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