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   auto_bitmap blocks;
 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         return NULL;
 162
 163       bitmap_set_bit (blocks, useblock->index);
 164     }
 165   commondom = BASIC_BLOCK_FOR_FN (cfun, bitmap_first_set_bit (blocks));
 166   EXECUTE_IF_SET_IN_BITMAP (blocks, 0, j, bi)
 167     commondom = nearest_common_dominator (CDI_DOMINATORS, commondom,
 168                                           BASIC_BLOCK_FOR_FN (cfun, j));
 169   return commondom;
 170 }
 171
 172 /* Given EARLY_BB and LATE_BB, two blocks in a path through the dominator
 173    tree, return the best basic block between them (inclusive) to place
 174    statements.
 175
 176    We want the most control dependent block in the shallowest loop nest.
 177
 178    If the resulting block is in a shallower loop nest, then use it.  Else
 179    only use the resulting block if it has significantly lower execution
 180    frequency than EARLY_BB to avoid gratutious statement movement.  We
 181    consider statements with VOPS more desirable to move.
 182
 183    This pass would obviously benefit from PDO as it utilizes block
 184    frequencies.  It would also benefit from recomputing frequencies
 185    if profile data is not available since frequencies often get out
 186    of sync with reality.  */
 187
 188 static basic_block
 189 select_best_block (basic_block early_bb,
 190                    basic_block late_bb,
 191                    gimple *stmt)
 192 {
 193   basic_block best_bb = late_bb;
 194   basic_block temp_bb = late_bb;
 195   int threshold;
 196
 197   while (temp_bb != early_bb)
 198     {
 199       /* If we've moved into a lower loop nest, then that becomes
 200          our best block.  */
 201       if (bb_loop_depth (temp_bb) < bb_loop_depth (best_bb))
 202         best_bb = temp_bb;
 203
 204       /* Walk up the dominator tree, hopefully we'll find a shallower
 205          loop nest.  */
 206       temp_bb = get_immediate_dominator (CDI_DOMINATORS, temp_bb);
 207     }
 208
 209   /* If we found a shallower loop nest, then we always consider that
 210      a win.  This will always give us the most control dependent block
 211      within that loop nest.  */
 212   if (bb_loop_depth (best_bb) < bb_loop_depth (early_bb))
 213     return best_bb;
 214
 215   /* Get the sinking threshold.  If the statement to be moved has memory
 216      operands, then increase the threshold by 7% as those are even more
 217      profitable to avoid, clamping at 100%.  */
 218   threshold = PARAM_VALUE (PARAM_SINK_FREQUENCY_THRESHOLD);
 219   if (gimple_vuse (stmt) || gimple_vdef (stmt))
 220     {
 221       threshold += 7;
 222       if (threshold > 100)
 223         threshold = 100;
 224     }
 225
 226   /* If BEST_BB is at the same nesting level, then require it to have
 227      significantly lower execution frequency to avoid gratutious movement.  */
 228   if (bb_loop_depth (best_bb) == bb_loop_depth (early_bb)
 229       && best_bb->frequency < (early_bb->frequency * threshold / 100.0))
 230     return best_bb;
 231
 232   /* No better block found, so return EARLY_BB, which happens to be the
 233      statement's original block.  */
 234   return early_bb;
 235 }
 236
 237 /* Given a statement (STMT) and the basic block it is currently in (FROMBB),
 238    determine the location to sink the statement to, if any.
 239    Returns true if there is such location; in that case, TOGSI points to the
 240    statement before that STMT should be moved.  */
 241
 242 static bool
 243 statement_sink_location (gimple *stmt, basic_block frombb,
 244                          gimple_stmt_iterator *togsi, bool *zero_uses_p)
 245 {
 246   gimple *use;
 247   use_operand_p one_use = NULL_USE_OPERAND_P;
 248   basic_block sinkbb;
 249   use_operand_p use_p;
 250   def_operand_p def_p;
 251   ssa_op_iter iter;
 252   imm_use_iterator imm_iter;
 253
 254   *zero_uses_p = false;
 255
 256   /* We only can sink assignments and non-looping const/pure calls.  */
 257   int cf;
 258   if (!is_gimple_assign (stmt)
 259       && (!is_gimple_call (stmt)
 260           || !((cf = gimple_call_flags (stmt)) & (ECF_CONST|ECF_PURE))
 261           || (cf & ECF_LOOPING_CONST_OR_PURE)))
 262     return false;
 263
 264   /* We only can sink stmts with a single definition.  */
 265   def_p = single_ssa_def_operand (stmt, SSA_OP_ALL_DEFS);
 266   if (def_p == NULL_DEF_OPERAND_P)
 267     return false;
 268
 269   /* There are a few classes of things we can't or don't move, some because we
 270      don't have code to handle it, some because it's not profitable and some
 271      because it's not legal.
 272
 273      We can't sink things that may be global stores, at least not without
 274      calculating a lot more information, because we may cause it to no longer
 275      be seen by an external routine that needs it depending on where it gets
 276      moved to.
 277
 278      We can't sink statements that end basic blocks without splitting the
 279      incoming edge for the sink location to place it there.
 280
 281      We can't sink statements that have volatile operands.
 282
 283      We don't want to sink dead code, so anything with 0 immediate uses is not
 284      sunk.
 285
 286      Don't sink BLKmode assignments if current function has any local explicit
 287      register variables, as BLKmode assignments may involve memcpy or memset
 288      calls or, on some targets, inline expansion thereof that sometimes need
 289      to use specific hard registers.
 290
 291   */
 292   if (stmt_ends_bb_p (stmt)
 293       || gimple_has_side_effects (stmt)
 294       || (cfun->has_local_explicit_reg_vars
 295           && TYPE_MODE (TREE_TYPE (gimple_get_lhs (stmt))) == BLKmode))
 296     return false;
 297
 298   /* Return if there are no immediate uses of this stmt.  */
 299   if (has_zero_uses (DEF_FROM_PTR (def_p)))
 300     {
 301       *zero_uses_p = true;
 302       return false;
 303     }
 304
 305   if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (DEF_FROM_PTR (def_p)))
 306     return false;
 307
 308   FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_ALL_USES)
 309     {
 310       tree use = USE_FROM_PTR (use_p);
 311       if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (use))
 312         return false;
 313     }
 314
 315   use = NULL;
 316
 317   /* If stmt is a store the one and only use needs to be the VOP
 318      merging PHI node.  */
 319   if (virtual_operand_p (DEF_FROM_PTR (def_p)))
 320     {
 321       FOR_EACH_IMM_USE_FAST (use_p, imm_iter, DEF_FROM_PTR (def_p))
 322         {
 323           gimple *use_stmt = USE_STMT (use_p);
 324
 325           /* A killing definition is not a use.  */
 326           if ((gimple_has_lhs (use_stmt)
 327                && operand_equal_p (gimple_get_lhs (stmt),
 328                                    gimple_get_lhs (use_stmt), 0))
 329               || stmt_kills_ref_p (use_stmt, gimple_get_lhs (stmt)))
 330             {
 331               /* If use_stmt is or might be a nop assignment then USE_STMT
 332                  acts as a use as well as definition.  */
 333               if (stmt != use_stmt
 334                   && ref_maybe_used_by_stmt_p (use_stmt,
 335                                                gimple_get_lhs (stmt)))
 336                 return false;
 337               continue;
 338             }
 339
 340           if (gimple_code (use_stmt) != GIMPLE_PHI)
 341             return false;
 342
 343           if (use
 344               && use != use_stmt)
 345             return false;
 346
 347           use = use_stmt;
 348         }
 349       if (!use)
 350         return false;
 351     }
 352   /* If all the immediate uses are not in the same place, find the nearest
 353      common dominator of all the immediate uses.  For PHI nodes, we have to
 354      find the nearest common dominator of all of the predecessor blocks, since
 355      that is where insertion would have to take place.  */
 356   else if (gimple_vuse (stmt)
 357            || !all_immediate_uses_same_place (def_p))
 358     {
 359       bool debug_stmts = false;
 360       basic_block commondom = nearest_common_dominator_of_uses (def_p,
 361                                                                 &debug_stmts);
 362
 363       if (commondom == frombb)
 364         return false;
 365
 366       /* If this is a load then do not sink past any stores.
 367          ???  This is overly simple but cheap.  We basically look
 368          for an existing load with the same VUSE in the path to one
 369          of the sink candidate blocks and we adjust commondom to the
 370          nearest to commondom.  */
 371       if (gimple_vuse (stmt))
 372         {
 373           /* Do not sink loads from hard registers.  */
 374           if (gimple_assign_single_p (stmt)
 375               && TREE_CODE (gimple_assign_rhs1 (stmt)) == VAR_DECL
 376               && DECL_HARD_REGISTER (gimple_assign_rhs1 (stmt)))
 377             return false;
 378
 379           imm_use_iterator imm_iter;
 380           use_operand_p use_p;
 381           basic_block found = NULL;
 382           FOR_EACH_IMM_USE_FAST (use_p, imm_iter, gimple_vuse (stmt))
 383             {
 384               gimple *use_stmt = USE_STMT (use_p);
 385               basic_block bb = gimple_bb (use_stmt);
 386               /* For PHI nodes the block we know sth about
 387                  is the incoming block with the use.  */
 388               if (gimple_code (use_stmt) == GIMPLE_PHI)
 389                 bb = EDGE_PRED (bb, PHI_ARG_INDEX_FROM_USE (use_p))->src;
 390               /* Any dominator of commondom would be ok with
 391                  adjusting commondom to that block.  */
 392               bb = nearest_common_dominator (CDI_DOMINATORS, bb, commondom);
 393               if (!found)
 394                 found = bb;
 395               else if (dominated_by_p (CDI_DOMINATORS, bb, found))
 396                 found = bb;
 397               /* If we can't improve, stop.  */
 398               if (found == commondom)
 399                 break;
 400             }
 401           commondom = found;
 402           if (commondom == frombb)
 403             return false;
 404         }
 405
 406       /* Our common dominator has to be dominated by frombb in order to be a
 407          trivially safe place to put this statement, since it has multiple
 408          uses.  */
 409       if (!dominated_by_p (CDI_DOMINATORS, commondom, frombb))
 410         return false;
 411
 412       commondom = select_best_block (frombb, commondom, stmt);
 413
 414       if (commondom == frombb)
 415         return false;
 416
 417       *togsi = gsi_after_labels (commondom);
 418
 419       return true;
 420     }
 421   else
 422     {
 423       FOR_EACH_IMM_USE_FAST (one_use, imm_iter, DEF_FROM_PTR (def_p))
 424         {
 425           if (is_gimple_debug (USE_STMT (one_use)))
 426             continue;
 427           break;
 428         }
 429       use = USE_STMT (one_use);
 430
 431       if (gimple_code (use) != GIMPLE_PHI)
 432         {
 433           sinkbb = gimple_bb (use);
 434           sinkbb = select_best_block (frombb, gimple_bb (use), stmt);
 435
 436           if (sinkbb == frombb)
 437             return false;
 438
 439           *togsi = gsi_for_stmt (use);
 440
 441           return true;
 442         }
 443     }
 444
 445   sinkbb = find_bb_for_arg (as_a <gphi *> (use), DEF_FROM_PTR (def_p));
 446
 447   /* This can happen if there are multiple uses in a PHI.  */
 448   if (!sinkbb)
 449     return false;
 450
 451   sinkbb = select_best_block (frombb, sinkbb, stmt);
 452   if (!sinkbb || sinkbb == frombb)
 453     return false;
 454
 455   /* If the latch block is empty, don't make it non-empty by sinking
 456      something into it.  */
 457   if (sinkbb == frombb->loop_father->latch
 458       && empty_block_p (sinkbb))
 459     return false;
 460
 461   *togsi = gsi_after_labels (sinkbb);
 462
 463   return true;
 464 }
 465
 466 /* Perform code sinking on BB */
 467
 468 static void
 469 sink_code_in_bb (basic_block bb)
 470 {
 471   basic_block son;
 472   gimple_stmt_iterator gsi;
 473   edge_iterator ei;
 474   edge e;
 475   bool last = true;
 476
 477   /* If this block doesn't dominate anything, there can't be any place to sink
 478      the statements to.  */
 479   if (first_dom_son (CDI_DOMINATORS, bb) == NULL)
 480     goto earlyout;
 481
 482   /* We can't move things across abnormal edges, so don't try.  */
 483   FOR_EACH_EDGE (e, ei, bb->succs)
 484     if (e->flags & EDGE_ABNORMAL)
 485       goto earlyout;
 486
 487   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
 488     {
 489       gimple *stmt = gsi_stmt (gsi);
 490       gimple_stmt_iterator togsi;
 491       bool zero_uses_p;
 492
 493       if (!statement_sink_location (stmt, bb, &togsi, &zero_uses_p))
 494         {
 495           gimple_stmt_iterator saved = gsi;
 496           if (!gsi_end_p (gsi))
 497             gsi_prev (&gsi);
 498           /* If we face a dead stmt remove it as it possibly blocks
 499              sinking of uses.  */
 500           if (zero_uses_p
 501               && ! gimple_vdef (stmt))
 502             {
 503               gsi_remove (&saved, true);
 504               release_defs (stmt);
 505             }
 506           else
 507             last = false;
 508           continue;
 509         }
 510       if (dump_file)
 511         {
 512           fprintf (dump_file, "Sinking ");
 513           print_gimple_stmt (dump_file, stmt, 0, TDF_VOPS);
 514           fprintf (dump_file, " from bb %d to bb %d\n",
 515                    bb->index, (gsi_bb (togsi))->index);
 516         }
 517
 518       /* Update virtual operands of statements in the path we
 519          do not sink to.  */
 520       if (gimple_vdef (stmt))
 521         {
 522           imm_use_iterator iter;
 523           use_operand_p use_p;
 524           gimple *vuse_stmt;
 525
 526           FOR_EACH_IMM_USE_STMT (vuse_stmt, iter, gimple_vdef (stmt))
 527             if (gimple_code (vuse_stmt) != GIMPLE_PHI)
 528               FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
 529                 SET_USE (use_p, gimple_vuse (stmt));
 530         }
 531
 532       /* If this is the end of the basic block, we need to insert at the end
 533          of the basic block.  */
 534       if (gsi_end_p (togsi))
 535         gsi_move_to_bb_end (&gsi, gsi_bb (togsi));
 536       else
 537         gsi_move_before (&gsi, &togsi);
 538
 539       sink_stats.sunk++;
 540
 541       /* If we've just removed the last statement of the BB, the
 542          gsi_end_p() test below would fail, but gsi_prev() would have
 543          succeeded, and we want it to succeed.  So we keep track of
 544          whether we're at the last statement and pick up the new last
 545          statement.  */
 546       if (last)
 547         {
 548           gsi = gsi_last_bb (bb);
 549           continue;
 550         }
 551
 552       last = false;
 553       if (!gsi_end_p (gsi))
 554         gsi_prev (&gsi);
 555
 556     }
 557  earlyout:
 558   for (son = first_dom_son (CDI_POST_DOMINATORS, bb);
 559        son;
 560        son = next_dom_son (CDI_POST_DOMINATORS, son))
 561     {
 562       sink_code_in_bb (son);
 563     }
 564 }
 565
 566 /* Perform code sinking.
 567    This moves code down the flowgraph when we know it would be
 568    profitable to do so, or it wouldn't increase the number of
 569    executions of the statement.
 570
 571    IE given
 572
 573    a_1 = b + c;
 574    if (<something>)
 575    {
 576    }
 577    else
 578    {
 579      foo (&b, &c);
 580      a_5 = b + c;
 581    }
 582    a_6 = PHI (a_5, a_1);
 583    USE a_6.
 584
 585    we'll transform this into:
 586
 587    if (<something>)
 588    {
 589       a_1 = b + c;
 590    }
 591    else
 592    {
 593       foo (&b, &c);
 594       a_5 = b + c;
 595    }
 596    a_6 = PHI (a_5, a_1);
 597    USE a_6.
 598
 599    Note that this reduces the number of computations of a = b + c to 1
 600    when we take the else edge, instead of 2.
 601 */
 602 namespace {
 603
 604 const pass_data pass_data_sink_code =
 605 {
 606   GIMPLE_PASS, /* type */
 607   "sink", /* name */
 608   OPTGROUP_NONE, /* optinfo_flags */
 609   TV_TREE_SINK, /* tv_id */
 610   /* PROP_no_crit_edges is ensured by running split_critical_edges in
 611      pass_data_sink_code::execute ().  */
 612   ( PROP_cfg | PROP_ssa ), /* properties_required */
 613   0, /* properties_provided */
 614   0, /* properties_destroyed */
 615   0, /* todo_flags_start */
 616   TODO_update_ssa, /* todo_flags_finish */
 617 };
 618
 619 class pass_sink_code : public gimple_opt_pass
 620 {
 621 public:
 622   pass_sink_code (gcc::context *ctxt)
 623     : gimple_opt_pass (pass_data_sink_code, ctxt)
 624   {}
 625
 626   /* opt_pass methods: */
 627   virtual bool gate (function *) { return flag_tree_sink != 0; }
 628   virtual unsigned int execute (function *);
 629
 630 }; // class pass_sink_code
 631
 632 unsigned int
 633 pass_sink_code::execute (function *fun)
 634 {
 635   loop_optimizer_init (LOOPS_NORMAL);
 636   split_critical_edges ();
 637   connect_infinite_loops_to_exit ();
 638   memset (&sink_stats, 0, sizeof (sink_stats));
 639   calculate_dominance_info (CDI_DOMINATORS);
 640   calculate_dominance_info (CDI_POST_DOMINATORS);
 641   sink_code_in_bb (EXIT_BLOCK_PTR_FOR_FN (fun));
 642   statistics_counter_event (fun, "Sunk statements", sink_stats.sunk);
 643   free_dominance_info (CDI_POST_DOMINATORS);
 644   remove_fake_exit_edges ();
 645   loop_optimizer_finalize ();
 646
 647   return 0;
 648 }
 649
 650 } // anon namespace
 651
 652 gimple_opt_pass *
 653 make_pass_sink_code (gcc::context *ctxt)
 654 {
 655   return new pass_sink_code (ctxt);
 656 }