gcc/tree-ssa-phiprop.c

   1 /* Backward propagation of indirect loads through PHIs.
   2    Copyright (C) 2007-2015 Free Software Foundation, Inc.
   3    Contributed by Richard Guenther <rguenther@suse.de>
   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 "tm.h"
  25 #include "input.h"
  26 #include "alias.h"
  27 #include "symtab.h"
  28 #include "tree.h"
  29 #include "fold-const.h"
  30 #include "tm_p.h"
  31 #include "predict.h"
  32 #include "hard-reg-set.h"
  33 #include "input.h"
  34 #include "function.h"
  35 #include "dominance.h"
  36 #include "cfg.h"
  37 #include "basic-block.h"
  38 #include "gimple-pretty-print.h"
  39 #include "tree-ssa-alias.h"
  40 #include "internal-fn.h"
  41 #include "tree-eh.h"
  42 #include "gimple-expr.h"
  43 #include "is-a.h"
  44 #include "gimple.h"
  45 #include "gimplify.h"
  46 #include "gimple-iterator.h"
  47 #include "gimple-ssa.h"
  48 #include "tree-phinodes.h"
  49 #include "ssa-iterators.h"
  50 #include "stringpool.h"
  51 #include "tree-ssanames.h"
  52 #include "tree-pass.h"
  53 #include "langhooks.h"
  54 #include "flags.h"
  55
  56 /* This pass propagates indirect loads through the PHI node for its
  57    address to make the load source possibly non-addressable and to
  58    allow for PHI optimization to trigger.
  59
  60    For example the pass changes
  61
  62      # addr_1 = PHI <&a, &b>
  63      tmp_1 = *addr_1;
  64
  65    to
  66
  67      # tmp_1 = PHI <a, b>
  68
  69    but also handles more complex scenarios like
  70
  71      D.2077_2 = &this_1(D)->a1;
  72      ...
  73
  74      # b_12 = PHI <&c(2), D.2077_2(3)>
  75      D.2114_13 = *b_12;
  76      ...
  77
  78      # b_15 = PHI <b_12(4), &b(5)>
  79      D.2080_5 = &this_1(D)->a0;
  80      ...
  81
  82      # b_18 = PHI <D.2080_5(6), &c(7)>
  83      ...
  84
  85      # b_21 = PHI <b_15(8), b_18(9)>
  86      D.2076_8 = *b_21;
  87
  88    where the addresses loaded are defined by PHIs itself.
  89    The above happens for
  90
  91      std::max(std::min(a0, c), std::min(std::max(a1, c), b))
  92
  93    where this pass transforms it to a form later PHI optimization
  94    recognizes and transforms it to the simple
  95
  96      D.2109_10 = this_1(D)->a1;
  97      D.2110_11 = c;
  98      D.2114_31 = MAX_EXPR <D.2109_10, D.2110_11>;
  99      D.2115_14 = b;
 100      D.2125_17 = MIN_EXPR <D.2115_14, D.2114_31>;
 101      D.2119_16 = this_1(D)->a0;
 102      D.2124_32 = MIN_EXPR <D.2110_11, D.2119_16>;
 103      D.2076_33 = MAX_EXPR <D.2125_17, D.2124_32>;
 104
 105    The pass does a dominator walk processing loads using a basic-block
 106    local analysis and stores the result for use by transformations on
 107    dominated basic-blocks.  */
 108
 109
 110 /* Structure to keep track of the value of a dereferenced PHI result
 111    and the virtual operand used for that dereference.  */
 112
 113 struct phiprop_d
 114 {
 115   tree value;
 116   tree vuse;
 117 };
 118
 119 /* Verify if the value recorded for NAME in PHIVN is still valid at
 120    the start of basic block BB.  */
 121
 122 static bool
 123 phivn_valid_p (struct phiprop_d *phivn, tree name, basic_block bb)
 124 {
 125   tree vuse = phivn[SSA_NAME_VERSION (name)].vuse;
 126   gimple use_stmt;
 127   imm_use_iterator ui2;
 128   bool ok = true;
 129
 130   /* The def stmts of the virtual uses need to be dominated by bb.  */
 131   gcc_assert (vuse != NULL_TREE);
 132
 133   FOR_EACH_IMM_USE_STMT (use_stmt, ui2, vuse)
 134     {
 135       /* If BB does not dominate a VDEF, the value is invalid.  */
 136       if ((gimple_vdef (use_stmt) != NULL_TREE
 137            || gimple_code (use_stmt) == GIMPLE_PHI)
 138           && !dominated_by_p (CDI_DOMINATORS, gimple_bb (use_stmt), bb))
 139         {
 140           ok = false;
 141           BREAK_FROM_IMM_USE_STMT (ui2);
 142         }
 143     }
 144
 145   return ok;
 146 }
 147
 148 /* Insert a new phi node for the dereference of PHI at basic_block
 149    BB with the virtual operands from USE_STMT.  */
 150
 151 static tree
 152 phiprop_insert_phi (basic_block bb, gphi *phi, gimple use_stmt,
 153                     struct phiprop_d *phivn, size_t n)
 154 {
 155   tree res;
 156   gphi *new_phi;
 157   edge_iterator ei;
 158   edge e;
 159
 160   gcc_assert (is_gimple_assign (use_stmt)
 161               && gimple_assign_rhs_code (use_stmt) == MEM_REF);
 162
 163   /* Build a new PHI node to replace the definition of
 164      the indirect reference lhs.  */
 165   res = gimple_assign_lhs (use_stmt);
 166   new_phi = create_phi_node (res, bb);
 167
 168   if (dump_file && (dump_flags & TDF_DETAILS))
 169     {
 170       fprintf (dump_file, "Inserting PHI for result of load ");
 171       print_gimple_stmt (dump_file, use_stmt, 0, 0);
 172     }
 173
 174   /* Add PHI arguments for each edge inserting loads of the
 175      addressable operands.  */
 176   FOR_EACH_EDGE (e, ei, bb->preds)
 177     {
 178       tree old_arg, new_var;
 179       gassign *tmp;
 180       source_location locus;
 181
 182       old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
 183       locus = gimple_phi_arg_location_from_edge (phi, e);
 184       while (TREE_CODE (old_arg) == SSA_NAME
 185              && (SSA_NAME_VERSION (old_arg) >= n
 186                  || phivn[SSA_NAME_VERSION (old_arg)].value == NULL_TREE))
 187         {
 188           gimple def_stmt = SSA_NAME_DEF_STMT (old_arg);
 189           old_arg = gimple_assign_rhs1 (def_stmt);
 190           locus = gimple_location (def_stmt);
 191         }
 192
 193       if (TREE_CODE (old_arg) == SSA_NAME)
 194         {
 195           if (dump_file && (dump_flags & TDF_DETAILS))
 196             {
 197               fprintf (dump_file, "  for edge defining ");
 198               print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e), 0);
 199               fprintf (dump_file, " reusing PHI result ");
 200               print_generic_expr (dump_file,
 201                                   phivn[SSA_NAME_VERSION (old_arg)].value, 0);
 202               fprintf (dump_file, "\n");
 203             }
 204           /* Reuse a formerly created dereference.  */
 205           new_var = phivn[SSA_NAME_VERSION (old_arg)].value;
 206         }
 207       else
 208         {
 209           tree rhs = gimple_assign_rhs1 (use_stmt);
 210           gcc_assert (TREE_CODE (old_arg) == ADDR_EXPR);
 211           new_var = make_ssa_name (TREE_TYPE (rhs));
 212           if (!is_gimple_min_invariant (old_arg))
 213             old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
 214           else
 215             old_arg = unshare_expr (old_arg);
 216           tmp = gimple_build_assign (new_var,
 217                                      fold_build2 (MEM_REF, TREE_TYPE (rhs),
 218                                                   old_arg,
 219                                                   TREE_OPERAND (rhs, 1)));
 220           gimple_set_location (tmp, locus);
 221
 222           gsi_insert_on_edge (e, tmp);
 223           update_stmt (tmp);
 224
 225           if (dump_file && (dump_flags & TDF_DETAILS))
 226             {
 227               fprintf (dump_file, "  for edge defining ");
 228               print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e), 0);
 229               fprintf (dump_file, " inserting load ");
 230               print_gimple_stmt (dump_file, tmp, 0, 0);
 231             }
 232         }
 233
 234       add_phi_arg (new_phi, new_var, e, locus);
 235     }
 236
 237   update_stmt (new_phi);
 238
 239   if (dump_file && (dump_flags & TDF_DETAILS))
 240     print_gimple_stmt (dump_file, new_phi, 0, 0);
 241
 242   return res;
 243 }
 244
 245 /* Propagate between the phi node arguments of PHI in BB and phi result
 246    users.  For now this matches
 247         # p_2 = PHI <&x, &y>
 248       <Lx>:;
 249         p_3 = p_2;
 250         z_2 = *p_3;
 251    and converts it to
 252         # z_2 = PHI <x, y>
 253       <Lx>:;
 254    Returns true if a transformation was done and edge insertions
 255    need to be committed.  Global data PHIVN and N is used to track
 256    past transformation results.  We need to be especially careful here
 257    with aliasing issues as we are moving memory reads.  */
 258
 259 static bool
 260 propagate_with_phi (basic_block bb, gphi *phi, struct phiprop_d *phivn,
 261                     size_t n)
 262 {
 263   tree ptr = PHI_RESULT (phi);
 264   gimple use_stmt;
 265   tree res = NULL_TREE;
 266   gimple_stmt_iterator gsi;
 267   imm_use_iterator ui;
 268   use_operand_p arg_p, use;
 269   ssa_op_iter i;
 270   bool phi_inserted;
 271   tree type = NULL_TREE;
 272
 273   if (!POINTER_TYPE_P (TREE_TYPE (ptr))
 274       || !is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr))))
 275     return false;
 276
 277   /* Check if we can "cheaply" dereference all phi arguments.  */
 278   FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
 279     {
 280       tree arg = USE_FROM_PTR (arg_p);
 281       /* Walk the ssa chain until we reach a ssa name we already
 282          created a value for or we reach a definition of the form
 283          ssa_name_n = &var;  */
 284       while (TREE_CODE (arg) == SSA_NAME
 285              && !SSA_NAME_IS_DEFAULT_DEF (arg)
 286              && (SSA_NAME_VERSION (arg) >= n
 287                  || phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
 288         {
 289           gimple def_stmt = SSA_NAME_DEF_STMT (arg);
 290           if (!gimple_assign_single_p (def_stmt))
 291             return false;
 292           arg = gimple_assign_rhs1 (def_stmt);
 293         }
 294       if (TREE_CODE (arg) != ADDR_EXPR
 295           && !(TREE_CODE (arg) == SSA_NAME
 296                && SSA_NAME_VERSION (arg) < n
 297                && phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
 298                && (!type
 299                    || types_compatible_p
 300                        (type, TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value)))
 301                && phivn_valid_p (phivn, arg, bb)))
 302         return false;
 303       if (!type
 304           && TREE_CODE (arg) == SSA_NAME)
 305         type = TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value);
 306     }
 307
 308   /* Find a dereferencing use.  First follow (single use) ssa
 309      copy chains for ptr.  */
 310   while (single_imm_use (ptr, &use, &use_stmt)
 311          && gimple_assign_ssa_name_copy_p (use_stmt))
 312     ptr = gimple_assign_lhs (use_stmt);
 313
 314   /* Replace the first dereference of *ptr if there is one and if we
 315      can move the loads to the place of the ptr phi node.  */
 316   phi_inserted = false;
 317   FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
 318     {
 319       gimple def_stmt;
 320       tree vuse;
 321
 322       /* Only replace loads in blocks that post-dominate the PHI node.  That
 323          makes sure we don't end up speculating loads.  */
 324       if (!dominated_by_p (CDI_POST_DOMINATORS,
 325                            bb, gimple_bb (use_stmt)))
 326         continue;
 327
 328       /* Check whether this is a load of *ptr.  */
 329       if (!(is_gimple_assign (use_stmt)
 330             && TREE_CODE (gimple_assign_lhs (use_stmt)) == SSA_NAME
 331             && gimple_assign_rhs_code (use_stmt) == MEM_REF
 332             && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
 333             && integer_zerop (TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 1))
 334             && (!type
 335                 || types_compatible_p
 336                      (TREE_TYPE (gimple_assign_lhs (use_stmt)), type))
 337             /* We cannot replace a load that may throw or is volatile.  */
 338             && !stmt_can_throw_internal (use_stmt)))
 339         continue;
 340
 341       /* Check if we can move the loads.  The def stmt of the virtual use
 342          needs to be in a different basic block dominating bb.  */
 343       vuse = gimple_vuse (use_stmt);
 344       def_stmt = SSA_NAME_DEF_STMT (vuse);
 345       if (!SSA_NAME_IS_DEFAULT_DEF (vuse)
 346           && (gimple_bb (def_stmt) == bb
 347               || !dominated_by_p (CDI_DOMINATORS,
 348                                   bb, gimple_bb (def_stmt))))
 349         goto next;
 350
 351       /* Found a proper dereference.  Insert a phi node if this
 352          is the first load transformation.  */
 353       if (!phi_inserted)
 354         {
 355           res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
 356           type = TREE_TYPE (res);
 357
 358           /* Remember the value we created for *ptr.  */
 359           phivn[SSA_NAME_VERSION (ptr)].value = res;
 360           phivn[SSA_NAME_VERSION (ptr)].vuse = vuse;
 361
 362           /* Remove old stmt.  The phi is taken care of by DCE, if we
 363              want to delete it here we also have to delete all intermediate
 364              copies.  */
 365           gsi = gsi_for_stmt (use_stmt);
 366           gsi_remove (&gsi, true);
 367
 368           phi_inserted = true;
 369         }
 370       else
 371         {
 372           /* Further replacements are easy, just make a copy out of the
 373              load.  */
 374           gimple_assign_set_rhs1 (use_stmt, res);
 375           update_stmt (use_stmt);
 376         }
 377
 378 next:;
 379       /* Continue searching for a proper dereference.  */
 380     }
 381
 382   return phi_inserted;
 383 }
 384
 385 /* Main entry for phiprop pass.  */
 386
 387 namespace {
 388
 389 const pass_data pass_data_phiprop =
 390 {
 391   GIMPLE_PASS, /* type */
 392   "phiprop", /* name */
 393   OPTGROUP_NONE, /* optinfo_flags */
 394   TV_TREE_PHIPROP, /* tv_id */
 395   ( PROP_cfg | PROP_ssa ), /* properties_required */
 396   0, /* properties_provided */
 397   0, /* properties_destroyed */
 398   0, /* todo_flags_start */
 399   TODO_update_ssa, /* todo_flags_finish */
 400 };
 401
 402 class pass_phiprop : public gimple_opt_pass
 403 {
 404 public:
 405   pass_phiprop (gcc::context *ctxt)
 406     : gimple_opt_pass (pass_data_phiprop, ctxt)
 407   {}
 408
 409   /* opt_pass methods: */
 410   virtual bool gate (function *) { return flag_tree_phiprop; }
 411   virtual unsigned int execute (function *);
 412
 413 }; // class pass_phiprop
 414
 415 unsigned int
 416 pass_phiprop::execute (function *fun)
 417 {
 418   vec<basic_block> bbs;
 419   struct phiprop_d *phivn;
 420   bool did_something = false;
 421   basic_block bb;
 422   gphi_iterator gsi;
 423   unsigned i;
 424   size_t n;
 425
 426   calculate_dominance_info (CDI_DOMINATORS);
 427   calculate_dominance_info (CDI_POST_DOMINATORS);
 428
 429   n = num_ssa_names;
 430   phivn = XCNEWVEC (struct phiprop_d, n);
 431
 432   /* Walk the dominator tree in preorder.  */
 433   bbs = get_all_dominated_blocks (CDI_DOMINATORS,
 434                                   single_succ (ENTRY_BLOCK_PTR_FOR_FN (fun)));
 435   FOR_EACH_VEC_ELT (bbs, i, bb)
 436     for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 437       did_something |= propagate_with_phi (bb, gsi.phi (), phivn, n);
 438
 439   if (did_something)
 440     gsi_commit_edge_inserts ();
 441
 442   bbs.release ();
 443   free (phivn);
 444
 445   free_dominance_info (CDI_POST_DOMINATORS);
 446
 447   return 0;
 448 }
 449
 450 } // anon namespace
 451
 452 gimple_opt_pass *
 453 make_pass_phiprop (gcc::context *ctxt)
 454 {
 455   return new pass_phiprop (ctxt);
 456 }