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