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