmt,
 338                                                gimple_get_lhs (stmt)))
 339                 return false;
 340               continue;
 341             }
 342
 343           if (gimple_code (use_stmt) != GIMPLE_PHI)
 344             return false;
 345
 346           if (use
 347               && use != use_stmt)
 348             return false;
 349
 350           use = use_stmt;
 351         }
 352       if (!use)
 353         return false;
 354     }
 355   /* If all the immediate uses are not in the same place, find the nearest
 356      common dominator of all the immediate uses.  For PHI nodes, we have to
 357      find the nearest common dominator of all of the predecessor blocks, since
 358      that is where insertion would have to take place.  */
 359   else if (gimple_vuse (stmt)
 360            || !all_immediate_uses_same_place (def_p))
 361     {
 362       bool debug_stmts = false;
 363       basic_block commondom = nearest_common_dominator_of_uses (def_p,
 364                                                                 &debug_stmts);
 365
 366       if (commondom == frombb)
 367         return false;
 368
 369       /* If this is a load then do not sink past any stores.
 370          ???  This is overly simple but cheap.  We basically look
 371          for an existing load with the same VUSE in the path to one
 372          of the sink candidate blocks and we adjust commondom to the
 373          nearest to commondom.  */
 374       if (gimple_vuse (stmt))
 375         {
 376           /* Do not sink loads from hard registers.  */
 377           if (gimple_assign_single_p (stmt)
 378               && TREE_CODE (gimple_assign_rhs1 (stmt)) == VAR_DECL
 379               && DECL_HARD_REGISTER (gimple_assign_rhs1 (stmt)))
 380             return false;
 381
 382           imm_use_iterator imm_iter;
 383           use_operand_p use_p;
 384           basic_block found = NULL;
 385           FOR_EACH_IMM_USE_FAST (use_p, imm_iter, gimple_vuse (stmt))
 386             {
 387               gimple *use_stmt = USE_STMT (use_p);
 388               basic_block bb = gimple_bb (use_stmt);
 389               /* For PHI nodes the block we know sth about
 390                  is the incoming block with the use.  */
 391               if (gimple_code (use_stmt) == GIMPLE_PHI)
 392                 bb = EDGE_PRED (bb, PHI_ARG_INDEX_FROM_USE (use_p))->src;
 393               /* Any dominator of commondom would be ok with
 394                  adjusting commondom to that block.  */
 395               bb = nearest_common_dominator (CDI_DOMINATORS, bb, commondom);
 396               if (!found)
 397                 found = bb;
 398               else if (dominated_by_p (CDI_DOMINATORS, bb, found))
 399                 found = bb;
 400               /* If we can't improve, stop.  */
 401               if (found == commondom)
 402                 break;
 403             }
 404           commondom = found;
 405           if (commondom == frombb)
 406             return false;
 407         }
 408
 409       /* Our common dominator has to be dominated by frombb in order to be a
 410          trivially safe place to put this statement, since it has multiple
 411          uses.  */
 412       if (!dominated_by_p (CDI_DOMINATORS, commondom, frombb))
 413         return false;
 414
 415       commondom = select_best_block (frombb, commondom, stmt);
 416
 417       if (commondom == frombb)
 418         return false;
 419
 420       *togsi = gsi_after_labels (commondom);
 421
 422       return true;
 423     }
 424   else
 425     {
 426       FOR_EACH_IMM_USE_FAST (one_use, imm_iter, DEF_FROM_PTR (def_p))
 427         {
 428           if (is_gimple_debug (USE_STMT (one_use)))
 429             continue;
 430           break;
 431         }
 432       use = USE_STMT (one_use);
 433
 434       if (gimple_code (use) != GIMPLE_PHI)
 435         {
 436           sinkbb = gimple_bb (use);
 437           sinkbb = select_best_block (frombb, gimple_bb (use), stmt);
 438
 439           if (sinkbb == frombb)
 440             return false;
 441
 442           *togsi = gsi_for_stmt (use);
 443
 444           return true;
 445         }
 446     }
 447
 448   sinkbb = find_bb_for_arg (as_a <gphi *> (use), DEF_FROM_PTR (def_p));
 449
 450   /* This can happen if there are multiple uses in a PHI.  */
 451   if (!sinkbb)
 452     return false;
 453
 454   sinkbb = select_best_block (frombb, sinkbb, stmt);
 455   if (!sinkbb || sinkbb == frombb)
 456     return false;
 457
 458   /* If the latch block is empty, don't make it non-empty by sinking
 459      something into it.  */
 460   if (sinkbb == frombb->loop_father->latch
 461       && empty_block_p (sinkbb))
 462     return false;
 463
 464   *togsi = gsi_after_labels (sinkbb);
 465
 466   return true;
 467 }
 468
 469 /* Perform code sinking on BB */
 470
 471 static void
 472 sink_code_in_bb (basic_block bb)
 473 {
 474   basic_block son;
 475   gimple_stmt_iterator gsi;
 476   edge_iterator ei;
 477   edge e;
 478   bool last = true;
 479
 480   /* If this block doesn't dominate anything, there can't be any place to sink
 481      the statements to.  */
 482   if (first_dom_son (CDI_DOMINATORS, bb) == NULL)
 483     goto earlyout;
 484
 485   /* We can't move things across abnormal edges, so don't try.  */
 486   FOR_EACH_EDGE (e, ei, bb->succs)
 487     if (e->flags & EDGE_ABNORMAL)
 488       goto earlyout;
 489
 490   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
 491     {
 492       gimple *stmt = gsi_stmt (gsi);
 493       gimple_stmt_iterator togsi;
 494       bool zero_uses_p;
 495
 496       if (!statement_sink_location (stmt, bb, &togsi, &zero_uses_p))
 497         {
 498           gimple_stmt_iterator saved = gsi;
 499           if (!gsi_end_p (gsi))
 500             gsi_prev (&gsi);
 501           /* If we face a dead stmt remove it as it possibly blocks
 502              sinking of uses.  */
 503           if (zero_uses_p
 504               && ! gimple_vdef (stmt))
 505             {
 506               gsi_remove (&saved, true);
 507               release_defs (stmt);
 508             }
 509           else
 510             last = false;
 511           continue;
 512         }
 513       if (dump_file)
 514         {
 515           fprintf (dump_file, "Sinking ");
 516           print_gimple_stmt (dump_file, stmt, 0, TDF_VOPS);
 517           fprintf (dump_file, " from bb %d to bb %d\n",
 518                    bb->index, (gsi_bb (togsi))->index);
 519         }
 520
 521       /* Update virtual operands of statements in the path we
 522          do not sink to.  */
 523       if (gimple_vdef (stmt))
 524         {
 525           imm_use_iterator iter;
 526           use_operand_p use_p;
 527           gimple *vuse_stmt;
 528
 529           FOR_EACH_IMM_USE_STMT (vuse_stmt, iter, gimple_vdef (stmt))
 530             if (gimple_code (vuse_stmt) != GIMPLE_PHI)
 531               FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
 532                 SET_USE (use_p, gimple_vuse (stmt));
 533         }
 534
 535       /* If this is the end of the basic block, we need to insert at the end
 536          of the basic block.  */
 537       if (gsi_end_p (togsi))
 538         gsi_move_to_bb_end (&gsi, gsi_bb (togsi));
 539       else
 540         gsi_move_before (&gsi, &togsi);
 541
 542       sink_stats.sunk++;
 543
 544       /* If we've just removed the last statement of the BB, the
 545          gsi_end_p() test below would fail, but gsi_prev() would have
 546          succeeded, and we want it to succeed.  So we keep track of
 547          whether we're at the last statement and pick up the new last
 548          statement.  */
 549       if (last)
 550         {
 551           gsi = gsi_last_bb (bb);
 552           continue;
 553         }
 554
 555       last = false;
 556       if (!gsi_end_p (gsi))
 557         gsi_prev (&gsi);
 558
 559     }
 560  earlyout:
 561   for (son = first_dom_son (CDI_POST_DOMINATORS, bb);
 562        son;
 563        son = next_dom_son (CDI_POST_DOMINATORS, son))
 564     {
 565       sink_code_in_bb (son);
 566     }
 567 }
 568
 569 /* Perform code sinking.
 570    This moves code down the flowgraph when we know it would be
 571    profitable to do so, or it wouldn't increase the number of
 572    executions of the statement.
 573
 574    IE given
 575
 576    a_1 = b + c;
 577    if (<something>)
 578    {
 579    }
 580    else
 581    {
 582      foo (&b, &c);
 583      a_5 = b + c;
 584    }
 585    a_6 = PHI (a_5, a_1);
 586    USE a_6.
 587
 588    we'll transform this into:
 589
 590    if (<something>)
 591    {
 592       a_1 = b + c;
 593    }
 594    else
 595    {
 596       foo (&b, &c);
 597       a_5 = b + c;
 598    }
 599    a_6 = PHI (a_5, a_1);
 600    USE a_6.
 601
 602    Note that this reduces the number of computations of a = b + c to 1
 603    when we take the else edge, instead of 2.
 604 */
 605 namespace {
 606
 607 const pass_data pass_data_sink_code =
 608 {
 609   GIMPLE_PASS, /* type */
 610   "sink", /* name */
 611   OPTGROUP_NONE, /* optinfo_flags */
 612   TV_TREE_SINK, /* tv_id */
 613   /* PROP_no_crit_edges is ensured by running split_critical_edges in
 614      pass_data_sink_code::execute ().  */
 615   ( PROP_cfg | PROP_ssa ), /* properties_required */
 616   0, /* properties_provided */
 617   0, /* properties_destroyed */
 618   0, /* todo_flags_start */
 619   TODO_update_ssa, /* todo_flags_finish */
 620 };
 621
 622 class pass_sink_code : public gimple_opt_pass
 623 {
 624 public:
 625   pass_sink_code (gcc::context *ctxt)
 626     : gimple_opt_pass (pass_data_sink_code, ctxt)
 627   {}
 628
 629   /* opt_pass methods: */
 630   virtual bool gate (function *) { return flag_tree_sink != 0; }
 631   virtual unsigned int execute (function *);
 632
 633 }; // class pass_sink_code
 634
 635 unsigned int
 636 pass_sink_code::execute (function *fun)
 637 {
 638   loop_optimizer_init (LOOPS_NORMAL);
 639   split_critical_edges ();
 640   connect_infinite_loops_to_exit ();
 641   memset (&sink_stats, 0, sizeof (sink_stats));
 642   calculate_dominance_info (CDI_DOMINATORS);
 643   calculate_dominance_info (CDI_POST_DOMINATORS);
 644   sink_code_in_bb (EXIT_BLOCK_PTR_FOR_FN (fun));
 645   statistics_counter_event (fun, "Sunk statements", sink_stats.sunk);
 646   free_dominance_info (CDI_POST_DOMINATORS);
 647   remove_fake_exit_edges ();
 648   loop_optimizer_finalize ();
 649
 650   return 0;
 651 }
 652
 653 } // anon namespace
 654
 655 gimple_opt_pass *
 656 make_pass_sink_code (gcc::context *ctxt)
 657 {
 658   return new pass_sink_code (ctxt);
 659 }