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