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