gcc/tree-ssa-phiprop.c

   1 /* Backward propagation of indirect loads through PHIs.
   2    Copyright (C) 2007-2014 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-ssa-alias.h"
  30 #include "internal-fn.h"
  31 #include "tree-eh.h"
  32 #include "gimple-expr.h"
  33 #include "is-a.h"
  34 #include "gimple.h"
  35 #include "gimplify.h"
  36 #include "gimple-iterator.h"
  37 #include "gimple-ssa.h"
  38 #include "tree-phinodes.h"
  39 #include "ssa-iterators.h"
  40 #include "stringpool.h"
  41 #include "tree-ssanames.h"
  42 #include "tree-pass.h"
  43 #include "langhooks.h"
  44 #include "flags.h"
  45
  46 /* This pass propagates indirect loads through the PHI node for its
  47    address to make the load source possibly non-addressable and to
  48    allow for PHI optimization to trigger.
  49
  50    For example the pass changes
  51
  52      # addr_1 = PHI <&a, &b>
  53      tmp_1 = *addr_1;
  54
  55    to
  56
  57      # tmp_1 = PHI <a, b>
  58
  59    but also handles more complex scenarios like
  60
  61      D.2077_2 = &this_1(D)->a1;
  62      ...
  63
  64      # b_12 = PHI <&c(2), D.2077_2(3)>
  65      D.2114_13 = *b_12;
  66      ...
  67
  68      # b_15 = PHI <b_12(4), &b(5)>
  69      D.2080_5 = &this_1(D)->a0;
  70      ...
  71
  72      # b_18 = PHI <D.2080_5(6), &c(7)>
  73      ...
  74
  75      # b_21 = PHI <b_15(8), b_18(9)>
  76      D.2076_8 = *b_21;
  77
  78    where the addresses loaded are defined by PHIs itself.
  79    The above happens for
  80
  81      std::max(std::min(a0, c), std::min(std::max(a1, c), b))
  82
  83    where this pass transforms it to a form later PHI optimization
  84    recognizes and transforms it to the simple
  85
  86      D.2109_10 = this_1(D)->a1;
  87      D.2110_11 = c;
  88      D.2114_31 = MAX_EXPR <D.2109_10, D.2110_11>;
  89      D.2115_14 = b;
  90      D.2125_17 = MIN_EXPR <D.2115_14, D.2114_31>;
  91      D.2119_16 = this_1(D)->a0;
  92      D.2124_32 = MIN_EXPR <D.2110_11, D.2119_16>;
  93      D.2076_33 = MAX_EXPR <D.2125_17, D.2124_32>;
  94
  95    The pass does a dominator walk processing loads using a basic-block
  96    local analysis and stores the result for use by transformations on
  97    dominated basic-blocks.  */
  98
  99
 100 /* Structure to keep track of the value of a dereferenced PHI result
 101    and the virtual operand used for that dereference.  */
 102
 103 struct phiprop_d
 104 {
 105   tree value;
 106   tree vuse;
 107 };
 108
 109 /* Verify if the value recorded for NAME in PHIVN is still valid at
 110    the start of basic block BB.  */
 111
 112 static bool
 113 phivn_valid_p (struct phiprop_d *phivn, tree name, basic_block bb)
 114 {
 115   tree vuse = phivn[SSA_NAME_VERSION (name)].vuse;
 116   gimple use_stmt;
 117   imm_use_iterator ui2;
 118   bool ok = true;
 119
 120   /* The def stmts of the virtual uses need to be dominated by bb.  */
 121   gcc_assert (vuse != NULL_TREE);
 122
 123   FOR_EACH_IMM_USE_STMT (use_stmt, ui2, vuse)
 124     {
 125       /* If BB does not dominate a VDEF, the value is invalid.  */
 126       if ((gimple_vdef (use_stmt) != NULL_TREE
 127            || gimple_code (use_stmt) == GIMPLE_PHI)
 128           && !dominated_by_p (CDI_DOMINATORS, gimple_bb (use_stmt), bb))
 129         {
 130           ok = false;
 131           BREAK_FROM_IMM_USE_STMT (ui2);
 132         }
 133     }
 134
 135   return ok;
 136 }
 137
 138 /* Insert a new phi node for the dereference of PHI at basic_block
 139    BB with the virtual operands from USE_STMT.  */
 140
 141 static tree
 142 phiprop_insert_phi (basic_block bb, gimple phi, gimple use_stmt,
 143                     struct phiprop_d *phivn, size_t n)
 144 {
 145   tree res;
 146   gimple new_phi;
 147   edge_iterator ei;
 148   edge e;
 149
 150   gcc_assert (is_gimple_assign (use_stmt)
 151               && gimple_assign_rhs_code (use_stmt) == MEM_REF);
 152
 153   /* Build a new PHI node to replace the definition of
 154      the indirect reference lhs.  */
 155   res = gimple_assign_lhs (use_stmt);
 156   new_phi = create_phi_node (res, bb);
 157
 158   if (dump_file && (dump_flags & TDF_DETAILS))
 159     {
 160       fprintf (dump_file, "Inserting PHI for result of load ");
 161       print_gimple_stmt (dump_file, use_stmt, 0, 0);
 162     }
 163
 164   /* Add PHI arguments for each edge inserting loads of the
 165      addressable operands.  */
 166   FOR_EACH_EDGE (e, ei, bb->preds)
 167     {
 168       tree old_arg, new_var;
 169       gimple tmp;
 170       source_location locus;
 171
 172       old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
 173       locus = gimple_phi_arg_location_from_edge (phi, e);
 174       while (TREE_CODE (old_arg) == SSA_NAME
 175              && (SSA_NAME_VERSION (old_arg) >= n
 176                  || phivn[SSA_NAME_VERSION (old_arg)].value == NULL_TREE))
 177         {
 178           gimple def_stmt = SSA_NAME_DEF_STMT (old_arg);
 179           old_arg = gimple_assign_rhs1 (def_stmt);
 180           locus = gimple_location (def_stmt);
 181         }
 182
 183       if (TREE_CODE (old_arg) == SSA_NAME)
 184         {
 185           if (dump_file && (dump_flags & TDF_DETAILS))
 186             {
 187               fprintf (dump_file, "  for edge defining ");
 188               print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e), 0);
 189               fprintf (dump_file, " reusing PHI result ");
 190               print_generic_expr (dump_file,
 191                                   phivn[SSA_NAME_VERSION (old_arg)].value, 0);
 192               fprintf (dump_file, "\n");
 193             }
 194           /* Reuse a formerly created dereference.  */
 195           new_var = phivn[SSA_NAME_VERSION (old_arg)].value;
 196         }
 197       else
 198         {
 199           tree rhs = gimple_assign_rhs1 (use_stmt);
 200           gcc_assert (TREE_CODE (old_arg) == ADDR_EXPR);
 201           new_var = make_ssa_name (TREE_TYPE (rhs), NULL);
 202           if (!is_gimple_min_invariant (old_arg))
 203             old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
 204           else
 205             old_arg = unshare_expr (old_arg);
 206           tmp = gimple_build_assign (new_var,
 207                                      fold_build2 (MEM_REF, TREE_TYPE (rhs),
 208                                                   old_arg,
 209                                                   TREE_OPERAND (rhs, 1)));
 210           gimple_set_location (tmp, locus);
 211
 212           gsi_insert_on_edge (e, tmp);
 213           update_stmt (tmp);
 214
 215           if (dump_file && (dump_flags & TDF_DETAILS))
 216             {
 217               fprintf (dump_file, "  for edge defining ");
 218               print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e), 0);
 219               fprintf (dump_file, " inserting load ");
 220               print_gimple_stmt (dump_file, tmp, 0, 0);
 221             }
 222         }
 223
 224       add_phi_arg (new_phi, new_var, e, locus);
 225     }
 226
 227   update_stmt (new_phi);
 228
 229   if (dump_file && (dump_flags & TDF_DETAILS))
 230     print_gimple_stmt (dump_file, new_phi, 0, 0);
 231
 232   return res;
 233 }
 234
 235 /* Propagate between the phi node arguments of PHI in BB and phi result
 236    users.  For now this matches
 237         # p_2 = PHI <&x, &y>
 238       <Lx>:;
 239         p_3 = p_2;
 240         z_2 = *p_3;
 241    and converts it to
 242         # z_2 = PHI <x, y>
 243       <Lx>:;
 244    Returns true if a transformation was done and edge insertions
 245    need to be committed.  Global data PHIVN and N is used to track
 246    past transformation results.  We need to be especially careful here
 247    with aliasing issues as we are moving memory reads.  */
 248
 249 static bool
 250 propagate_with_phi (basic_block bb, gimple phi, struct phiprop_d *phivn,
 251                     size_t n)
 252 {
 253   tree ptr = PHI_RESULT (phi);
 254   gimple use_stmt;
 255   tree res = NULL_TREE;
 256   gimple_stmt_iterator gsi;
 257   imm_use_iterator ui;
 258   use_operand_p arg_p, use;
 259   ssa_op_iter i;
 260   bool phi_inserted;
 261   tree type = NULL_TREE;
 262
 263   if (!POINTER_TYPE_P (TREE_TYPE (ptr))
 264       || !is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr))))
 265     return false;
 266
 267   /* Check if we can "cheaply" dereference all phi arguments.  */
 268   FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
 269     {
 270       tree arg = USE_FROM_PTR (arg_p);
 271       /* Walk the ssa chain until we reach a ssa name we already
 272          created a value for or we reach a definition of the form
 273          ssa_name_n = &var;  */
 274       while (TREE_CODE (arg) == SSA_NAME
 275              && !SSA_NAME_IS_DEFAULT_DEF (arg)
 276              && (SSA_NAME_VERSION (arg) >= n
 277                  || phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
 278         {
 279           gimple def_stmt = SSA_NAME_DEF_STMT (arg);
 280           if (!gimple_assign_single_p (def_stmt))
 281             return false;
 282           arg = gimple_assign_rhs1 (def_stmt);
 283         }
 284       if (TREE_CODE (arg) != ADDR_EXPR
 285           && !(TREE_CODE (arg) == SSA_NAME
 286                && SSA_NAME_VERSION (arg) < n
 287                && phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
 288                && (!type
 289                    || types_compatible_p
 290                        (type, TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value)))
 291                && phivn_valid_p (phivn, arg, bb)))
 292         return false;
 293       if (!type
 294           && TREE_CODE (arg) == SSA_NAME)
 295         type = TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value);
 296     }
 297
 298   /* Find a dereferencing use.  First follow (single use) ssa
 299      copy chains for ptr.  */
 300   while (single_imm_use (ptr, &use, &use_stmt)
 301          && gimple_assign_ssa_name_copy_p (use_stmt))
 302     ptr = gimple_assign_lhs (use_stmt);
 303
 304   /* Replace the first dereference of *ptr if there is one and if we
 305      can move the loads to the place of the ptr phi node.  */
 306   phi_inserted = false;
 307   FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
 308     {
 309       gimple def_stmt;
 310       tree vuse;
 311
 312       /* Only replace loads in blocks that post-dominate the PHI node.  That
 313          makes sure we don't end up speculating loads.  */
 314       if (!dominated_by_p (CDI_POST_DOMINATORS,
 315                            bb, gimple_bb (use_stmt)))
 316         continue;
 317
 318       /* Check whether this is a load of *ptr.  */
 319       if (!(is_gimple_assign (use_stmt)
 320             && TREE_CODE (gimple_assign_lhs (use_stmt)) == SSA_NAME
 321             && gimple_assign_rhs_code (use_stmt) == MEM_REF
 322             && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
 323             && integer_zerop (TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 1))
 324             && (!type
 325                 || types_compatible_p
 326                      (TREE_TYPE (gimple_assign_lhs (use_stmt)), type))
 327             /* We cannot replace a load that may throw or is volatile.  */
 328             && !stmt_can_throw_internal (use_stmt)))
 329         continue;
 330
 331       /* Check if we can move the loads.  The def stmt of the virtual use
 332          needs to be in a different basic block dominating bb.  */
 333       vuse = gimple_vuse (use_stmt);
 334       def_stmt = SSA_NAME_DEF_STMT (vuse);
 335       if (!SSA_NAME_IS_DEFAULT_DEF (vuse)
 336           && (gimple_bb (def_stmt) == bb
 337               || !dominated_by_p (CDI_DOMINATORS,
 338                                   bb, gimple_bb (def_stmt))))
 339         goto next;
 340
 341       /* Found a proper dereference.  Insert a phi node if this
 342          is the first load transformation.  */
 343       if (!phi_inserted)
 344         {
 345           res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
 346           type = TREE_TYPE (res);
 347
 348           /* Remember the value we created for *ptr.  */
 349           phivn[SSA_NAME_VERSION (ptr)].value = res;
 350           phivn[SSA_NAME_VERSION (ptr)].vuse = vuse;
 351
 352           /* Remove old stmt.  The phi is taken care of by DCE, if we
 353              want to delete it here we also have to delete all intermediate
 354              copies.  */
 355           gsi = gsi_for_stmt (use_stmt);
 356           gsi_remove (&gsi, true);
 357
 358           phi_inserted = true;
 359         }
 360       else
 361         {
 362           /* Further replacements are easy, just make a copy out of the
 363              load.  */
 364           gimple_assign_set_rhs1 (use_stmt, res);
 365           update_stmt (use_stmt);
 366         }
 367
 368 next:;
 369       /* Continue searching for a proper dereference.  */
 370     }
 371
 372   return phi_inserted;
 373 }
 374
 375 /* Main entry for phiprop pass.  */
 376
 377 namespace {
 378
 379 const pass_data pass_data_phiprop =
 380 {
 381   GIMPLE_PASS, /* type */
 382   "phiprop", /* name */
 383   OPTGROUP_NONE, /* optinfo_flags */
 384   TV_TREE_PHIPROP, /* tv_id */
 385   ( PROP_cfg | PROP_ssa ), /* properties_required */
 386   0, /* properties_provided */
 387   0, /* properties_destroyed */
 388   0, /* todo_flags_start */
 389   TODO_update_ssa, /* todo_flags_finish */
 390 };
 391
 392 class pass_phiprop : public gimple_opt_pass
 393 {
 394 public:
 395   pass_phiprop (gcc::context *ctxt)
 396     : gimple_opt_pass (pass_data_phiprop, ctxt)
 397   {}
 398
 399   /* opt_pass methods: */
 400   virtual bool gate (function *) { return flag_tree_phiprop; }
 401   virtual unsigned int execute (function *);
 402
 403 }; // class pass_phiprop
 404
 405 unsigned int
 406 pass_phiprop::execute (function *fun)
 407 {
 408   vec<basic_block> bbs;
 409   struct phiprop_d *phivn;
 410   bool did_something = false;
 411   basic_block bb;
 412   gimple_stmt_iterator gsi;
 413   unsigned i;
 414   size_t n;
 415
 416   calculate_dominance_info (CDI_DOMINATORS);
 417   calculate_dominance_info (CDI_POST_DOMINATORS);
 418
 419   n = num_ssa_names;
 420   phivn = XCNEWVEC (struct phiprop_d, n);
 421
 422   /* Walk the dominator tree in preorder.  */
 423   bbs = get_all_dominated_blocks (CDI_DOMINATORS,
 424                                   single_succ (ENTRY_BLOCK_PTR_FOR_FN (fun)));
 425   FOR_EACH_VEC_ELT (bbs, i, bb)
 426     for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 427       did_something |= propagate_with_phi (bb, gsi_stmt (gsi), phivn, n);
 428
 429   if (did_something)
 430     gsi_commit_edge_inserts ();
 431
 432   bbs.release ();
 433   free (phivn);
 434
 435   free_dominance_info (CDI_POST_DOMINATORS);
 436
 437   return 0;
 438 }
 439
 440 } // anon namespace
 441
 442 gimple_opt_pass *
 443 make_pass_phiprop (gcc::context *ctxt)
 444 {
 445   return new pass_phiprop (ctxt);
 446 }