gcc/tree-ssa-phiprop.c

   1 /* Backward propagation of indirect loads through PHIs.
   2    Copyright (C) 2007-2013 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 "tree.h"
  26 #include "tm_p.h"
  27 #include "basic-block.h"
  28 #include "gimple-pretty-print.h"
  29 #include "gimplify.h"
  30 #include "gimple-iterator.h"
  31 #include "gimple-ssa.h"
  32 #include "tree-phinodes.h"
  33 #include "ssa-iterators.h"
  34 #include "tree-ssanames.h"
  35 #include "tree-pass.h"
  36 #include "langhooks.h"
  37 #include "flags.h"
  38
  39 /* This pass propagates indirect loads through the PHI node for its
  40    address to make the load source possibly non-addressable and to
  41    allow for PHI optimization to trigger.
  42
  43    For example the pass changes
  44
  45      # addr_1 = PHI <&a, &b>
  46      tmp_1 = *addr_1;
  47
  48    to
  49
  50      # tmp_1 = PHI <a, b>
  51
  52    but also handles more complex scenarios like
  53
  54      D.2077_2 = &this_1(D)->a1;
  55      ...
  56
  57      # b_12 = PHI <&c(2), D.2077_2(3)>
  58      D.2114_13 = *b_12;
  59      ...
  60
  61      # b_15 = PHI <b_12(4), &b(5)>
  62      D.2080_5 = &this_1(D)->a0;
  63      ...
  64
  65      # b_18 = PHI <D.2080_5(6), &c(7)>
  66      ...
  67
  68      # b_21 = PHI <b_15(8), b_18(9)>
  69      D.2076_8 = *b_21;
  70
  71    where the addresses loaded are defined by PHIs itself.
  72    The above happens for
  73
  74      std::max(std::min(a0, c), std::min(std::max(a1, c), b))
  75
  76    where this pass transforms it to a form later PHI optimization
  77    recognizes and transforms it to the simple
  78
  79      D.2109_10 = this_1(D)->a1;
  80      D.2110_11 = c;
  81      D.2114_31 = MAX_EXPR <D.2109_10, D.2110_11>;
  82      D.2115_14 = b;
  83      D.2125_17 = MIN_EXPR <D.2115_14, D.2114_31>;
  84      D.2119_16 = this_1(D)->a0;
  85      D.2124_32 = MIN_EXPR <D.2110_11, D.2119_16>;
  86      D.2076_33 = MAX_EXPR <D.2125_17, D.2124_32>;
  87
  88    The pass does a dominator walk processing loads using a basic-block
  89    local analysis and stores the result for use by transformations on
  90    dominated basic-blocks.  */
  91
  92
  93 /* Structure to keep track of the value of a dereferenced PHI result
  94    and the virtual operand used for that dereference.  */
  95
  96 struct phiprop_d
  97 {
  98   tree value;
  99   tree vuse;
 100 };
 101
 102 /* Verify if the value recorded for NAME in PHIVN is still valid at
 103    the start of basic block BB.  */
 104
 105 static bool
 106 phivn_valid_p (struct phiprop_d *phivn, tree name, basic_block bb)
 107 {
 108   tree vuse = phivn[SSA_NAME_VERSION (name)].vuse;
 109   gimple use_stmt;
 110   imm_use_iterator ui2;
 111   bool ok = true;
 112
 113   /* The def stmts of the virtual uses need to be dominated by bb.  */
 114   gcc_assert (vuse != NULL_TREE);
 115
 116   FOR_EACH_IMM_USE_STMT (use_stmt, ui2, vuse)
 117     {
 118       /* If BB does not dominate a VDEF, the value is invalid.  */
 119       if ((gimple_vdef (use_stmt) != NULL_TREE
 120            || gimple_code (use_stmt) == GIMPLE_PHI)
 121           && !dominated_by_p (CDI_DOMINATORS, gimple_bb (use_stmt), bb))
 122         {
 123           ok = false;
 124           BREAK_FROM_IMM_USE_STMT (ui2);
 125         }
 126     }
 127
 128   return ok;
 129 }
 130
 131 /* Insert a new phi node for the dereference of PHI at basic_block
 132    BB with the virtual operands from USE_STMT.  */
 133
 134 static tree
 135 phiprop_insert_phi (basic_block bb, gimple phi, gimple use_stmt,
 136                     struct phiprop_d *phivn, size_t n)
 137 {
 138   tree res;
 139   gimple new_phi;
 140   edge_iterator ei;
 141   edge e;
 142
 143   gcc_assert (is_gimple_assign (use_stmt)
 144               && gimple_assign_rhs_code (use_stmt) == MEM_REF);
 145
 146   /* Build a new PHI node to replace the definition of
 147      the indirect reference lhs.  */
 148   res = gimple_assign_lhs (use_stmt);
 149   new_phi = create_phi_node (res, bb);
 150
 151   if (dump_file && (dump_flags & TDF_DETAILS))
 152     {
 153       fprintf (dump_file, "Inserting PHI for result of load ");
 154       print_gimple_stmt (dump_file, use_stmt, 0, 0);
 155     }
 156
 157   /* Add PHI arguments for each edge inserting loads of the
 158      addressable operands.  */
 159   FOR_EACH_EDGE (e, ei, bb->preds)
 160     {
 161       tree old_arg, new_var;
 162       gimple tmp;
 163       source_location locus;
 164
 165       old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
 166       locus = gimple_phi_arg_location_from_edge (phi, e);
 167       while (TREE_CODE (old_arg) == SSA_NAME
 168              && (SSA_NAME_VERSION (old_arg) >= n
 169                  || phivn[SSA_NAME_VERSION (old_arg)].value == NULL_TREE))
 170         {
 171           gimple def_stmt = SSA_NAME_DEF_STMT (old_arg);
 172           old_arg = gimple_assign_rhs1 (def_stmt);
 173           locus = gimple_location (def_stmt);
 174         }
 175
 176       if (TREE_CODE (old_arg) == SSA_NAME)
 177         {
 178           if (dump_file && (dump_flags & TDF_DETAILS))
 179             {
 180               fprintf (dump_file, "  for edge defining ");
 181               print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e), 0);
 182               fprintf (dump_file, " reusing PHI result ");
 183               print_generic_expr (dump_file,
 184                                   phivn[SSA_NAME_VERSION (old_arg)].value, 0);
 185               fprintf (dump_file, "\n");
 186             }
 187           /* Reuse a formerly created dereference.  */
 188           new_var = phivn[SSA_NAME_VERSION (old_arg)].value;
 189         }
 190       else
 191         {
 192           tree rhs = gimple_assign_rhs1 (use_stmt);
 193           gcc_assert (TREE_CODE (old_arg) == ADDR_EXPR);
 194           new_var = make_ssa_name (TREE_TYPE (rhs), NULL);
 195           if (!is_gimple_min_invariant (old_arg))
 196             old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
 197           else
 198             old_arg = unshare_expr (old_arg);
 199           tmp = gimple_build_assign (new_var,
 200                                      fold_build2 (MEM_REF, TREE_TYPE (rhs),
 201                                                   old_arg,
 202                                                   TREE_OPERAND (rhs, 1)));
 203           gimple_set_location (tmp, locus);
 204
 205           gsi_insert_on_edge (e, tmp);
 206           update_stmt (tmp);
 207
 208           if (dump_file && (dump_flags & TDF_DETAILS))
 209             {
 210               fprintf (dump_file, "  for edge defining ");
 211               print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e), 0);
 212               fprintf (dump_file, " inserting load ");
 213               print_gimple_stmt (dump_file, tmp, 0, 0);
 214             }
 215         }
 216
 217       add_phi_arg (new_phi, new_var, e, locus);
 218     }
 219
 220   update_stmt (new_phi);
 221
 222   if (dump_file && (dump_flags & TDF_DETAILS))
 223     print_gimple_stmt (dump_file, new_phi, 0, 0);
 224
 225   return res;
 226 }
 227
 228 /* Propagate between the phi node arguments of PHI in BB and phi result
 229    users.  For now this matches
 230         # p_2 = PHI <&x, &y>
 231       <Lx>:;
 232         p_3 = p_2;
 233         z_2 = *p_3;
 234    and converts it to
 235         # z_2 = PHI <x, y>
 236       <Lx>:;
 237    Returns true if a transformation was done and edge insertions
 238    need to be committed.  Global data PHIVN and N is used to track
 239    past transformation results.  We need to be especially careful here
 240    with aliasing issues as we are moving memory reads.  */
 241
 242 static bool
 243 propagate_with_phi (basic_block bb, gimple phi, struct phiprop_d *phivn,
 244                     size_t n)
 245 {
 246   tree ptr = PHI_RESULT (phi);
 247   gimple use_stmt;
 248   tree res = NULL_TREE;
 249   gimple_stmt_iterator gsi;
 250   imm_use_iterator ui;
 251   use_operand_p arg_p, use;
 252   ssa_op_iter i;
 253   bool phi_inserted;
 254   tree type = NULL_TREE;
 255
 256   if (!POINTER_TYPE_P (TREE_TYPE (ptr))
 257       || !is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr))))
 258     return false;
 259
 260   /* Check if we can "cheaply" dereference all phi arguments.  */
 261   FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
 262     {
 263       tree arg = USE_FROM_PTR (arg_p);
 264       /* Walk the ssa chain until we reach a ssa name we already
 265          created a value for or we reach a definition of the form
 266          ssa_name_n = &var;  */
 267       while (TREE_CODE (arg) == SSA_NAME
 268              && !SSA_NAME_IS_DEFAULT_DEF (arg)
 269              && (SSA_NAME_VERSION (arg) >= n
 270                  || phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
 271         {
 272           gimple def_stmt = SSA_NAME_DEF_STMT (arg);
 273           if (!gimple_assign_single_p (def_stmt))
 274             return false;
 275           arg = gimple_assign_rhs1 (def_stmt);
 276         }
 277       if (TREE_CODE (arg) != ADDR_EXPR
 278           && !(TREE_CODE (arg) == SSA_NAME
 279                && SSA_NAME_VERSION (arg) < n
 280                && phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
 281                && (!type
 282                    || types_compatible_p
 283                        (type, TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value)))
 284                && phivn_valid_p (phivn, arg, bb)))
 285         return false;
 286       if (!type
 287           && TREE_CODE (arg) == SSA_NAME)
 288         type = TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value);
 289     }
 290
 291   /* Find a dereferencing use.  First follow (single use) ssa
 292      copy chains for ptr.  */
 293   while (single_imm_use (ptr, &use, &use_stmt)
 294          && gimple_assign_ssa_name_copy_p (use_stmt))
 295     ptr = gimple_assign_lhs (use_stmt);
 296
 297   /* Replace the first dereference of *ptr if there is one and if we
 298      can move the loads to the place of the ptr phi node.  */
 299   phi_inserted = false;
 300   FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
 301     {
 302       gimple def_stmt;
 303       tree vuse;
 304
 305       /* Check whether this is a load of *ptr.  */
 306       if (!(is_gimple_assign (use_stmt)
 307             && TREE_CODE (gimple_assign_lhs (use_stmt)) == SSA_NAME
 308             && gimple_assign_rhs_code (use_stmt) == MEM_REF
 309             && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
 310             && integer_zerop (TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 1))
 311             && (!type
 312                 || types_compatible_p
 313                      (TREE_TYPE (gimple_assign_lhs (use_stmt)), type))
 314             /* We cannot replace a load that may throw or is volatile.  */
 315             && !stmt_can_throw_internal (use_stmt)))
 316         continue;
 317
 318       /* Check if we can move the loads.  The def stmt of the virtual use
 319          needs to be in a different basic block dominating bb.  */
 320       vuse = gimple_vuse (use_stmt);
 321       def_stmt = SSA_NAME_DEF_STMT (vuse);
 322       if (!SSA_NAME_IS_DEFAULT_DEF (vuse)
 323           && (gimple_bb (def_stmt) == bb
 324               || !dominated_by_p (CDI_DOMINATORS,
 325                                   bb, gimple_bb (def_stmt))))
 326         goto next;
 327
 328       /* Found a proper dereference.  Insert a phi node if this
 329          is the first load transformation.  */
 330       if (!phi_inserted)
 331         {
 332           res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
 333           type = TREE_TYPE (res);
 334
 335           /* Remember the value we created for *ptr.  */
 336           phivn[SSA_NAME_VERSION (ptr)].value = res;
 337           phivn[SSA_NAME_VERSION (ptr)].vuse = vuse;
 338
 339           /* Remove old stmt.  The phi is taken care of by DCE, if we
 340              want to delete it here we also have to delete all intermediate
 341              copies.  */
 342           gsi = gsi_for_stmt (use_stmt);
 343           gsi_remove (&gsi, true);
 344
 345           phi_inserted = true;
 346         }
 347       else
 348         {
 349           /* Further replacements are easy, just make a copy out of the
 350              load.  */
 351           gimple_assign_set_rhs1 (use_stmt, res);
 352           update_stmt (use_stmt);
 353         }
 354
 355 next:;
 356       /* Continue searching for a proper dereference.  */
 357     }
 358
 359   return phi_inserted;
 360 }
 361
 362 /* Main entry for phiprop pass.  */
 363
 364 static unsigned int
 365 tree_ssa_phiprop (void)
 366 {
 367   vec<basic_block> bbs;
 368   struct phiprop_d *phivn;
 369   bool did_something = false;
 370   basic_block bb;
 371   gimple_stmt_iterator gsi;
 372   unsigned i;
 373   size_t n;
 374
 375   calculate_dominance_info (CDI_DOMINATORS);
 376
 377   n = num_ssa_names;
 378   phivn = XCNEWVEC (struct phiprop_d, n);
 379
 380   /* Walk the dominator tree in preorder.  */
 381   bbs = get_all_dominated_blocks (CDI_DOMINATORS,
 382                                   single_succ (ENTRY_BLOCK_PTR));
 383   FOR_EACH_VEC_ELT (bbs, i, bb)
 384     for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 385       did_something |= propagate_with_phi (bb, gsi_stmt (gsi), phivn, n);
 386
 387   if (did_something)
 388     gsi_commit_edge_inserts ();
 389
 390   bbs.release ();
 391   free (phivn);
 392
 393   return 0;
 394 }
 395
 396 static bool
 397 gate_phiprop (void)
 398 {
 399   return flag_tree_phiprop;
 400 }
 401
 402 namespace {
 403
 404 const pass_data pass_data_phiprop =
 405 {
 406   GIMPLE_PASS, /* type */
 407   "phiprop", /* name */
 408   OPTGROUP_NONE, /* optinfo_flags */
 409   true, /* has_gate */
 410   true, /* has_execute */
 411   TV_TREE_PHIPROP, /* tv_id */
 412   ( PROP_cfg | PROP_ssa ), /* properties_required */
 413   0, /* properties_provided */
 414   0, /* properties_destroyed */
 415   0, /* todo_flags_start */
 416   ( TODO_update_ssa | TODO_verify_ssa ), /* todo_flags_finish */
 417 };
 418
 419 class pass_phiprop : public gimple_opt_pass
 420 {
 421 public:
 422   pass_phiprop (gcc::context *ctxt)
 423     : gimple_opt_pass (pass_data_phiprop, ctxt)
 424   {}
 425
 426   /* opt_pass methods: */
 427   bool gate () { return gate_phiprop (); }
 428   unsigned int execute () { return tree_ssa_phiprop (); }
 429
 430 }; // class pass_phiprop
 431
 432 } // anon namespace
 433
 434 gimple_opt_pass *
 435 make_pass_phiprop (gcc::context *ctxt)
 436 {
 437   return new pass_phiprop (ctxt);
 438 }