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
 163       old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
 164       while (TREE_CODE (old_arg) == SSA_NAME
 165              && (SSA_NAME_VERSION (old_arg) >= n
 166                  || phivn[SSA_NAME_VERSION (old_arg)].value == NULL_TREE))
 167         {
 168           gimple def_stmt = SSA_NAME_DEF_STMT (old_arg);
 169           old_arg = gimple_assign_rhs1 (def_stmt);
 170         }
 171
 172       if (TREE_CODE (old_arg) == SSA_NAME)
 173         {
 174           if (dump_file && (dump_flags & TDF_DETAILS))
 175             {
 176               fprintf (dump_file, "  for edge defining ");
 177               print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e), 0);
 178               fprintf (dump_file, " reusing PHI result ");
 179               print_generic_expr (dump_file,
 180                                   phivn[SSA_NAME_VERSION (old_arg)].value, 0);
 181               fprintf (dump_file, "\n");
 182             }
 183           /* Reuse a formerly created dereference.  */
 184           new_var = phivn[SSA_NAME_VERSION (old_arg)].value;
 185         }
 186       else
 187         {
 188           gcc_assert (TREE_CODE (old_arg) == ADDR_EXPR);
 189           old_arg = TREE_OPERAND (old_arg, 0);
 190           new_var = create_tmp_var (TREE_TYPE (old_arg), NULL);
 191           tmp = gimple_build_assign (new_var, unshare_expr (old_arg));
 192           if (TREE_CODE (TREE_TYPE (old_arg)) == COMPLEX_TYPE
 193               || TREE_CODE (TREE_TYPE (old_arg)) == VECTOR_TYPE)
 194             DECL_GIMPLE_REG_P (new_var) = 1;
 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
 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);
 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
 250   if (!POINTER_TYPE_P (TREE_TYPE (ptr))
 251       || !is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr))))
 252     return false;
 253
 254   /* Check if we can "cheaply" dereference all phi arguments.  */
 255   FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
 256     {
 257       tree arg = USE_FROM_PTR (arg_p);
 258       /* Walk the ssa chain until we reach a ssa name we already
 259          created a value for or we reach a definition of the form
 260          ssa_name_n = &var;  */
 261       while (TREE_CODE (arg) == SSA_NAME
 262              && !SSA_NAME_IS_DEFAULT_DEF (arg)
 263              && (SSA_NAME_VERSION (arg) >= n
 264                  || phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
 265         {
 266           gimple def_stmt = SSA_NAME_DEF_STMT (arg);
 267           if (!gimple_assign_single_p (def_stmt))
 268             return false;
 269           arg = gimple_assign_rhs1 (def_stmt);
 270         }
 271       if ((TREE_CODE (arg) != ADDR_EXPR
 272            /* Avoid to have to decay *&a to a[0] later.  */
 273            || !is_gimple_reg_type (TREE_TYPE (TREE_OPERAND (arg, 0))))
 274           && !(TREE_CODE (arg) == SSA_NAME
 275                && SSA_NAME_VERSION (arg) < n
 276                && phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
 277                && phivn_valid_p (phivn, arg, bb)))
 278         return false;
 279     }
 280
 281   /* Find a dereferencing use.  First follow (single use) ssa
 282      copy chains for ptr.  */
 283   while (single_imm_use (ptr, &use, &use_stmt)
 284          && gimple_assign_ssa_name_copy_p (use_stmt))
 285     ptr = gimple_assign_lhs (use_stmt);
 286
 287   /* Replace the first dereference of *ptr if there is one and if we
 288      can move the loads to the place of the ptr phi node.  */
 289   phi_inserted = false;
 290   FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
 291     {
 292       gimple def_stmt;
 293       tree vuse;
 294
 295       /* Check whether this is a load of *ptr.  */
 296       if (!(is_gimple_assign (use_stmt)
 297             && TREE_CODE (gimple_assign_lhs (use_stmt)) == SSA_NAME
 298             && gimple_assign_rhs_code (use_stmt) == INDIRECT_REF
 299             && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
 300             /* We cannot replace a load that may throw or is volatile.  */
 301             && !stmt_can_throw_internal (use_stmt)))
 302         continue;
 303
 304       /* Check if we can move the loads.  The def stmt of the virtual use
 305          needs to be in a different basic block dominating bb.  */
 306       vuse = gimple_vuse (use_stmt);
 307       def_stmt = SSA_NAME_DEF_STMT (vuse);
 308       if (!SSA_NAME_IS_DEFAULT_DEF (vuse)
 309           && (gimple_bb (def_stmt) == bb
 310               || !dominated_by_p (CDI_DOMINATORS,
 311                                   bb, gimple_bb (def_stmt))))
 312         goto next;
 313
 314       /* Found a proper dereference.  Insert a phi node if this
 315          is the first load transformation.  */
 316       if (!phi_inserted)
 317         {
 318           res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
 319
 320           /* Remember the value we created for *ptr.  */
 321           phivn[SSA_NAME_VERSION (ptr)].value = res;
 322           phivn[SSA_NAME_VERSION (ptr)].vuse = vuse;
 323
 324           /* Remove old stmt.  The phi is taken care of by DCE, if we
 325              want to delete it here we also have to delete all intermediate
 326              copies.  */
 327           gsi = gsi_for_stmt (use_stmt);
 328           gsi_remove (&gsi, false);
 329
 330           phi_inserted = true;
 331         }
 332       else
 333         {
 334           /* Further replacements are easy, just make a copy out of the
 335              load.  */
 336           gimple_assign_set_rhs1 (use_stmt, res);
 337           update_stmt (use_stmt);
 338         }
 339
 340 next:;
 341       /* Continue searching for a proper dereference.  */
 342     }
 343
 344   return phi_inserted;
 345 }
 346
 347 /* Main entry for phiprop pass.  */
 348
 349 static unsigned int
 350 tree_ssa_phiprop (void)
 351 {
 352   VEC(basic_block, heap) *bbs;
 353   struct phiprop_d *phivn;
 354   bool did_something = false;
 355   basic_block bb;
 356   gimple_stmt_iterator gsi;
 357   unsigned i;
 358   size_t n;
 359
 360   calculate_dominance_info (CDI_DOMINATORS);
 361
 362   n = num_ssa_names;
 363   phivn = XCNEWVEC (struct phiprop_d, n);
 364
 365   /* Walk the dominator tree in preorder.  */
 366   bbs = get_all_dominated_blocks (CDI_DOMINATORS,
 367                                   single_succ (ENTRY_BLOCK_PTR));
 368   for (i = 0; VEC_iterate (basic_block, bbs, i, bb); ++i)
 369     for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 370       did_something |= propagate_with_phi (bb, gsi_stmt (gsi), phivn, n);
 371
 372   if (did_something)
 373     gsi_commit_edge_inserts ();
 374
 375   VEC_free (basic_block, heap, bbs);
 376   free (phivn);
 377
 378   return 0;
 379 }
 380
 381 static bool
 382 gate_phiprop (void)
 383 {
 384   return flag_tree_phiprop;
 385 }
 386
 387 struct gimple_opt_pass pass_phiprop =
 388 {
 389  {
 390   GIMPLE_PASS,
 391   "phiprop",                    /* name */
 392   gate_phiprop,                 /* gate */
 393   tree_ssa_phiprop,             /* execute */
 394   NULL,                         /* sub */
 395   NULL,                         /* next */
 396   0,                            /* static_pass_number */
 397   TV_TREE_PHIPROP,              /* tv_id */
 398   PROP_cfg | PROP_ssa,          /* properties_required */
 399   0,                            /* properties_provided */
 400   0,                            /* properties_destroyed */
 401   0,                            /* todo_flags_start */
 402   TODO_dump_func
 403   | TODO_ggc_collect
 404   | TODO_update_ssa
 405   | TODO_verify_ssa             /* todo_flags_finish */
 406  }
 407 };