gcc/tree-ssa-phiprop.c

   1 /* Backward propagation of indirect loads through PHIs.
   2    Copyright (C) 2007, 2008 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 "ggc.h"
  26 #include "tree.h"
  27 #include "rtl.h"
  28 #include "tm_p.h"
  29 #include "basic-block.h"
  30 #include "timevar.h"
  31 #include "diagnostic.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_var (TREE_TYPE (old_arg), NULL);
 194           tmp = gimple_build_assign (new_var, unshare_expr (old_arg));
 195           if (TREE_CODE (TREE_TYPE (old_arg)) == COMPLEX_TYPE
 196               || TREE_CODE (TREE_TYPE (old_arg)) == VECTOR_TYPE)
 197             DECL_GIMPLE_REG_P (new_var) = 1;
 198           gcc_assert (is_gimple_reg (new_var));
 199           add_referenced_var (new_var);
 200           new_var = make_ssa_name (new_var, tmp);
 201           gimple_assign_set_lhs (tmp, new_var);
 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
 254   if (!POINTER_TYPE_P (TREE_TYPE (ptr))
 255       || !is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr))))
 256     return false;
 257
 258   /* Check if we can "cheaply" dereference all phi arguments.  */
 259   FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
 260     {
 261       tree arg = USE_FROM_PTR (arg_p);
 262       /* Walk the ssa chain until we reach a ssa name we already
 263          created a value for or we reach a definition of the form
 264          ssa_name_n = &var;  */
 265       while (TREE_CODE (arg) == SSA_NAME
 266              && !SSA_NAME_IS_DEFAULT_DEF (arg)
 267              && (SSA_NAME_VERSION (arg) >= n
 268                  || phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
 269         {
 270           gimple def_stmt = SSA_NAME_DEF_STMT (arg);
 271           if (!gimple_assign_single_p (def_stmt))
 272             return false;
 273           arg = gimple_assign_rhs1 (def_stmt);
 274         }
 275       if ((TREE_CODE (arg) != ADDR_EXPR
 276            /* Avoid to have to decay *&a to a[0] later.  */
 277            || !is_gimple_reg_type (TREE_TYPE (TREE_OPERAND (arg, 0))))
 278           && !(TREE_CODE (arg) == SSA_NAME
 279                && SSA_NAME_VERSION (arg) < n
 280                && phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
 281                && phivn_valid_p (phivn, arg, bb)))
 282         return false;
 283     }
 284
 285   /* Find a dereferencing use.  First follow (single use) ssa
 286      copy chains for ptr.  */
 287   while (single_imm_use (ptr, &use, &use_stmt)
 288          && gimple_assign_ssa_name_copy_p (use_stmt))
 289     ptr = gimple_assign_lhs (use_stmt);
 290
 291   /* Replace the first dereference of *ptr if there is one and if we
 292      can move the loads to the place of the ptr phi node.  */
 293   phi_inserted = false;
 294   FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
 295     {
 296       gimple def_stmt;
 297       tree vuse;
 298
 299       /* Check whether this is a load of *ptr.  */
 300       if (!(is_gimple_assign (use_stmt)
 301             && TREE_CODE (gimple_assign_lhs (use_stmt)) == SSA_NAME
 302             && gimple_assign_rhs_code (use_stmt) == INDIRECT_REF
 303             && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
 304             /* We cannot replace a load that may throw or is volatile.  */
 305             && !stmt_can_throw_internal (use_stmt)))
 306         continue;
 307
 308       /* Check if we can move the loads.  The def stmt of the virtual use
 309          needs to be in a different basic block dominating bb.  */
 310       vuse = gimple_vuse (use_stmt);
 311       def_stmt = SSA_NAME_DEF_STMT (vuse);
 312       if (!SSA_NAME_IS_DEFAULT_DEF (vuse)
 313           && (gimple_bb (def_stmt) == bb
 314               || !dominated_by_p (CDI_DOMINATORS,
 315                                   bb, gimple_bb (def_stmt))))
 316         goto next;
 317
 318       /* Found a proper dereference.  Insert a phi node if this
 319          is the first load transformation.  */
 320       if (!phi_inserted)
 321         {
 322           res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
 323
 324           /* Remember the value we created for *ptr.  */
 325           phivn[SSA_NAME_VERSION (ptr)].value = res;
 326           phivn[SSA_NAME_VERSION (ptr)].vuse = vuse;
 327
 328           /* Remove old stmt.  The phi is taken care of by DCE, if we
 329              want to delete it here we also have to delete all intermediate
 330              copies.  */
 331           gsi = gsi_for_stmt (use_stmt);
 332           gsi_remove (&gsi, false);
 333
 334           phi_inserted = true;
 335         }
 336       else
 337         {
 338           /* Further replacements are easy, just make a copy out of the
 339              load.  */
 340           gimple_assign_set_rhs1 (use_stmt, res);
 341           update_stmt (use_stmt);
 342         }
 343
 344 next:;
 345       /* Continue searching for a proper dereference.  */
 346     }
 347
 348   return phi_inserted;
 349 }
 350
 351 /* Main entry for phiprop pass.  */
 352
 353 static unsigned int
 354 tree_ssa_phiprop (void)
 355 {
 356   VEC(basic_block, heap) *bbs;
 357   struct phiprop_d *phivn;
 358   bool did_something = false;
 359   basic_block bb;
 360   gimple_stmt_iterator gsi;
 361   unsigned i;
 362   size_t n;
 363
 364   calculate_dominance_info (CDI_DOMINATORS);
 365
 366   n = num_ssa_names;
 367   phivn = XCNEWVEC (struct phiprop_d, n);
 368
 369   /* Walk the dominator tree in preorder.  */
 370   bbs = get_all_dominated_blocks (CDI_DOMINATORS,
 371                                   single_succ (ENTRY_BLOCK_PTR));
 372   for (i = 0; VEC_iterate (basic_block, bbs, i, bb); ++i)
 373     for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 374       did_something |= propagate_with_phi (bb, gsi_stmt (gsi), phivn, n);
 375
 376   if (did_something)
 377     gsi_commit_edge_inserts ();
 378
 379   VEC_free (basic_block, heap, bbs);
 380   free (phivn);
 381
 382   return 0;
 383 }
 384
 385 static bool
 386 gate_phiprop (void)
 387 {
 388   return flag_tree_phiprop;
 389 }
 390
 391 struct gimple_opt_pass pass_phiprop =
 392 {
 393  {
 394   GIMPLE_PASS,
 395   "phiprop",                    /* name */
 396   gate_phiprop,                 /* gate */
 397   tree_ssa_phiprop,             /* execute */
 398   NULL,                         /* sub */
 399   NULL,                         /* next */
 400   0,                            /* static_pass_number */
 401   TV_TREE_PHIPROP,              /* tv_id */
 402   PROP_cfg | PROP_ssa,          /* properties_required */
 403   0,                            /* properties_provided */
 404   0,                            /* properties_destroyed */
 405   0,                            /* todo_flags_start */
 406   TODO_dump_func
 407   | TODO_ggc_collect
 408   | TODO_update_ssa
 409   | TODO_verify_ssa             /* todo_flags_finish */
 410  }
 411 };