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