gcc/tree-ssa-phiprop.c

   1 /* Backward propagation of indirect loads through PHIs.
   2    Copyright (C) 2007, 2008, 2009, 2010 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 "timevar.h"
  29 #include "diagnostic.h"
  30 #include "tree-pretty-print.h"
  31 #include "gimple-pretty-print.h"
  32 #include "tree-flow.h"
  33 #include "tree-pass.h"
  34 #include "tree-dump.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) == INDIRECT_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   SSA_NAME_DEF_STMT (res) = 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           gcc_assert (TREE_CODE (old_arg) == ADDR_EXPR);
 192           old_arg = TREE_OPERAND (old_arg, 0);
 193           new_var = create_tmp_reg (TREE_TYPE (old_arg), NULL);
 194           tmp = gimple_build_assign (new_var, unshare_expr (old_arg));
 195           gcc_assert (is_gimple_reg (new_var));
 196           add_referenced_var (new_var);
 197           new_var = make_ssa_name (new_var, tmp);
 198           gimple_assign_set_lhs (tmp, new_var);
 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, gimple 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
 251   if (!POINTER_TYPE_P (TREE_TYPE (ptr))
 252       || !is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr))))
 253     return false;
 254
 255   /* Check if we can "cheaply" dereference all phi arguments.  */
 256   FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
 257     {
 258       tree arg = USE_FROM_PTR (arg_p);
 259       /* Walk the ssa chain until we reach a ssa name we already
 260          created a value for or we reach a definition of the form
 261          ssa_name_n = &var;  */
 262       while (TREE_CODE (arg) == SSA_NAME
 263              && !SSA_NAME_IS_DEFAULT_DEF (arg)
 264              && (SSA_NAME_VERSION (arg) >= n
 265                  || phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
 266         {
 267           gimple def_stmt = SSA_NAME_DEF_STMT (arg);
 268           if (!gimple_assign_single_p (def_stmt))
 269             return false;
 270           arg = gimple_assign_rhs1 (def_stmt);
 271         }
 272       if ((TREE_CODE (arg) != ADDR_EXPR
 273            /* Avoid to have to decay *&a to a[0] later.  */
 274            || !is_gimple_reg_type (TREE_TYPE (TREE_OPERAND (arg, 0))))
 275           && !(TREE_CODE (arg) == SSA_NAME
 276                && SSA_NAME_VERSION (arg) < n
 277                && phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
 278                && phivn_valid_p (phivn, arg, bb)))
 279         return false;
 280     }
 281
 282   /* Find a dereferencing use.  First follow (single use) ssa
 283      copy chains for ptr.  */
 284   while (single_imm_use (ptr, &use, &use_stmt)
 285          && gimple_assign_ssa_name_copy_p (use_stmt))
 286     ptr = gimple_assign_lhs (use_stmt);
 287
 288   /* Replace the first dereference of *ptr if there is one and if we
 289      can move the loads to the place of the ptr phi node.  */
 290   phi_inserted = false;
 291   FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
 292     {
 293       gimple def_stmt;
 294       tree vuse;
 295
 296       /* Check whether this is a load of *ptr.  */
 297       if (!(is_gimple_assign (use_stmt)
 298             && TREE_CODE (gimple_assign_lhs (use_stmt)) == SSA_NAME
 299             && gimple_assign_rhs_code (use_stmt) == INDIRECT_REF
 300             && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
 301             /* We cannot replace a load that may throw or is volatile.  */
 302             && !stmt_can_throw_internal (use_stmt)))
 303         continue;
 304
 305       /* Check if we can move the loads.  The def stmt of the virtual use
 306          needs to be in a different basic block dominating bb.  */
 307       vuse = gimple_vuse (use_stmt);
 308       def_stmt = SSA_NAME_DEF_STMT (vuse);
 309       if (!SSA_NAME_IS_DEFAULT_DEF (vuse)
 310           && (gimple_bb (def_stmt) == bb
 311               || !dominated_by_p (CDI_DOMINATORS,
 312                                   bb, gimple_bb (def_stmt))))
 313         goto next;
 314
 315       /* Found a proper dereference.  Insert a phi node if this
 316          is the first load transformation.  */
 317       if (!phi_inserted)
 318         {
 319           res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
 320
 321           /* Remember the value we created for *ptr.  */
 322           phivn[SSA_NAME_VERSION (ptr)].value = res;
 323           phivn[SSA_NAME_VERSION (ptr)].vuse = vuse;
 324
 325           /* Remove old stmt.  The phi is taken care of by DCE, if we
 326              want to delete it here we also have to delete all intermediate
 327              copies.  */
 328           gsi = gsi_for_stmt (use_stmt);
 329           gsi_remove (&gsi, false);
 330
 331           phi_inserted = true;
 332         }
 333       else
 334         {
 335           /* Further replacements are easy, just make a copy out of the
 336              load.  */
 337           gimple_assign_set_rhs1 (use_stmt, res);
 338           update_stmt (use_stmt);
 339         }
 340
 341 next:;
 342       /* Continue searching for a proper dereference.  */
 343     }
 344
 345   return phi_inserted;
 346 }
 347
 348 /* Main entry for phiprop pass.  */
 349
 350 static unsigned int
 351 tree_ssa_phiprop (void)
 352 {
 353   VEC(basic_block, heap) *bbs;
 354   struct phiprop_d *phivn;
 355   bool did_something = false;
 356   basic_block bb;
 357   gimple_stmt_iterator gsi;
 358   unsigned i;
 359   size_t n;
 360
 361   calculate_dominance_info (CDI_DOMINATORS);
 362
 363   n = num_ssa_names;
 364   phivn = XCNEWVEC (struct phiprop_d, n);
 365
 366   /* Walk the dominator tree in preorder.  */
 367   bbs = get_all_dominated_blocks (CDI_DOMINATORS,
 368                                   single_succ (ENTRY_BLOCK_PTR));
 369   for (i = 0; VEC_iterate (basic_block, bbs, i, bb); ++i)
 370     for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 371       did_something |= propagate_with_phi (bb, gsi_stmt (gsi), phivn, n);
 372
 373   if (did_something)
 374     gsi_commit_edge_inserts ();
 375
 376   VEC_free (basic_block, heap, bbs);
 377   free (phivn);
 378
 379   return 0;
 380 }
 381
 382 static bool
 383 gate_phiprop (void)
 384 {
 385   return flag_tree_phiprop;
 386 }
 387
 388 struct gimple_opt_pass pass_phiprop =
 389 {
 390  {
 391   GIMPLE_PASS,
 392   "phiprop",                    /* name */
 393   gate_phiprop,                 /* gate */
 394   tree_ssa_phiprop,             /* execute */
 395   NULL,                         /* sub */
 396   NULL,                         /* next */
 397   0,                            /* static_pass_number */
 398   TV_TREE_PHIPROP,              /* tv_id */
 399   PROP_cfg | PROP_ssa,          /* properties_required */
 400   0,                            /* properties_provided */
 401   0,                            /* properties_destroyed */
 402   0,                            /* todo_flags_start */
 403   TODO_dump_func
 404   | TODO_ggc_collect
 405   | TODO_update_ssa
 406   | TODO_verify_ssa             /* todo_flags_finish */
 407  }
 408 };